Posted By Michael Johnston,
Thursday, March 01, 2007
Updated: Sunday, February 10, 2013
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From the beginning, the National Electrical Code has included specific rules that are essential for protection of persons and property. Wiring and protection is covered more specifically in chapter 2 and is so titled. Article 250 provides the specific rules for grounding and bonding electrical systems and equipment. To understand how the grounding and bonding rules apply to electrical installations, one must establish a thorough knowledge of how grounding and bonding functions from a performance standpoint. In other words, what is intended to be accomplished when a rule requires grounding, and what must be accomplished when the Code requires bonding? (see figures 1 and 2).
Figure 1. Grounding means connected to the earth.
Figure 2. Bonding means connected together
Both grounding and bonding are fundamentally necessary for electrical safety. This article provides a look at extensive work completed over a period of approximately one year that resulted in significant improvements in understandability and usability in the NEC specifically related to grounding and bonding rules. It should be understood that as of this writing, the NEC Technical Committees have completed their work on all proposals and comments for the 2008 NEC, but this work could still be impacted by any appeals that may be filed in accordance with the NFPA Regulations Governing Committee Projects.
Figure 3. Purpose of the equipment grounding conductor is to ground equipment and provide an effective path for ground fault current.
Proposal 5-1 in the 2004 NEC Report on Proposals created quite a stir during the code development process in the 2005 NEC cycle. This proposal introduced a concept of changing the defined term equipment grounding conductor to the term equipment bonding conductor. This is where it all started. The substantiation with the proposal clearly identified some points that warranted serious consideration. As one can imagine, this proposal was met with a wide variety of reactions. Such a proposed change in the NEC flies in the face of tradition and faced skepticism and resistance. Many Code traditionalists saw this proposal as unnecessary and a change that would create confusion and unnecessary work in many other industry and product standards, which is definitely understandable. Others viewed this proposed change with optimism and an open-minded approach to the concepts, and realized the proposed revisions were technically correct in several ways. The challenges presented were similar to those that the industry faced when grounding and bonding rules were first developed. It was clear, based on the initial reaction of nineteen of the code-making panels, that the proposal had merit. Additionally, this proposal identified necessary revisions to defined grounding and bonding words and terms in Article 100. The proposal was ultimately rejected and the change never happened in the 2005 NEC; however, this proposal generated considerable interest and concern from the members of code-making panel 5, which is responsible for Articles 200, 250, 280, and 285. After much deliberation and discussion about this concept and some identified areas in need of improvement, it was determined that the chairman of CMP-5 would recommend to the NEC Technical Correlating Committee (TCC) that a special task group be assembled to explore all of the defined words and terms related to grounding and bonding and verify their accuracy and functionality, and to review their use in Article 250 and other rules throughout the entire Code. The recommendation was viewed favorably by the correlating committee resulting in a TCC assigned Task Group on Grounding and Bonding. A task group chair, who was also a representative of the TCC, was appointed to lead this group in achieving their established set of specific objectives. This provided an excellent conduit for continuous communication and observations by the TCC of the work and progress. The task group was then assembled including seven key members of CMP-5 along with several other key members of other NEC technical committees. There were also two members of the Technical Correlating Committee included in this working group which totaled 18 members.
Photo 1. Bonding through conduit or tubing fittings
The Task Ahead
The Technical Correlating Committee provided clear directives to explore several significant issues identified in Proposal 5-1 and Comment 5-1 in the 2005 NEC cycle regarding grounding and bonding terminology defined and used the Code. The following represents the scope of the assigned Task Group on Grounding and Bonding.
- To explore the issues identified by Proposal 5-1 and Comment 5-1 in the 2005 NEC cycle
- To consider developing proposals for the 2008 NEC to establish consistent use of the terms grounding and bonding as discussed in the identified proposals and comments during the 2005 NEC development process
- To consider other codes and standards, such as the Canadian Electrical Code (CE Code), Part I and International Electrotechnical Commission (IEC) 60364 in an effort to harmonize the definitions and use of the terms grounding and bonding
- To consider the inter-relationship of the NEC with product standards and the National Electrical Safety Code (NESC)
Figure 4. Equipment grounding conductors perform bonding and grounding functions.
Before the work could begin, it was important to get all members of the group on the same level of understanding. One of the most important tasks for this working group was to establish a clear and common understanding of all defined grounding and bonding words and terms. Another essential objective of this group was to review the performance requirements provided in 250.4. Section 250.4(A) and (B) provide the descriptive performance objectives of grounding and bonding. As the work began, it was clear that not all members in the group viewed grounding and bonding in the same fashion, and these members all carry extensive levels of Code experience. The result was a meaningful open dialog for all task group members to share ideas and benefit from each other’s input and experience. The first few months of work were necessary to establish this common understanding before any productive work could begin in achieving the established objectives of the assignment. Each member of the working group quickly realized that not everyone has the same understanding of grounding and bonding, which is also the case in the electrical industry as a whole. For this work to be productive and of benefit to the NEC, progressive thinking and being open to sharing all ideas became an eye-opening realization for the group.
Rules Using Defined Words and Terms
Table 1. Revised grounding and bonding defined words and terms
The definitions in the NEC assist users with proper application of the rules. The Code rules should mean what they imply by definition. When defined words and terms are not used consistently within the rules, it can lead to inconsistent and incorrect application of the requirements. With everyone in the group on the same page, the task of reviewing each of the defined grounding or bonding words and terms was underway. It was soon realized that some definitions needed revision, another was considered for deletion, and new ones were considered. Table 1 shows a a summary of the grounding or bonding words and terms that were affected by the work of this task group.
Photo 2. Bonding using equipment bonding jumpers
The work of the task group on these defined words and terms was primarily in an effort to reduce the definitions to their simplest form. As the work progressed, it was soon realized that many of these definitions were not accurate, and included performance requirements that were already contained in other performance requirements incorporated into 250.4. Keeping defined words and terms in a simple form helps assure that where these words or terms are used in Code rules, they will be accurate in meaning. In recent editions of the NEC, clear performance text was incorporated into Article 250 that explains the purpose of grounding and bonding. With this understanding, the task group realized that keeping the defined words and terms in their simplest form would be beneficial in the essential code-wide work that would follow. The following are the revised definitions of the grounding and bonding words or terms provided in table one.
Article 100 Definitions
Bonded (Bonding).Connected to establish electrical continuity and conductivity.
Ground. The earth.
Grounded (Grounding).Connected (connecting) to ground or to a conductive body that extends the ground connection.
Grounded, Effectively.Definition was deleted because the term is subjective and there are no specific parameters to use in making determinations as to whether or not an entity is effectively grounded. Instances where it was used in previous editions of the Code have been revised to remove the word "effectively” from the phrase. The term grounded, by definition, means connected to the earth. The direct connection to the earth through grounding electrodes is not always effective and varies based on geographical location or seasonal conditions and so forth. The word "effective” is used in the performance rules in Section 250.4 that relate to the effectiveness of the ground-fault current path necessary to facilitate overcurrent device operation, which is appropriate, measurable, and remains unchanged as a result of deleting the definition. The term effectively bonded, which was never defined, was also revised to remove the word "effectively” from the phrase. The performance criteria for bonding and what it is intended to accomplish is already provided in Section 250.4 and 250.90. There are conditions covered by the Code where bonding is required solely to minimize differences of potential between conductive parts such as for health care facilities, swimming pools and similar installations, agricultural buildings, and so forth. The definition of bonding has been revised to simplify its meaning as covered above.
Grounding Electrode.A conductive object through which a direct connection to earth is established.
Grounding Electrode Conductor.The conductor used to connect the grounding electrode(s) to a system conductor or to equipment.
Grounding Conductor, Equipment (EGC).The conductive path installed to connect normally non-current-carrying parts of equipment together, and to the system grounded conductor or to the grounding electrode conductor, or both.
FPN No. 1: It is recognized that the equipment grounding conductor also performs bonding.
FPN No. 2: See 250.118 for a list of acceptable equipment grounding conductors.
Ungrounded.Not connected to ground or to a conductive body that extends the ground connection.
Section 250.2 Definition
Ground Fault.An unintentional, electrically conducting connection between a normally current-carrying conductor of an electrical circuit and the normally non–current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or earth.
FPN: Unintentional grounding connections to the grounded conductors on the load side of the service disconnecting means or the load side of a separately derived system, creates one type of ground fault condition addressed in the definition of ground fault.
Responsibilities for Definitions
Photo 3. Connection to ground through a grounding electrode conductor
Sometimes timing is everything. The 2008 NEC development process included a shift in responsibility for any technical definitions that fall under the scope of responsibility of certain NEC technical committees. Traditionally, all of the definitions in Article 100 were the responsibility of code-making panel 1. Action by the NEC Technical Correlating Committee (TCC) results in each code-making panel being responsible for definitions of words and terms that are under their responsibility, but these definitions will continue to be located in Article 100. Definitions that are general in nature will continue to be assigned to code-making panel 1. As a result, CMP-5 was responsible for reviewing and acting on all proposals to revise definitions of grounding and bonding words and terms. The panel acted favorably to all definition revisions that resulted from the work of the task group.
