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Grounding & Bonding

Posted By Leslie Stoch, Sunday, November 01, 1998
Updated: Friday, August 24, 2012

The Canadian Electrical Code has a long and precise definition for grounding as: "a permanent and continuous conducting path to the earth with sufficient ampacity to carry any fault current liable to be imposed upon it, and of sufficiently low impedance to limit the voltage rise above ground and to facilitate the operation of the protective devices in the circuit.” When we talk about grounding, we are usually thinking about electrical systems.

The code has an equally precise, but not as lengthy definition for bonding as: "a low impedance path obtained by permanently joining all non-current-carrying metal parts to assure electrical continuity and having the capacity to conduct safely any current likely to be imposed upon it. When we talk about bonding, we are usually thinking about metallic items such as equipment enclosures.

The rules for grounding and bonding are designed to protect people and property against the dangers of electrical shocks and electrically caused fires by:

  • Limiting electrical circuit voltages
  • Minimizing differences in potential between metal objects such as equipment enclosures and other grounded metal.
  • Making sure that fuses and breakers trip promptly when they should
  • Reducing voltage surges caused by lightning or other causes.

The code specifies that alternating current systems must be connected to ground for voltages up to 150 phase to ground or if the system carries a neutral.

The code also provides a number of exceptions where it would be more hazardous or impractical to operate with a grounded electrical system.

One such exception is electrical arc furnaces. It would be easy to see why a grounded system would be impracticable and maybe even hazardous, since the return currents might find many paths back to its source.

Another exception would be isolated circuits such as wiring to the underwater speakers in a swimming pool. If these circuits were to be grounded, under fault conditions, there could be a higher possibility of current flow in the water, which could be very hazardous to swimmers.

A third exception to the rule is an electric crane running across a hazardous area containing combustible fibres. It would be easy to see why it is advisable to eliminate some of the arcing and sparking risks so as to minimize the possibility of fire.

The code has some very specific requirements for grounding an electrical system. The system must be connected to ground once at the owner’s main substation or by the electrical utility and once again in the owner’s main service. The code prohibits any connections to ground once past the owner’s main electrical service.

This requirement has two main purposes:

  • To maintain control of return currents by ensuring that they flow only on the insulated neutral conductor and not select random paths back to their source; and
  • To ensure that the ground fault protection in switchboards will operate accurately and when it should.

The code is also very specific about bonding. It specifies that except for double insulated devices, metallic electrical equipment enclosures and raceways must be bonded to each other and to ground by a number of acceptable means such as bonding wires, busbars, metallic cable sheaths or metallic raceways.

The code prohibits using the identified circuit conductor (neutral) as the means of bonding electrical enclosures. Once again, there are several valid reasons:

  • To maintain control of return currents as above; and
  • To avoid grounding the electrical system past the main service, which the code prohibits; and
  • To avoid the possibility that equipment enclosures could become live should the identified conductor (neutral) become accidentally disconnected.

The code goes to great lengths to ensure that all related metal objects remain at ground potential and do not develop differences in potential to each other. For these reasons, it specifies that in addition to the above, we must also bond to ground all of the following with a building:

  • Metal water piping when not used for grounding the electrical system
  • Metal gas piping
  • Metal waste piping
  • Metal supports for computer room floors

In dairy barns and other agricultural buildings where small differences in potential can have harmful effects upon animals (tingle voltages), all metal is bonded together, including all water piping, stanchions, water bowls, vacuum lines an even the floors where cattle are milked.

Another area where very small potential differences can be dangerous to people is that area in or around swimming pools. Section 68 of the code includes some very precise requirements for bonding all of the metal objects in the vicinity of a pool, in particular when electrical equipment is used within three metres of the pool.

As in previous articles, you should seek the advice of your local inspection authorities for an exact interpretation of any of the above as they apply in each province or territory.

Some Special Requirements

Several months ago in two previous articles, we covered some of the standard grounding and bond fundamentals contained in the Canadian Electrical Code. Now let’s look at some less usual or more interesting code requirements covering some special circumstances. It’s unlikely that all of our readers will have had an opportunity to employ all of the following.

One excellent example is Rule 18-132(2), which deals with bonding electrical equipment in Class I, Division 1 hazardous locations. Such locations contain dangerous concentrations of flammable gases or vapours which may ignite or explode on occurrence of a spark due to any cause including a loose bonding connection.

You may not be aware that Rule 18-132(2) has some exceptional bonding rules for electrical circuits, not only within the hazardous location, but also within the non-hazardous location from which the hazardous location circuit is supplied. (See Sketch A). This rule refers us to Rule 10-606(1)(a), (c) and (d), and Rule 10-60(2) (Means of Assuring Continuity at Service Equipment).