Table 2. Summary of proposals submitted by the task group
The work of TCC Grounding and Bonding Task Group also included a global NEC review and analysis to address how each of the grounding or bonding words or terms were used in Article 250 and submit proposals for necessary revisions as required. The task group had the responsibility to specifically address how each word or term was currently being used throughout the rest of the rules in the Code. Where revisions were necessary, the task group developed and submitted proposals to each NEC Technical Committee. In many cases, revisions were made to include more specific direction and prescriptive language for users that clarifies what is meant by a rule that includes the words "shall be grounded.” The accepted changes in this instance resulted in changing the term shall be grounded to shall be connected to an equipment grounded conductor where that was the original intention of the requirement. This resulted in better Code that is not subjective and is understood and enforceable. The Grounding and Bonding Task Group developed 28 proposals for the 2008 NEC as provided in table 2.
Each of the code-making panels acted favorably to the proposals resulting from the efforts of the task group; however, not all of the proposals were accepted without some modification. There were some instances where each technical committee had to accept in principle the proposed revision and make slight adjustments necessary for specific functionality. The efforts to make such extensive code-wide revisions involved all of the NEC technical committees embracing the concepts and efforts put forth by the Task Group on Grounding and Bonding.
Photo 4. Ungrounded portable generator
People are generally resistant to change, which is human nature. Breaking tradition is more difficult for some than others. Sometimes change is necessary and meaningful. In the electrical code-making process, change is continuous because of the inherent dynamics of the NEC. Changes should never be made just for the sake of change; there must be purpose (reason), an objective (goal), and a positive result (good code). Good code is code that is understandable, practical, and enforceable. The 2008 Code has experienced a series of changes related to grounding and bonding definitions and rules. The concepts and objectives of these revised definitions and rules are retained, clarified, and improved as a result of extensive work by an assigned group of many dedicated professionals to achieve these objectives. See IAEI’s book, Analysis of Changes 2008 NEC for additional information about these and many other significant changes ultimately incorporated into this new edition of the Code.
Read more by Michael Johnston
Posted By Tim Crnko,
Thursday, March 01, 2007
Updated: Sunday, February 10, 2013
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This article provides readers with essential information about basic operation and basic time-current characteristics of branch-circuit-rated, low-voltage fuses and circuit breakers. These overcurrent protective devices (OCPDs) are typically used in main service disconnects, feeders and branch circuits of residential, commercial, institutional, and industrial electrical systems. There are other OCPDs used, such as relays and supplementary OCPDs, which this article does not directly address. However, many of these principles presented also apply to the other types of devices. This article explains the basics, and as you might suspect, there are product designs for fuses and circuit breakers where the operation principles are more complex and may deviate from what is presented. However, you need to walk before you run. Part II, which will be in the May/June issue, will cover important information regarding OCPD ratings, application in designs, and NEC compliance aspects.
Why Is Overcurrent Protection So Important?
Figure 1. Oscillograph representation of a fault
The author remembers having a conversation some years ago with a well-known industry expert who is very knowledgeable in the National Electrical Code. This expert views grounding and bonding and overcurrent protection as the two most important protective principles in the Code. Grounding and bonding is important for two reasons: (1) improper grounding and bonding can kill people and pose a fire hazard and (2) adequate grounding and bonding helps ensure the overcurrent protective devices operate in a reasonable time by providing a low impedance and effective path for fault current. Overcurrent protection is important to the overall objective of electrical safety. If the designer, installer, maintainer or inspector does not get overcurrent protection right, there can be the threat of fires and personal safety hazards due to (1) long-time thermal ignition of materials from improper overload protection, (2) explosive ignition and flash hazard from improper short-circuit protection or (3) the explosive ignition and flash hazards from improper voltage-rated or improper interrupting-rated overcurrent protective devices.
Figure 2. Example of fuse time-current characteristics
The proper selection of overcurrent protective devices entails many considerations, some mandatory and some discretionary. The mandatory considerations include complying with NEC requirements and ensuring OCPDs are applied within their ratings and limits per their capabilities, which are typically evidenced by specific product standard listing and labeling [110.3(A)(1)].
OCPDs are intended to protect against the effects of potentially harmful overcurrents. An overcurrent is either an overload current or a short-circuit current, which often is referred to as fault current. Overload current is an excessive current relative to normal operating current, but one that is confined to the normal conductive path provided by the conductors and other components and loads of the distribution system. As the name implies, a short-circuit current is one which flows outside the normal conducting path. Article 100 has definitions for overcurrent and overload. One of the important overcurrent protection principles that typically holds true is that the higher the overcurrent magnitude, the faster the overcurrent must be interrupted.
Figure 3. Example of fuse minimum melt and total clear band
Overloads are most typically between one and six times the normal current level. Most often, they are caused by harmless temporary surge currents that occur when motors start up or transformers are energized. Harmful sustained overloads can result from defective motors (such as worn motor bearings), overloaded equipment, or too many loads on one circuit. Such sustained overloads are destructive and must be cut off by protective devices before they damage the distribution system or system loads. However, since they are of relatively low magnitude, removal of the overload current within a few seconds to many minutes will generally prevent circuit or equipment damage. A sustained overload current results in overheating of conductors and other components and will cause deterioration of insulation, which may eventually result in severe damage and short circuits if not interrupted.
Short-Circuit or Ground-Fault Currents
Whereas overload currents occur at rather modest levels, short-circuit or ground-fault currents occur in a wide range of current magnitude. For instance, a fault may be a lower level ground fault (a high impedance fault between phase and ground), a high-level ground fault (a low impedance fault between phase and ground), a high-level bolted three-phase fault (a low impedance fault between all three phases), or a moderate to high level three- phase arcing fault (a moderate or low impedance fault, through air, between all three phases). Since the load is faulted out of the circuit, the circuit impedance is drastically reduced. Since I (current) = E (voltage) divided by Z (impedance), the resulting lower impedance causes the immediate increase in the current (see figure 1). Fault currents can be many hundreds of times larger than the normal operating current. A high-level fault may be 50,000 A (or larger). If not cut off within a matter of a few thousandths of a second, damage and destruction can become rampant; there can be severe insulation damage, melting of conductors, vaporization of metal, ionization of gases, arcing, and fires. Simultaneously, high-level short-circuit currents can develop huge magnetic-field stresses. The magnetic forces between bus bars and other conductors can be many hundreds of pounds per linear foot; even heavy bracing may not be adequate to keep them from being warped or distorted beyond repair. In the last 10 years or so, the industry has begun recognizing the severe flash hazards and blast hazards to personnel due to arcing fault current.
Figure 4. Typical 100A, 600V, Class RK1, dual-element, time-delay fuse
If you understand the physical properties and how the devices operate, you may retain the information better and understand the reasons for specific requirements. The following is a brief, simplified version. There are many types of circuit breakers and fuses, but all follow common, basic principles.
Figure 5. Overload operation
Let’s start with the principle that OCPDs are intended to continuously carry the load current, and if there is an overcurrent condition, their purpose is to open in time to prevent extensive damage to the circuit components. This is a requirement under fault conditions in Section 110.10. The allowable speed of response of an overcurrent protective device can vary depending on the magnitude of overcurrent. If the overcurrent is a light overload, it may be permissible to permit the current to flow for many minutes. As a matter of fact, some circuit components, such as motors, primary winding of transformers and capacitors, have harmless high starting or energizing inrush current which can be many times greater than the normal full load current. So the application of OCPDs on these circuits require that the OCPD permit intentional overload currents for a period of time without opening. If the overcurrent is a faulted circuit, rapid OCPD response is desired to minimize circuit component or equipment damage. The examples in figures 2 and 3 illustrate OCPD time-current characteristics via a circuit diagram with ammeter readings and OCPD opening times for various overcurrents. For higher levels of overcurrent, the OCPD operates faster. Also, this example illustrates that the OCPD characteristics can be represented by time-current characteristic curves. See figures 2 and 3, and for the overcurrents depicted in figure 2 determine the opening times from the curve in figure 3. On the time-current curve, the horizontal axis is the amount of current in amperes and the vertical axis is time in seconds. Note: both the current axis and time axis are logarithmic scale, which is the typical representation for OCPD time-current characteristics. The fuse time-current characteristic is properly represented by a tolerance band with the minimum melt curve as the boundary on the left and the total clear curve as the boundary on the right. So for a given overcurrent value, the fuse opening time is represented by a range. For instance in figure 2, the example with a 500 A overcurrent, the fuse will open somewhere between 10 and 17 seconds (see figure 3). Most fuse manufacturers provide minimum melt fuse curves and total clear fuse curves on separate pages. For simplicity, some users just want a fuse represented by a single line curve not a band, so manufacturers may also represent fuses via an average melt curve. An average melt curve, if overlaid, would fall between the minimum melt and total clear curves.
Figure 6. During short-circuit operation
Fuse operation is based on basic thermal principles. As current flows through a fuse, the resistance of the fuse element creates heat. If the current is below the amp rating of the fuse, the fuse will carry the current continuously (dependent on sizing per the NEC). In this case, the fuse operates in a thermally stable condition and the internal temperature does not reach a point where the fuse opens. The thermal energy created by the current flowing through the fuse element dissipates to the ambient. In overcurrent conditions, the internal temperature of the fuse elevates; the dissipation of thermal energy is less than the thermal energy created. Whether the fuse opens or how fast it takes to open is dependent on the amount of overcurrent and the duration of the overcurrent condition. The following is a series of illustrations to explain how fuses operate. Shown is a dual-element, time-delay fuse construction. There are other type constructions, but the principles are similar. Figure 4 shows a typical 100 A, 600 V, Class RK1, dual-element, time-delay fuse which has a 300,000 A interrupting rating. Artistic liberty is taken to illustrate the internal portion of this fuse. The real fuse has a non-transparent tube and special small granular, arc quenching material completely filling the internal space (see figure 4).
Figure 7. After short-circuit current interruption
Figure 5 illustrates how a dual-element fuse operates in the overload range. Under sustained overload conditions, the trigger spring fractures the calibrated fusing alloy and releases the "connector.” The insets represent a model of the overload element before and after. The calibrated fusing alloy connecting the short-circuit element to the overload element fractures at a specific temperature due to a persistent overload current. The coiled spring pushes the connector from the short-circuit element and the circuit is interrupted.
Figure 8. Example of current-limitation for fault current
Figures 6 and 7 illustrate a fuse operation in the short-circuit current range. A short-circuit current causes the restricted portions of the short-circuit element to vaporize and arcing commences (figure 6: the arcing is depicted by animation). The arcs burn back the element at the points of the arcing. Longer arcs result, which assist in reducing the current. Also, the special arc quenching filler material contributes to extinguishing the arcing current. The clearing time of a fuse under short-circuit current conditions is the time it takes to melt or vaporize the fuse element’s restricted portions plus the arcing time. The time to melt or vaporize depends on the fuse design and current magnitude. The time duration from the point of the fuse element melting or vaporization until the current is interrupted is rather fast. Typically, this time will be a fraction of a half cycle. For current-limiting fuses in their current-limiting range, the total time to clear is ½ cycle or less (melting plus clearing).
Figure 9. Illustrates various fuse characteristic curves
The special small granular, arc-quenching material plays an important part in the interruption process. Figure 7 shows an actual photo of the internal fuse element after interrupting a fault. The filler assists in quenching the arcs; the filler material absorbs the thermal energy of the arcs, bonds together and creates an insulating barrier. This process helps in forcing the current to zero. It is this entire process that enables fuses to be current-limiting. What does this mean? When the fault current is in the fuse’s current limiting range, the fuse cuts-off the current before it reaches its first peak current value by vaporizing the restricted portions of the fuse element. Then the current is forced to zero via the process with arcing and the filler quenching the arcing before the first ½ cycle of the fault current. Current-limitation greatly reduces the energy that is released in the circuit (see figure 8).
The interruption process is critical for a fuse. To have sufficient voltage rating and interrupting rating a fuse must be designed properly. Critical in achieving a specific voltage rating are the number of restricted portions or neck-down sections in series. For the fuse shown in this example, there are five restricted portions in series and this fuse is rated 600 Vac. If this fuse were misapplied in a 1500 V circuit and the fuse tried to interrupt, the arcing at the restricted portions would probably continue until so much energy was released that the fuse could violently rupture. There are not enough restricted portions in series for this 600 V fuse to interrupt 1500 V. Similarly, when a fuse attempts to interrupt high fault currents, the fuse must be designed to withstand the tremendous pressure produced inside the fuse body as a result of the rapid vaporization and arcing of a portion of the fuse element. If a fuse tries to interrupt a fault current greater than its interrupting rating, the fuse can violently rupture. Section 110.9 requires that the available short-circuit current at the line terminals does not exceed a fuse’s interrupting rating or a circuit breaker’s interrupting rating. This is a matter of safety.
Figure 10. Circuit breaker operating functions
Several different fuse characteristic types have evolved over the years, each having different time-current characteristics and different degrees of current-limitation under short-circuit conditions. For instance there are non-time-delay fuses (for non-inductive loads), time-delay fuses (for motor loads and now used for most general-purpose applications and even static loads), high-speed fuses (often referenced as semiconductor fuses used for the protection of power electronics). Figure 9 illustrates the minimum melt time-current curve characteristic for three 100 A, 600 V fuse types:
- High-speed fuse
- Class J time-delay fuse
- Class RK5 time-delay fuse
Circuit Breaker Operation
Circuit breakers are mechanical overcurrent protective devices. All circuit breakers share three common operating functions:
- Current sensing means:
- Unlatching mechanism: mechanical
- Current/voltage interruption means (both)
A. Contact parting: mechanical
B. Arc chutes
To interrupt an overcurrent, the chain of events is significantly different from that of a fuse. First, the circuit breaker senses the overcurrent. If the overcurrent persists for too long, the sensing means causes or signals the unlatching of the contact mechanism. The unlatching function permits a mechanism to start the contacts to part. As the contacts start to part, the current is stretched through the air and arcing between the contacts commences. The further the contacts separate the longer the arc, which aids in interrupting the overcurrent. However, in most cases, especially for fault current, the contacts alone are not sufficient to interrupt. The arcing is thrown to the arc chutes which aid in stretching and cooling the arc so that interruption can be made. Figure 10 shows a simplified model with the three operating functions shown for a thermal magnetic circuit breaker, which is the most commonly used circuit breaker. Also, it should be noted that there are various contact mechanism designs that can significantly affect the interruption process.
Circuit Breaker Overload Operation
Figures 11A and 11B illustrate circuit breaker operation by the thermal bimetal element sensing a persistent overload. The bimetal element senses overload conditions similar to the sensor in a HVAC bimetal thermostat. In some circuit breakers, the overload sensing function is performed by electronic means. In either case, the unlatching and interruption process is the same as illustrated in figures 11A and 11B. Figure 11A illustrates, as the overload persists, the bimetal sensing element bends. If the overload persists too long, the force exerted by the bimetal sensor on the trip bar becomes sufficient to unlatch the circuit breaker. Figure 11B shows that once a circuit breaker is unlatched it is on its way to opening. The spring-loaded contacts separate and the overload is cleared. There can be some arcing as the contacts open, but the arcing is not as prominent as when a short-circuit current is interrupted.
Figure 11a. Circuit breaker senses overload and unlatches
Figure 11b. Circuit breaker contacts open and clear overload
Circuit Breaker Instantaneous Trip Operation
Figures 12A, 12B, and 12C illustrate circuit breaker instantaneous trip operation due to a short-circuit current. The magnetic element senses higher level overcurrent conditions. This element is often referred to as the instantaneous trip, which means the circuit breaker is opening without intentional delay. In some circuit breakers, the instantaneous trip function is performed by electronic means. In either case, the unlatching and interruption process is the same as illustrated in figures 12B and 12C.
Figure 12a. Circuit breaker instantaneous trip sensing and unlatching
Figure 12b. Circuit breaker contacts part and arcing
Figure 12c. Circuit breaker contacts open and fault cleared
Figure 12A illustrates the operation under a short-circuit condition. The high rate of change of the current causes the trip bar to be pulled toward the magnetic element. If the fault current is high enough, the strong force causes the trip bar to exert enough force to unlatch the circuit breaker. This is a rapid event and is referred to as instantaneous trip.
Figure 12B shows that once unlatched, the contacts are permitted to start to open. It is important to understand that once a circuit breaker is unlatched it is designed to open; however, the current interruption does not commence until the contacts start to part. As the contacts start to part, the current continues to flow through the air (arcing current) between the stationary contact and the movable contact. At some point, the arc is thrown to the arc chutes, which stretch and cool the arc. The speed of the contacts opening depends on the circuit breaker design. The total time of the current interruption for circuit breaker instantaneous tripping is dependent on the specific design and condition of the mechanisms. Smaller amp rated circuit breakers may clear in ½ to 1 cycle. Larger amp rated circuit breakers may clear in a range typically from 1 to 3 cycles depending on the design. Circuit breakers that are listed and marked as current-limiting can interrupt in a ½ cycle or less when the fault current is in the circuit breaker’s current-limiting range.
With the assistance of the arc chutes, the current gets interrupted when the current approaches zero in the normal course of the alternating current and the contacts travel a sufficient distance (see figure 12C). There can be a tremendous amount of energy released at the contact interruption path and arc chutes during the current interruption process. Circuit breakers are designed to have specific interrupting ratings at specific voltage ratings. For instance, a circuit breaker may have a 14,000 A interrupting rating at 480 Vac and 25,000 A at 240 Vac. If a circuit breaker is misapplied by installing it in a circuit with an available short-circuit current greater than the circuit breakers interrupting rating, the circuit breaker can violently rupture when attempting to interrupt.
Typical Circuit Breaker Time-Current Curve
Figure 13. 400 A molded case circuit breaker time-current curve
Circuit breaker curves are represented in various formats as time-current curves. Figure 13 illustrates a 400 A molded case circuit breaker curve. This is an older style circuit breaker time-current curve representation and the author has not seen curves published with this much detail in recent time. The newer curves do not provide the unlatching time or unlatching curve for the instantaneous trip. However, this curve format is good for learning how a circuit breaker functions. Once you understand there is an unlatching curve, you can interpret the modern curves to make evaluations, if necessary.
The shaded "Overload Operation” portion represents the characteristics of the overload protection with the bimetal element as described in figures 11A and 11B. Notice the representation is a tolerance band not a line curve. This is similar to the fuse tolerance band. If an overload persists long enough, the circuit breaker is intended to open at some point within that "Overload Operation” band. For instance, a 1000 A overload current would be expected to be interrupted between 70 seconds and 300 seconds (see figure 13).