In simpler terms this combination of CEC rules tells us that circuit bonding in Class I, Division 1 hazardous locations and the rest of each circuit originating outside the hazardous location, must satisfy the special CEC conditions normally reserved for bonding on the supply side of customer main services.

As a direct result, electrical circuits in Class I, Division 1 hazardous locations and all the way back to their points of supply are restricted to the following methods of bonding and bonding connections:

  • Bonding conductors
  • Threaded couplings and bosses
  • Grounding bushings and bonding jumpers
  • Standard locknuts and bushings are insufficient and you will notice that EMT connections are not anywhere included in this list.

I have always interpreted the above rules as applying to only a complete circuit from its source (such as a distribution panelboard or motor control centre) in a non-hazardous location to electrical equipment in a hazardous location. But do these rules also affect the feeders upstream from these sources and perhaps back to the main substation? I hope someone can enlighten me. The code is not very specific on that point and I’m afraid you’ll have to discuss that issue with your local inspector for the best interpretation.

Livestock buildings and facilities also necessitate special grounding and bonding practices. CEC Rule 10-402(4) specifies that livestock waterers must be bonded with minimum #6 AWG copper conductors. Also, Rule 10-406(5) requires that all metal such as water pipes, vacuum lines, water bowls and stanchions must be bonded with #6AWG copper conductors.

For what reasons do we need these heavy wire sizes and special precautions? Tingle voltages (between electrical objects and the floor or between metal objects) in livestock buildings or other areas may originate from electrical ground faults in farm buildings or neutral potentials from primary electrical distribution systems. Cattle are extremely sensitive to very small voltages and will refuse to drink if tingle voltages are present in water bowls. They may also withhold milk during milking. Both of these actions along with their general unease in the presence of tingle voltages may lead to deterioration in health and a loss of production.

For these reasons and for safety reasons the CEC has created some special grounding and bonding requirements in livestock areas where these conditions may exist. To supplement the effectiveness of these rules, a device sometimes called tingle voltage filter is also available.

I am sure you already know that Rule 10-204 specifies that AC services, when required to be grounded, must only be grounded once at the customer’s main electrical service and once again back at the transformer supplying the service. Rule 10-204(1)(d) also prohibits any grounding or bonding connections to the system neutral downstream from the main service.

This sub-rule has two important purposes. It is designed to prevent grounding and bonding conductors, metal piping, ducts, cable sheaths or building structures from becoming parallel paths for unbalanced neutral currents. It also ensures that electrical system ground fault protection will operate effectively to eliminate dangerous and destructive arcing ground faults when called upon.

If that be the case, how do we treat an emergency standby generator which must have its neutral interconnected with the building system neutral? If the generator neutral is interconnected with its case inside the machine, there is no choice but to provide multi-pole switching to disconnect the generator neutral simultaneously with the phase conductors. If the neutral is isolated from the case and if not separately grounded, the neutral does not require disconnection from the electrical system neutral, but a separate grounding conductor must be installed to the generator case.

As with past articles, please consult your local electrical inspector in each province or territory for an exact interpretation of the above as applicable.

Read more by Leslie Stoch

Tags:  Canadian Code  November-December 1998 

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Safety Signs, Labels and Tags

Posted By David Young, Sunday, November 01, 1998
Updated: Friday, August 24, 2012

The National Electrical Safety Code® (NESC®) occasionally references other standards. For example, ANSI Z535.1-1991 through ANSI Z535.5-1991 inclusive are referenced many times within the NESC. Most of these references are made in a NOTE: following a rule. Rule 015D explains that a NOTE: indicates material provided for information or illustrative purposes only. When a standard is referenced in a NOTE:, compliance with the standard is not mandatory.

One of the references to ANSI® Z535.1-1991 through ANSI Z535.5-1991 inclusive is within a rule. Rule 411D states that "all warning signs and tags required by Part 4 shall comply with the provisions of ANSI Z535.1-1991 through ANSI Z535.5-l991 inclusive.” In the very next sentence, Rule 411D requires "permanent warning signs shall be displayed in conspicuous places at all entrances to electric supply stations, substations, and other enclosed walk-in areas containing exposed current-carrying parts.” In this case, where a reference is made within a rule, compliance with the standard is mandatory.

So, What is ANSI Z535?

ANSI Z535 is the American National Standard for Safety Signs, Labels and Tags. It is the standard "for the design, application, and use of signs, colors, and symbols intended to identify and warn against specific hazards and other accident prevention purposes.” The standard consists of five publications labeled ANSI Z535.1-1991 through ANSI Z535.5-1991.