Figure 14. Circuit breaker with overload protection and short-time delay setting
The shaded "Instantaneous Trip” portion represents the characteristics of the short-circuit protection with the magnetic element as described in figures 12A, 12B, and 12C. The band for a specific level of current represents the time of unlatching, parting of the contacts, and extinguishing the current/arcing. The average unlatching time for the instantaneous trip function is shown as a diagonal line; this corresponds to the unlatching described in figure 12A. Once a circuit breaker is unlatched, it still needs to part its contacts and extinguish the arcing; this corresponds to figures 12B and 12C. For instance, on this 400 A circuit breaker curve, a 10,000 A fault current would unlatch the circuit breaker in 0.0025 seconds. Then the contacts part and the current extinguished within 0.028 seconds (approximately 1½ cycles). Note: figure 13 shows the characteristics from 0.001 to 0.01 seconds to illustrate the circuit breaker’s unlatching characteristics. Most fuse and circuit breaker curves show characteristics from 0.01 seconds and greater.
Figure 15. Circuit breaker with overload protection, short-time delay, and instantaneous trip override
There is a variety of circuit breaker types for different application needs. For instance, there are instantaneous trip-only circuit breakers that are intended for motor branch circuit short-circuit protection. There are circuit breakers that have a short-time delay setting that are used either in lieu of the instantaneous trip element (see figure 14) or in conjunction with an instantaneous trip override (see figure 15).
Conclusion and Part II
The information in this Overcurrent Protection Basics, Part I, provided an understanding of how fuses and circuit breakers operate and on the basics of how to read time-current curves. In the next issue, Overcurrent Protection Basics, Part II, we leverage off this material to look at the important ratings for fuses and circuit breakers and other key important criteria that lay the foundation for a better understanding of overcurrent protection and code-compliance.
Read more by Tim Crnko
Posted By Ark Tsisserev,
Thursday, March 01, 2007
Updated: Sunday, February 10, 2013
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The object of the Code is very transparent on the fact that all prescriptive rules of the Code address the objective-based fundamental safety principles of the IEC Standard "Electrical Installations of Buildings”.
This standard, IEC 60364-I, comprises the following types of protection:
- against electric shock (direct and indirect contact);
- against thermal effect (high temperature or electric arc);
- against overcurrent (damage due to excessive temperatures or electromechanical stress caused by overcurrents likely to arise on live conductors);
- against fault currents (against dynamic and thermal effects of fault currents); and
- against over-voltage (harmful effects due of a fault between live parts of circuits supplied at different voltages).
Thus, it is quite obvious that the prescriptive requirements of the CE Code represent very focused acceptable means by which objective-based fundamental safety principles of IEC 60364-I may be accomplished. Rules of the installations are well coordinated and correlated between different sections of the Code. Rules of General Sections are applicable throughout the Code, unless they are specifically modified or amended by particular rules of Supplementary or Amendatory Sections. Sections 0 to 16 and Section 26 are considered to be "General Sections”, and other sections are "Supplementary or Amendatory”.
For example, Rule 6-206 (in General Section 6) mandates that a service box must be installed within the building being served, and that the deviation from this requirement may be entertained only under provisions of a special permission.
However, Section 36 (which is amendatory to General Sections) allows outdoor installation of the H.V. service boxes. Another example is Rule 14-104. Although, in general, this rule states that the rating or setting of overcurrent devices must not exceed the allowable ampacity of conductors that these o/c devices protect, the Rule further allows deviations from this requirement if the conditions of Table 13 are met, or if particular provisions for coordination between the allowable ampacity of conductors and the o/c devices protecting these conductors are governed by other rules of the Code.
In fact, these latter criteria could apply for General and Supplementary Sections. For instance, perfect examples of deviations from Rule 14-104 permitted by the Code are: Rules 26-208 and 26-210. These rules belong to another General Section (Section 26). But they govern installation of capacitors (of a very specific equipment that requires a unique coordination between the setting of the overcurrent devices and ampacity of conductors protected by these o/c devices), and as such these particular rules of Section 26 represent an exception to general provisions of Rule 14-104.
Other similar examples may be found in various supplementary sections (i.e., Rules 28-106 and 28-200 — for motors; Rule 62-114(7) and (8) — for space heating, etc.).
It is interesting to note that requirements of the installation Code (CEC, Part ) are also coordinated with provisions of the safety standards for electrical products (for electrical equipment, cables and wiring devices). In fact, each of these product standards states in its scope that the standard covers design and construction requirements of a specific type of electrical product that is intended to be installed in conformance with the CE Code, Part I.
No wonder that the installation Code is called the CEC, Part I, and each safety standard for an electrical product is called CSA Part II Standard.
So, this coordination perfectly demonstrates the fact that the safety requirements for design and construction of electrical products and installation rules represent two separate but complimentary parts of the single Canadian Electrical Code.
As it was mentioned above, the rules of the Code are correlated through the document to create a set of versatile and comprehensive requirements for safe electrical installations.
However, there are numerous relaxations from these rules in the body of the Code. These relaxations are manifested by notwithstanding clauses or statements such as "it shall be permitted”. These relaxing provisions are clearly linked to the specific conditions of installations, spelled in the rule. But there are also circumstances, where Code allows deviations from its prescriptive requirements based not on specific technical conditions, but rather on the fact that each such deviation may be obtained for the particular installation only from the authority that provides a regulatory enforcement of the installation.
Each situation where a deviation from the rules is deemed to be possible is conditional to provisions of Rule 2-030 "deviation of postponement”. Under this Rule, a designer (or an electrical contractor who obtained a permit for the electrical work) may approach the regulator with a request to deviate from a specific prescriptive requirement of the Code. Each such request must relate to a very particular installation (i.e., it must be unique, and not be used as a blanket approach), and it must be clearly substantiated that the relevant objective-based fundamental safety principles of the IEC 60364 (see above listed types of protection described by the IEC 60364) are fully met by that unique request.
Regulators evaluate each such request and may grant a special permission to deviate from the Code requirements for the particular installation.
However, the regulatory authority cannot grant a special permission for use of unapproved equipment. All equipment (except for provisions of Rule 16-222(2)) must be approved in order to be installed under rules of the CE Code, Part I. Code users should note that approved is the defined term, and this definition is clearly spelled out in the Code.
It should be also noted that the local regulatory authority must be consulted on a matter of deviations in every particular case of installation.
Read more by Ark Tsisserev
Posted By Jesse Abercrombie,
Thursday, March 01, 2007
Updated: Sunday, February 10, 2013
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Many electrical contractors and inspectors probably were not that familiar with the alternative minimum tax (AMT) a few years ago. However, in the past few years things have really changed. Even the name may trigger a lot of speculation. What, exactly, is this tax an "alternative” to? And, what does the word "minimum” mean? Is it the smallest possible tax that can be assessed? If so, who has to pay it?
Here is a little background on the AMT. First, there has been some type of minimum tax ever since 1969. Because many well-off individuals used credits and tax breaks to cut their tax liability to little or nothing, Congress passed laws requiring taxpayers to calculate their tax liability first under the conventional method and then under the AMT method—and then pay whichever tax is higher.
Although the AMT rates of 26 and 28 percent are lower than the top regular tax rates, the AMT rates are levied on a broader income base—one that excludes personal exemptions and many itemized expenses.
For many years, the AMT affected relatively few people, but that has begun to change. The number of people subject to the AMT will shoot up from 1.4 million in 2001 to about 30 million in 2010, according to the Tax Policy Center of the Brookings Institution and the Urban Institute. And if the tax cuts of 2001 and 2003 are extended, this number will climb to almost 40 million by 2014.
What is behind these big jumps?
Consider these two factors:
No adjustment for inflation—Most taxpayers have been shielded from the AMT by its large exemption. But this exemption is not adjusted for inflation, so, as wages and earnings rise each year, more and more people will be subject to the AMT. The exemption amounts for 2006 are:
- 58,000 if married filing jointly or as a surviving spouse
- $40,250 if single or a head of household
- $29,000 if married filing separately
However, barring Congressional action in the last few months of 2006, the basic AMT exemption is scheduled to decrease in 2007 to its prior levels of $45,000 for joint returns and $35,750 for unmarried taxpayers. If these exemption levels were to return, we could see a huge jump in the number of people paying the AMT in 2007.
New tax brackets—In 2003, Congress lowered the tax brackets. These lower brackets, combined with the available exemptions and deductions, mean that many middle- and upper-income taxpayers’ regular taxes will now be lower than the AMT, which means they will have to pay the AMT.
If you are subject to the AMT, you’ll have to deal with more complicated tax returns
Plus, tax planning is more difficult, because you can’t always predict when you will face the AMT; consequently, you could lose valuable tax breaks. For example, if you have a home-equity loan of up to $100,000, your interest is normally deductible under the regular tax calculations. But, if you’re forced to calculate your tax liability using the AMT formula, your home-equity loan may not be deductible, particularly if it’s used for purposes other than home improvement. Years ago selecting a good tax-free bond for my contractors was simple. Most contractors got involved in the bonds that they were working on. Recently some of their favorite projects do not make the best investments because they too can still be subject to AMT. The lesson here is if you are looking at tax-free bonds, make sure that they are not subject to AMT.
See your tax advisor
In October 2006, a presidential tax reform panel recommended eliminating the AMT, but no one can predict if this recommendation will ever become law.
In the meantime, see your tax adviser to determine if you are susceptible to the AMT; and, if so, what you can do about it. If you have a good understanding of how the AMT works, you won’t be surprised when tax time rolls around.
Read more by Jesse Abercrombie
Posted By Tatjana Dinic,
Thursday, March 01, 2007
Updated: Sunday, February 10, 2013
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The Electrical Safety Authority (ESA) is responsible for public electrical safety in Ontario, Canada, and operates as a Delegated Administrative Authority of the Government of Ontario. As part of its mandate ESA is given the authority to enforce the Ontario Electrical Safety Code (OESC). The Code defines the standard for safe electrical products and installations in Ontario, and when followed; protects public, workers, contractors and business owners.
What do we consider as Unsafe Electrical Products?
ESA considers three product categories defined as Unsafe Electrical Products:
- Unapproved electrical products
- Electrical products with suspected counterfeit manufacturer or certification label
- Certified but unsafe electrical products
Unapproved electrical productsare the products that have not been certified by a recognized certification agency. The OESC requires products to be certified by one of the recognized certification organizations accredited by the Standards Council of Canada.
Electrical products with a suspected counterfeit labelare products that bear a counterfeit certification label, or fake manufacturer label. For the past several years, ESA has worked in coalition with certification and enforcement agencies, along with manufacturers in various investigations. In all, counterfeit extension cords, power bars, lighting fixtures, breakers, power tools, etc., were discovered. For example, extension cords with counterfeit certification labels are most often identified with undersized wires that will overheat and fail. Very often, the product enclosure is made of flammable material. ESA’s biggest concern is that the users that have purchased those products may not be aware of the potential hazard these products present to their homes and families.
Certified but unsafe electrical productsare the products that have been certified by a recognized certification agency; however, they have failed in an unsafe manner or have some identified safety issues. In these cases, ESA initiates a product incident report (PIR). Some reported products have initiated a product recall or safety advisory.
Response process based on the risk assessment
Figure 1. Potential electric shock and fire hazards associated with unapproved infrared saunas
In order to manage the unsafe products issues, it is important to have a defined response process in place. This response process should not only allow for the initial assessment of potential risk elements, but also for prioritization, implementation of mitigating actions, periodic reassessment of risk, and mitigation priorities.
Therefore, ESA developed a response process for unsafe electrical products based on risk assessment. Unsafe products are rated with corresponding low to high risk assessment to determine the response strategy. High-risk products receive priority action and aggressive response strategies.
Identifying risk criteria for unsafe electrical products was a challenging task. The challenge lay within many factors that had to be considered for assessment. ESA staff, manufacturers, certification agencies plus other external stakeholders as well as other Canadian Provincial jurisdictions had an opportunity to comment and aid in the development of ESA’s risk assessment criteria for electrical products.
Figure 2. Recognized Agency Certification markings
Hazard identification is a key element in risk assessment. Hazard is defined as "the potential to cause harm.” Identification and analysis of electrical products, their mode of operation and failure, are essential in ensuring that all relevant hazards and potentially hazardous situations are addressed. It is important to consider all stages in the product life including installation, operation, maintenance and failure. Product standards require that if a product fails, it has to do so in a safe manner. The hazards that were considered included: shock, fire, fumes, heat, noise, and toxic substances.
Five Risk Groups are considered:
- User characteristics and human-device interaction
- Product design and characteristics
- Product source
- Perception of risk
User characteristics and human-device interaction
Risk factors considered under this category are:
- users qualifications/skill level,
- exposure, and the
- amount of personal contact/interface.
For example, electrical products used for hair care (curling irons) are rated as intensive in the category that considers the amount of contact between the human and a product. On the other hand, generators or patio string lights will be rated as minor or minimal in the same category.
Where is the product used? Risk factors considered under this category are:
- locations, and
- conditions, such as weather, humidity, air quality and temperature.
Product design and characteristics
Risk factors considered under this category are:
- adequacy or appropriateness of design and materials, equipment failure mode, product application, and
- product certification.
Electrical products that have not been evaluated by any certification agency will receive maximum points in this category.
Risk factors considered under this category are:
- manufacturer information,
- retailer information, and
- purchasing restriction versus availability.
Information based on past interaction and availability of a quality control program, such as ISO 9001, will be a deciding factor in determining the low or high point allocation.
Perception of risk
An individual’s perception of risk is amplified if the product or product incident has been or is:
- implicated in media,
- a catastrophic incident,
- a health care product,
- an incident involving a child, and
- product recalls and safety alerts are on file.
Figure 3. Recognized Field Evaluation markings
This category is ESA’s attempt to understand and anticipate people’s extreme aversion to some hazards, and their indifference to others. The items listed were identified to play a prominent role in decisions that people make. Experts tend, for a variety of good reasons, to focus on measurable, quantifiable attributes of risks. The public, on the other hand, focuses less on quantitative aspects of risk, and responds to the qualitative, and attributes like fairness and controllability.
Risk assessment for unapproved infrared sauna
In one of the investigations, it was identified that a potential electric shock and fire hazards exists associated with infrared saunas. Information collected identified the following: the item was unapproved; it was combustible; and had exposed live connections. For more information, see ESA safety alerts,http://www.esasafe.com/Alerts.php
Based on the risk assessment criteria identified above, the rating for unapproved infrared saunas was assessed and figure 1 represents the risk factor.
The total score for the unapproved infrared saunas is 100, making them a high-risk product based on ESA’s risk assessment.
The following were some key events that occurred once the assessment was identified as high risk: ESA identified the hazard to the manufacturer and requested immediate corrective action; laid charges against one manufacturer that did not comply by continuing to distribute unsafe saunas; and ESA issued a Safety Flash notice.
How does response process work?
Unapproved products.Upon receiving a complaint or information about the unapproved product, ESA would undergo a risk assessment and depending on the findings, initiate a response strategy based on the evaluated risk assessment. If the risk is identified to be low, ESA’s response may be a warning letter to the manufacturer or retailer/distributor. In the warning letter, ESA requests that the individual stop distributing or using the unapproved product, and confirm in writing compliance with the approval requirements. The party is advised that failure to respond and meet approval requirements may require ESA to conduct a formal investigation, whereby charges may be laid. A person or company that contravenes the OESC requirement for the product approval may be prosecuted and upon conviction, is subject to fines up to $50,000 and/or one-year imprisonment.
High-risk productsreceive ESA’s aggressive response. The response includes a warning letter, inspector verification, and immediate response from manufacturer or retailer. ESA will require immediate corrective action and inform the public. They will consider issuing a Safety Alert, preferably in conjunction with the manufacturer /distributor.
Products with a suspected counterfeit manufacturer or certification label
The response process for products with suspected counterfeit manufacturer or certification label involves cooperation between certification agency, manufacturer (if known), and the Royal Canadian Mounted Police (RCMP). The critical step in the process is confirmation from manufacturer or certification agency that the label is, in fact, counterfeit. The process continues following the risk assessment model but with differences in requested response time between high, medium, and low risk products. ESA’s priority goal is to remove the products with counterfeit labels from the market.
Certified but unsafe electrical products
After receiving a complaint about failures or safety issues of certified product, ESA initiates the categorization of the risk assessment, which will determine the response strategy. However, the process starts with issuing a product incident report (PIR) to the certification agency that certified the product. The certification system in Canada is based on the third party certification; therefore the certification agencies are responsible to investigate safety issues and compliance to safety standards. Recently, ESA has started sending PIRs to manufacturers and distributors, so that they are aware at the very beginning about the concerns. Recent experience shows that working with manufacturers and distributors expedites the investigation.
If several unsafe product failures have been reported, the product may be rated as high risk; the certification agency, manufacturer or distributor may be advised to take corrective action with products still currently on the market as well as with items that are in the consumer’s possession. This might require a safety alert, retrofit and/or recall issued to the public.
How does ESA track unsafe electrical products issues?
ESA has developed an Unsafe Products Database. From October 2005, ESA has responded to over 200 complaints about unsafe products. Its database contains information about the products, manufacturers and retailers. Its database reflects response process and identifies resolution. More than 80 percent of unapproved product complaints are closed. The file closure suggests that the unapproved products were removed from the Ontario stores; unapproved products were certified or field-evaluated. In a few cases, manufacturers or distributors who did not comply with the product approval requirements were prosecuted and charged. Only about 5 percent of the complaints were categorized as high risk requiring quick and direct intervention from all affected parties.
Informing the Public
In the last several years, ESA issued more than 20 Safety Alerts and Flash Safety notices about unsafe electrical products. ESA has established close working relationships with their safety partners in Ontario and across North America.
ESA’s recently redesigned web site has a section that is dedicated to Unsafe Electrical Product. The section "unsafe electrical products” provides information about regulations that define approval requirements; recognized certification and field evaluation marks and exceptions to approval requirements.
This link also contains Recalls (gathered from U.S. Consumer Product Safety Commission, CSA, UL, and manufacturers), Safety Alerts and Flash Notices. This ESA web site section represents one of the most useful sources in Canada for electrical product recalls and safety alerts. The information is updated on a regular basis as information is received.
For more information about ESA’s unsafe product initiative, visit http://www.esasafe.com/.
Read more by Tatjana Dinic
Posted By David Young,
Thursday, March 01, 2007
Updated: Sunday, February 10, 2013
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Electric utility rates vary greatly from utility to utility and from state to state. To protect your and your company’s wallets, it is very important to understand the rates by which you are being charged for electricity. The cost of electricity is so high that some commercial and industrial companies have entire departments whose sole responsibility is to study the utility rates and make load management recommendations for saving money. The work of these departments pays for itself many times over.
Load management is changing the way a company or household operates electrical devices to save money. I am not talking about just turning off lights when not needed. Yes, that does save money. What I am talking about is operating electrical equipment during off-peak hours when the cost of electricity is cheaper. I will explain off-peak and on-peak when I get into the actual rates in part two of this series. Unlike most consumables, the cost of electricity varies with the time of day, day of the week and month of the year.
Most companies just have someone pay the bill and do not realize for what they are paying. To give you an idea of what I am talking about, I want to share a true story with you. While working in the engineering department of a large electric utility, I was often asked by the marketing department to talk to customers with technical questions. In one such occasion, I was asked to talk to a commercial customer about the company’s electric bills. The customer was questioning why the electric bills at their 20,000 square foot warehouse suddenly had doubled. At the request of the customer, the utility had already checked the accuracy of the metering.
I asked the customer if there had been any recent changes to the facility. They said they had added a mezzanine, a partial second floor, within the warehouse so they could store more material. They expected some increase in their electric bill since additional lighting was installed under the mezzanine, but they did not expect their bills to double. They had a twenty-four-hour, seven-day- per-week operation. The building was not heated or air-conditioned. They said their only electrical load was the lighting.
We walked around the facility and stepped into the elevator to go up to the mezzanine— yes, a large elevator capable of lifting a fully loaded fork truck. They had forgotten about the elevator in our discussion of new loads. Though there was a staircase to the mezzanine level, they used the elevator even for pedestrian use. I asked them about the capacity of the elevator and the frequency of use. They said they only used it about a dozen times per day. Since the elevator was only on for a few minutes each time they operated it, they could not imagine how it would increase their electric bill significantly.
I sat down with them and explained the electric rate by which they were being billed. They were being billed an energy charge for the number of kilowatt hours (kWh) of energy they consumed and a demand charge for the maximum amount of power in kilowatts (kW) they used. Demand is how much power is being used by the electrical loads at any instant in time. If you were trying to decide what size generator to purchase to supply all the power for a facility, you would have to know the maximum demand of the facility. Fortunately for the customers of most utilities, utilities do not charge for the maximum demand. They usually charge for the maximum one-hour or fifteen-minute demand. For some utilities, the maximum one-hour demand is the maximum average demand for any rolling one-hour period. Some utilities use a clock hour. In this case, a commercial company can save a lot of money, for example, if they have a compressor that has to be on half the time, they operate it from half past the hour to half past the next hour. The resulting one-hour demand for the compressor is then half of what it would be if operated randomly.
Maximum fifteen-minute demand is the maximum average demand usually for any rolling fifteen-minute period. The maximum fifteen-minute demand is usually less than the peak demand and the maximum one-hour demand is usually less than the fifteen-minute demand. If the load is constant, all three demands are the same.
In the case of the 20,000 square foot warehouse, the demand was constant prior to the addition of the elevator. With the addition of the elevator, the demand charge had gone through the roof. I found out that though the elevator was used only a few times each day, they usually operated it several times in a fifteen-minute period. The demand charge was based upon the maximum fifteen-minute demand and, therefore, it went up.
They asked what they could do to reduce the cost. Since the lifting height limit of their fork trucks would permit lifting pallets to the mezzanine level, one of my suggestions was to remove a ten-foot-wide opening in the mezzanine wall and use the fork trucks to lift pallets directly to the mezzanine level. They could use a small walk-behind fork truck to move the pallets around on the mezzanine level. If they had to stack pallets on the mezzanine, they would have to keep a regular fork truck on the mezzanine.
With this scenario, I suggested they lock the elevator controls in the off position. I received a call from the president of the company about six months later. He was very happy with the lower electric bills and he thanked me for my assistance. At that point, he felt the cost of the elevator had been a waste. He was not happy with the elevator company and the electrical contractor for not warning him of the effect the elevator would have on the electric bills.
Next time, in Part 2, I will get into the details of typical electrical rates and how you can save money with load management both at home and on the job.
Read more by David Young
Posted By James W. Carpenter,
Thursday, March 01, 2007
Updated: Sunday, February 10, 2013
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The holidays are over and the stress of getting just the right gift for that special someone, of fighting the crowds at the stores, and wondering if the money will last is finally over. Now just the stress of paying the bills as they come in is on us. But was it not worth it to see the happy faces of the children and/or the grandchildren? Now it’s back to the regular grind. But it doesn’t have to be a regular grind. This year, 2007, should be an exciting year for us all. A new National Electrical Code will be published. New learning opportunities will abound— new challenges to meet and conquer, and a whole year to do those things that we have been putting off.
IAEI and its members will have many opportunities to get involved in many wide-ranging activities from educational opportunities at seminars, chapter, division, and the annual section meetings, to supporting the association by getting new members and encouraging present members to get in and stay active. Your International Office has already been hard at work in many different areas. Preparing for the 2008 Analysis of Changes is well underway. We have several updating projects under way. Study guides have been updated to the 2005 edition of the NEC and should be available in the first quarter of 2007. The One- and Two-Family Dwelling Electrical Systems book based on the 2005 NEC and the 2006 IRC will be available in March 2007. Seminars are being planned and scheduled for 2007 on various timely subjects. New ways of presenting subject material are being planned to compliment our standby classroom style of presentation. Distance learning, taking continuing education courses on the internet, will be expanded through IAEI’s and UL’s joint program on UL University. Our new codes and standards specialist, Mike Weitzel, has added to the productivity of the Education Department. As a result, the Publication Department’s staff has been under increased pressure to turn around the material for publication. They continue to do their high quality work.
The International Office finished 2006 under an existing membership software system that was being phased out, and started 2007 with a new system from another supplier. The changeover went smoothly, which is surprising since our online membership, ordering, and event registering were off-line for several weeks. The staff had to be trained, so during the first week of January 2007 you may have had difficulty getting to a staff member. We apologize for the difficulty but, hopefully, now things are even better than before. The services provided for you by the International Office are growing and more exciting things are being planned.
As IAEI and you continue into 2007, I would like to expand on my editorial in the January/February issue of the IAEI News. Last issue I asked for you to reflect on why you were in the electrical industry. This time let’s explore why you are a member of the International Association of Electrical Inspectors. In a survey that we did last year several comments from people were noted. They were comments such as:
- IAEI does an outstanding job of educating the electrical industry, particularly the electrical inspector. if you think education is expensive, try ignorance.
- I have been a member since 1956, and IAEI has been an important part of my career.
- The organization has helped me in my career.
- Good association for promoting the field
- Information from articles in the IAEI News.
- Of course, not all comments were complimentary to IAEI but that gives us areas to improve.
I asked last time for you to tell us your story about the reasons you are in the electrical trade. Now you should add to that story why you are a member of IAEI and what it means to you.
Each time the International Board of Directors, the Executive Committee, or the International Membership Committee meet, a discussion always takes place on how to present the benefits of membership to the public. More difficult for them is to identify those benefits. Everyone has different benefits that he or she considers most important. So how about at the next chapter or division meeting that you ask your fellow members what they consider as benefits of membership. Then let the International Office know. We can then better serve all the electrical industry.
Maybe most will cite IAEI’s participation in the development of codes and standards, representing the association among the electrical industry and the public, or collecting, interpreting, and disseminating information on subjects related to the profession. One caller recently told me that he wanted to join IAEI because his insurance agent told him that he could get better rates if he belonged to associations like IAEI. That, in itself, may not be an accurate reason to join IAEI, but if the member takes advantage of the benefits and opportunities to better himself, then certainly he or she would be a lesser risk for the insurance company. Whatever your reasons are, write them up and let us all know.
Read more by James W. Carpenter
Posted By Leslie Stoch,
Thursday, March 01, 2007
Updated: Sunday, February 10, 2013
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The Canadian Electrical Code defines a grounding electrode as: "a buried metal water-piping system or metal object or device buried in, or driven into, the ground to which a grounding conductor is electrically and mechanically connected.” In other words, it’s whatever metal objects the code allows you to drive into or bury in the earth and use for grounding electrical systems. The requirements for grounding electrodes up to 750 volts are found in Rule 10-700. This rule has been substantially rewritten in the 2006 Canadian Electrical Code.
Rule 10-700 of the 2006 CE Code opens by listing three different types of grounding electrodes, manufactured, field-assembled and in situ grounding electrodes that form a part of an existing infrastructure (for example a building). The rule provides several new definitions and an expansion in the number of permissible options for establishing a grounding electrode. Unfortunately, the new rule also may produce some new areas of possible confusion, since some parts of the rule are less prescriptive than in the past.
Manufactured grounding electrode, Rule 10-700(2)
Subrule 2(a) —A manufactured grounding electrode may consist of two ground rods, spaced no closer that 2 m apart, bonded together and driven full length into the earth. Except for some new verbiage, nothing much has changed here.
Subrule 2(b) —As before, it may also consist of an approved plate electrode buried in the earth, at least 600 mm below finished grade or encased in the bottom 50 mm of a concrete slab that is in direct contact with the earth, and not less than 600 mm below finished grade. A plate electrode must provide at least .2 square m surface area in contact with the earth. Once again, nothing has changed.
Field assembled grounding electrode Rule 10-700(3)
Subrule 2(a) —A field-assembled grounding electrode may consist of a bare copper conductor at least 6 m long, sized in accordance with Table 43 and installed in the bottom 50 mm of a concrete footing or foundation and not less than 600 mm below finished grade. This has sometimes been referred to as a "ufer ground,” named for the person who dreamed up the idea. Once again nothing has changed.
Subrule 2(b) —A bare copper conductor at least 6 m long, sized in accordance with Table 43 and buried in the earth at least 600 mm below finished grade is a brand-new alternative now permissible in the 2006 CE Code. The American National Electrical Code provides a similar option, but in the form of a "ground ring” surrounding a building and installed underground.
In situ grounding electrode
Rule 10-700(4) just specifies that an in situ grounding electrode is not considered electrical equipment, must be located at least 600 mm below finished grade and have a surface exposure to earth equivalent to a manufactured grounding electrode. The rule doesn’t say it must be metallic (but of course we knew this from the original definition in Section 0). This grounding selection is new and not specifically spelled out.
When we turn to Appendix B, we find that an in situ grounding electrode must have a surface area in contact with the earth at least as great as that of a manufactured grounding electrode. A helpful hint in Appendix B tells us that the necessary specifications for manufactured grounding electrodes may be found in the CSA Standard C22.2 No. 41 – OK if we all have access to this document.
Appendix B also provides a number of examples for in situ grounding electrodes including:
- An underground metal water system at least 600 mm below finished grade and extending at least 3 m beyond the building foundation, which has traditionally been recognized as a suitable grounding electrode; or
- And this is brand-new — the reinforcing steel of concrete slabs, foundations and pilings or metal pilings in contact with the earth and at least 600 mm below finished grade. Obviously, building reinforcing steel and steel pilings treated against corrosion would be unsuitable for use as grounding electrodes.
How does one determine that the requirements of the rule are met for the examples provided in Appendix B? No doubt the CSA standard does provide some data in the form of minimum metal surfaces required to be in contact with the earth. But how easily can we relate this data to the surface areas of different diameters and lengths of piping and building reinforcing steel? It seems to me that the electrical inspection authorities should work out the equivalencies and provide some guidelines to help reduce the inevitable number of uncertainties.
As with previous articles, you should always refer to the electrical inspection authority in each province or territory for a more precise interpretation of any of the above.
Read more by Leslie Stoch
Posted By Underwriters Laboratories,
Thursday, March 01, 2007
Updated: Sunday, February 10, 2013
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Question: UL code correlation database
I see that the 2006 White Book now includes an index that correlates the 2005 NEC to UL product categories, is there a UL code correlation database online that I can access?
The answer is yes; there is a UL code correlation database on UL.com. In the past several issues of the UL Question Corner, we discussed all the new features in the 2006 UL White Book that make it the companion tool to the NEC. One of those features is the Index of Product Categories Correlated to the 2005 NEC, the index is a code correlation index. UL took that data and incorporated that into an online version in a database form that correlates the 2005 NEC to UL product categories and also includes various building, mechanical and gas codes. If you are an electrical or a multi-discipline inspector, this database will be a onestop shop for determining which UL Listed products you should be looking for to determine compliance with the Code.
The UL Code Correlation Database is located on the Regulators page of UL.com by clicking on the UL Code Correlation Database button on the right hand side of the screen. The database can also be directly accessed atwww.ul.com/regulators/codelink.
The Code Correlation Database covers the following model codes: 2005 National Electrical Code (NEC), 2000, 2003 and 2006 International Building Code (IBC), UL Online Model Code Correlation Database 2003 and 2006 International Mechanical Code (IMC) and the 2003 and 2006 International Fuel Gas Code (IFGC). In the near future, the 2006 International Fire Code will also be added.
The database is simple to use:
1. Select the Model Code you would like to search from the drop down menu.
2. Enter the Code Section you are searching for or the UL Category Code. By entering the code section, you will see the UL product categories that apply to the specific code section. If you enter a UL category code instead of a code section, all the code sections will be shown that have been identified as pertaining to that category code.
In addition to the identification of the proper UL Product Category and Category Code, you will also be provided with the additional details, such as the identification of the standard that is used for certification and a link to the scope of the UL standard if applicable.
This database will be a big help to AHJs, plan reviewers, specifiers and designers by bringing a direct link between the Code and code-compliant product installations.
Let’s take an example of how it works. Suppose we are trying to locate transfer equipment for use in optional standby systems for NEC 702.6. First, locate the Model Code Correlation Database atwww.ul.com/regulators/codelink, then select the 2005 NEC from the Model Code pull down menu, then enter the Code Section 702.6 in the Code Section Number field and click submit.
The results show there are five UL product categories that may satisfy this Code requirement. Those are: Panelboards (QEUY), Enclosed Switches (WIAX), Automatic Transfer Switches for use in Optional Standby Systems (WPXT), Non Automatic Transfer Switches (WPYV), and Transfer Switches (WPTZ).
While the transfer switch categories may be obvious for compliance with Article 702, Panelboards (QEUY) and Enclosed Switches (WIAX) may not be so obvious. By clicking on the details link and then the Guide Information link for Panelboards (QEUY), we see that the Guide Information includes information regarding Article 702. This information states, "Some panelboards, constructed with interlocked main switching and overcurrent protective devices, have been investigated for use in optional standby systems in accordance with Article 702 of the NEC and are marked ‘Suitable for use in accordance with Article 702 of the National Electrical Code ANSI/NFPA 70,’ or, if provided within kit form, ‘Suitable for use in accordance with Article 702 of the National Electrical Code ANSI/NFPA 70 when provided with interlock kit Cat No. ____.’”
If we click on the details link for Enclosed Switches (WIAX) and then the Guide Information link we see the (WIAX) Guide Information also includes information regarding Article 702 suitability. The Guide Information states, "Some panelboards, constructed with interlocked main switching and overcurrent protective devices, have been investigated for use in optional standby systems in accordance with Article 702 of the NEC and are marked ‘Suitable for use in accordance with Article 702 of the National Electrical Code ANSI/NFPA 70,’ or, if provided within kit form, ‘Suitable for use in accordance with Article 702 of the National Electrical Code ANSI/NFPA 70 when provided with interlock kit Cat No. ____.”
The UL Model Code Correlation Database is just another tool UL provides to AHJs and installers to assist in determining code-compliant installations. If you have any questions on the UL Model Code Correlation Database, please contact Bob Eugene at Robert.Eugene@us.ul.com.
UL Question Corner
Posted By Michael Weitzel,
Thursday, March 01, 2007
Updated: Sunday, February 10, 2013
| Comments (0)
Typically, when length is a factor in the installation, so is voltage drop. A variety of installations may involve feeders or branch circuits of considerable length. These include such installations as industrial plants; airports; tollway, highway, turnpike or street lighting; electrically controlled irrigation machines (also known as center pivot irrigation machines); installations on docks, marinas, or boatyards; farms or ranches; and commercial, residential, or governmental structures. Electrical designers and installers as a whole are generally aware of the requirements in 210.19(A)(1), and fine print note (FPN) No. 4 that provides explanatory material relating to voltage drop for feeders and branch circuits and suggests the maximum percentage of voltage drop which will provide "reasonable efficiency of operation,” should not exceed 5% at the farthest outlet where power is required. Some regard and apply the information as a requirement of the Code, and others may ignore it because they figure, Hey, it’s not mandatory text, so it’s no big deal if I do it or not.
Generally, switchboards and panelboards will function with voltages that are slightly lower than standard nominal ratings with no ill effects to them. Utilization equipment, however, such as lighting, computer-data processing equipment, electronic equipment, and motors may malfunction or be severely damaged by under-voltage conditions. One important thing to consider is the manufacturer’s instructions that are included with the listing and labeling of the product. Even though both 210.19(A)(1) FPN No. 4 and 215.2(3) FPN No. 2 suggest percentages for maximum allowable voltage drop which will provide reasonable efficiency of operation for utilization equipment, Section 110.3(B) requires all listed and labeled electrical equipment to be installed and used in accordance with the manufacturer’s instructions included with the product’s listing and labeling. Nearly all electrical equipment and materials that are installed and used today are listed, labeled, and include installation instructions.
For example, in an outdoor area lighting installation such as those at sports complexes and so forth, luminaires (lighting fixtures) rated for operation at 240 volts ac, nominal, may have manufacturer’s instructions included with the product that permit an operating voltage of no less than 228 volts for proper operation. A 5% voltage drop from 240 volts nominal is 228 volts (240 V x .95 = 228 V), and meets the minimum voltage allowed per the manufacturer. A drop in voltage of more than 5% will cause the luminaires to malfunction. Utility system distribution voltages typically are regulated to be kept within a certain bandwidth, normally plus or minus 2% of nominal system voltage for the end user. In some parts of the country, utilities may generate and distribute voltages that may be as much as 4% higher than the nominal system voltage (240 V x 1.04 = 249.6 V). This slight variation of higher voltage is favorable to the designer/installer of electrical systems, helps with voltage-drop concerns, and will generally not harm utilization equipment, but cannot be counted on. It is wise to install electrical systems to a worst-case scenario and account for some slight drop in voltage, as distribution system loads may increase and power source feed locations may change in time. The point to remember is that electrical installations must meet at least the minimum requirements of the Code, and 110.3(B) is definitely one of the requirements that cannot be ignored.
Enter Article 250
Sizing electrical ungrounded (hot) and grounded (neutral) conductors for voltage drop is a necessity for the proper functioning of electrical equipment and as a requirement of the Code. However, equipment grounding conductors must also be considered for a properly functioning and code-compliant feeder or branch-circuit installation. These conductors are an essential part of a safe electrical installation, as they protect people and property from injury or damage.
Proper Sizing of Equipment Grounding Conductors for Voltage-Drop Situations
Of course, equipment grounding conductors (EGCs) may be installed in the form of busbars, metal raceways, or be considered as the outer metal sheath of cables of one type or another in accordance with 250.118, Types of Equipment Grounding Conductors, and other applicable Code rules. EGCs also may be in the form of solid or stranded, insulated, covered or bare wire type conductors. For simplicity and illustration, wire type conductors will be addressed in this article.
Generally, with few exceptions, all feeders and branch circuits require some sort of EGC. Feeders and branch circuits of extended length are required to have an EGC that will perform per 250.4(A)(5), and qualify as a "permanent, low-impedance circuit facilitating the operation of the overcurrent protective device or ground detector for high-impedance grounded systems.” Safety hazards to life and property are created when feeders or branch circuits are run a long distance and the EGCs are undersized when a ground fault occurs a long distance from the source of power. The overcurrent protective device may not see nor clear the fault, and the ground-fault current may endanger persons or damage property.
When ungrounded and grounded (neutral) conductors are run a long distance and are required to be increased a percentage above the standard size conductor in order to function properly, per Section 110.3(B), manufacturers’ or listing agencies’ instructions, EGCs must be increased in size also as they may be needed and must be ready to carry fault current to open an overcurrent device [also see 250.122(B)].
The minimum sizes for EGCs are found in NEC Table 250.122. The note at the bottom of the table requires that "where necessary to comply with 250.4(A)(5) or (B)(4), the equipment grounding conductor shall be sized larger than given in this table.” Section 250.122(F) applies where conductors are run in parallel. Individual EGCs and those installed in parallel must comply with the note at the bottom of Table 250.122.
Sample Voltage-Drop Calculation
This example is given as an illustration to resemble a real-world voltage drop scenario, and emphasize some items to consider when long runs of electrical feeders or branch circuits may be necessary or cannot be avoided. Installations including all conductors within the same cable(s) are commonly used; however, raceways are discussed here; though for simplicity, raceway sizes are not mentioned.
Obviously, as conductor sizes are increased, raceway sizes may need to be increased. Voltage drop for branch circuits and cable installations is reserved for another article, though most information here would apply. The load for the feeder has already been calculated per Article 220. Other real-world concerns such as customer preference, the cost and availability of conductors and electrical equipment such as large transformers, or installing a high-voltage feeder using overhead spans of smaller conductors are all factors that designers and installers must consider when designing any installation.
Figure 1. 2000-amp, 208Y/120V-feeder, distance = 1500 feet long
Example: A proposed 2000-amp 208Y/120 V feeder will be 750 feet long (for a total length of 1500 feet to the load and back) in its maximum length (including all branch circuits); have 5) 600 kcmil copper conductors per phase installed in non-metallic conduit (5 x 420 amperes = 2100 amperes), include a 1/0 grounded (neutral) conductor in each raceway (the neutral load has been sized per 220.61 at 620 amperes), (5 x 150 amperes = 750 amperes), and a required minimum equipment grounding conductor size of 250 kcmil to be installed in each raceway per Table 250.122 and Section 250.122(F). For our example, all conductors are copper, and their ampacities are taken from the 75°C column of Table 310.16 for THWN conductors (see figure 1).
The questions are, Are all conductors properly sized to account for voltage drop? and Are the proposed equipment grounding conductors adequate to facilitate the operation of the overcurrent device per the requirements in 250.4(A)(5)?
Utilizing the voltage drop/circular mil formula found on page 136 of volume 2 of Ferm’s Fast Finder, 2005 edition, for three-phase circuits for the ungrounded conductors:
(18.7 is known as the "K” factor, a value assigned for the resistance of a conductor at a certain temperature per foot rated to the circular mil size. Other examples of the use and calculation of the K factor are given in Ferm’s Fast Finder).
If the wire size of the ungrounded conductors is increased to 700 kcmil each:
If the wire size of the ungrounded conductors is increased to 750 kcmil each:
If the wire size of the ungrounded conductors is increased to 800 kcmil each:
If the wire size of the ungrounded conductors is increased to 900 kcmil each:
If the wire size of the ungrounded conductors is increased to 1,000 kcmil each:
If the wire size of the ungrounded conductors is increased to 1,250 kcmil each:
The grounded (neutral) 1/0 copper THWN conductors are rated at 150 amperes, and are 105,600 cm in size per chapter 9, Table 8, Conductor Properties. 105,600 cm x 2.08 (208%) = 219,648 cm, which is not a standard size conductor. The closest size above that value is 250 kcmil copper, (250,000 cm). One 250 kcmil grounded (neutral) conductor must be installed in each of the paralleled raceways.
Table 250.122 specifies the minimum size EGC for a 2000-amp feeder or branch circuit as 250 kcmil. 250,000 cm x 2.08 (208 %) = 520,000 cm. Again, chapter 9 Table 8, Conductor Properties, is consulted, and the closest size is 600 kcmil copper, which is required to be installed in each of the paralleled raceways.
Note: The original minimal size required by Code was 250 kcmil, but because of the long distance (1500 ft) of the feeder, a much larger EGC is required; in this case, over twice the original size, as a minimum.
Remember, the EGCs must be increased in size by the same proportion as the ungrounded conductors, per 250.122(B).
At this point, it is important to mention that provisions must be made for terminating these larger conductors in switchboards, panelboards, disconnect switches, or other electrical equipment. Manufacturers’ instructions, per 110.3(B), must be consulted, as well as the requirements in 110.14 for terminations, 312.6 for wire bending space, and space for installing, bending, and termination of all conductors. Large junction or splice boxes, indoor or outdoor termination enclosures, or wireways—any of which may contain power distribution blocks or similar devices—may be required at each end of the system in order to transition from a larger conductor size to account for voltage drop to/from a size that will terminate in the equipment and meet all Code requirements. Buss gutters may also be considered for use. Raceway, wireway, or auxiliary gutter fill, junction or pull box sizes, and all other applicable Code rules must be considered as part of the design work and prior to installation.
Sometimes transformers are used to step up the voltage in long feeders in order to reduce the size of the feeder conductors in the long run. In this case, a transformer would be necessary at both ends of the feeder as shown in figure 2.
The same feeder installed at 480Y/ 277 volts would be sized this way:
The load on the grounded (neutral) conductor for the feeder now rated at 480Y / 277 volts is:
In accordance with Table 310.16, 700 kcmil copper conductors paralleled twice (460 amperes x 2 = 920 amperes) are the minimum size required to serve the load. However, per 240.6, the next standard size overcurrent protective device size above 868 amperes is 1000 amperes, and the conductors are required to be sized per the overcurrent device size at a minimum. Table 310.16 indicates that 900 kcmil THWN copper conductors paralleled two times = 1,040 amps, and would be code-compliant for the installation; but with the long distance of the feeder, we must verify that all conductors are sized large enough to be code-compliant and properly sized to serve the load, including the grounded (neutral) conductor. The minimum size grounded (neutral) conductors sized per 310.4 as a minimum 1/0 copper (installed in parallel in each non-metallic raceway) = (150 amperes x 2 = 300 amperes), which under normal conditions would meet the requirements for the 269 amperes capacity for the conductors. The minimum size EGC—again required to be installed in each paralleled non-metallic raceway—per 250.122(F) must be verified for proper sizing. Generally, the minimum size EGC for a 1000 ampere feeder per Table 250.122 is 2/0 copper.
The calculation for the ungrounded conductors is as follows:
Because of the requirements of 240.6, the ungrounded conductors were increased in size from 700 kcmil to 900 kcmil copper. This is a 29% increase in size, and the grounded (neutral) conductors and equipment grounding conductors must be increased by the same proportion, per 250.122(B). For the 1500-foot distance of the feeder, because the voltage is higher at 480 volts (as opposed to 208 volts), the amperage of the load is less.
The minimum size grounded (neutral) conductor permitted to be installed in parallel is 1/0, in accordance with NEC 310.4. Each grounded conductor must be increased 29% to account for voltage drop, the same as all other conductors of this feeder. A 1/0 copper conductor = 105,600 cm x 1.29 (129%) = 136,224 cm. Based on chapter 9, Table 8, Conductor Properties, 136,224 cm is not a standard size conductor. From this table, you will find that the next larger size found is 3/0, which is 167,800 cm in size. A 3/0 copper grounded (neutral) conductor is then required to be installed in each of the two paralleled non-metallic raceways for this feeder.
The calculation for the grounded conductor is:
The minimum size EGC for a 1000-amp feeder is 2/0 copper, which must be increased in size by a minimum of 29%. The question is, How many circular mils (cm) in size is a 2/0 copper conductor? The answer is found in chapter 9, Table 8, Conductor Properties. From this table, find that a 2/0 conductor is 133,100 cm in size. 133,100 cm x 1.29 (129%) = 171,699 circular mils. This is not a standard size conductor. Note that the next higher and closest size is a 4/0 conductor, sized at 211,600 cm (circular mils). A 4/0 copper EGC is required to be installed in each of the two paralleled non-metallic raceways for this feeder.
Figure 2. The same feeder as figure 1, except now at 480Y/277V with step up and step down transformers to account for voltage drop
Once again, it is important to remember that the Code is not a design or specification manual, but it is a minimum standard for electrical installations.
In this article, we have discussed the voltage-drop requirements found in the NEC, illustrated the use of a formula for determining the amount of voltage drop on a long feeder or branch circuit, some items to consider when designing electrical installations that include the termination of long runs of conductors, and the proper sizing of equipment grounding conductors for installations where voltage drop is a factor.
Electrical installations must meet at least the minimum requirements of the Code. While the FPN following Section 210.19(B) and FPN No. 2 to 215.2(3) are explanatory material and not mandatory, 110.3(B) is definitely one of the requirements which cannot be ignored, and does apply relating to the voltage drop of conductors that supply utilization equipment.
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