ANSI Z535.2-1991 is the standard for environmental and facility safety signs. The warning signs spoken of in Rule 4llD would be considered "facility safety signs.” For a safety sign to be effective in alerting people of a hazard, the message must be easily recognizable and highly conspicuous. To achieve this, ANSI Z535.2-1991 recommends that safety signs be designed with three elements.

The first element is a signal word to get the person’s attention, i.e., DANGER, WARNING, CAUTION or NOTICE. The signal word designates the degree or level of safety alerting.

The word DANGER should only be used in an imminently hazardous situation which, if not avoided, will result in death or serious injury. This signal word is the one to use on a sign located inside an enclosure containing exposed line parts as recommended by Rule 381G2 for pad-mounted equipment. The hazard is life threatening and immediate.

The word WARNING should only be used in a potentially hazardous situation which, if not avoided, could result in death or serious injury. This signal word is the one to use on a sign located on the outside of the entrance to an enclosed walk-in area containing exposed line parts as required in Rule 411D. The hazard is life threatening but is not immediate. There is a door between the person and the hazard.

The word CAUTION should only be used in a potentially hazardous situation which, if not avoided, may result in minor or moderate injury or property damage. This is not a life-threatening situation. This signal word is the one to use on a sign alerting people that a passageway does not have 7 foot head room as required by Rule 112B or a low ceiling in a parking garage.

The word NOTICE should only be used to indicate a company policy directly or indirectly related to safety of personnel or protection of property. This signal word is the one to use on the sign that informs personnel that "Hard hats are required in this area” or "Check oil when refueling your vehicle.”

The colors used to display the signal word must comply with ANSI Z535.1-1991, i.e., white letters on red for DANGER, black letters on orange for WARNING, black letters on yellow for CAUTION, and white letters on blue for NOTICE.

The second element is a symbol or pictorial to promote greater or more rapid understanding. The use of symbols and pictorials is very important in getting the message across since the general population’s reading and comprehension skills vary. ANSI Z535.3-1991 covers the requirements for safety symbols. Symbols should be tested to insure that the people to whom the sign is directed understand what the symbol means. Symbol comprehension varies with location. A symbol that passes the test in New York may not pass the test in Florida. A suggested procedure for evaluation of symbols is included in the standard.

The third element is the message text. The message should identify the hazard, i.e., High Voltage, the location of the hazard, i.e., Inside, how to avoid the hazard, i.e., Keep Out! and the probable consequences of not avoiding the hazard, i.e., Can shock, burn or cause death. This example is the message you might use on an electrical hazard WARNING sign. ANSI Z535.2-1991 also gets into letter style, letter size, viewing distance, sign placement, illumination and the use of bilingual signs.

ANSI Z535.4-1991 covers product safety signs and labels. These are the kind of labels you would expect to see on your new chain saw. ANSI Z535.5-1991 covers accident prevention tags for temporary hazards. The blocking tags required by Rule 442E and 444C must comply with ANSI Z535.5-1991 because they are in Part 4.

ANSI Z535 has just recently been revised (1998). The significant change in the new edition is the addition of a safety alert symbol, an exclamation point inside a triangle, to the left of the DANGER, WARNING and CAUTION signal words involving personal injury. The safety alert symbol should not be used on a CAUTION sign intended to prevent property damage.

Read more by David Young

Tags:  November-December 1998  Other Code 

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Service & Main Bonding Jumpers

Posted By J. Philip Simmons , Sunday, November 01, 1998
Updated: Monday, August 27, 2012


Bonding (Bonded):"The permanent joining of metallic parts to form an electrically conductive path that will ensure electrical continuity and the capacity to conduct safely any current likely to be imposed.” N

Bonding jumper, Main:"The connection between the grounded circuit conductor and the equipment grounding conductor at the service.”N

Main bonding jumper

The main bonding jumper is one of the most critical elements in the safety grounding system. This conductor is the link between the grounded service conductor, the equipment grounding conductor and in some cases, the grounding electrode conductor. The primary purpose of the main bonding jumper is to carry the ground-fault current from the service enclosure as well as from the equipment grounding system that is returning to the source. In addition, where the grounding electrode conductor is connected directly to the grounded service conductor bus, the main bonding jumper ensures that the equipment grounding bus is at the same potential as the earth.

Figure 5-1. Main bonding jumper

Figure 5-1. Main bonding jumper

For a grounded system, Section 250-28 requires that an unspliced main bonding jumper be used to connect the equipment grounding conductor(s) and the service-disconnect enclosure to the grounded conductor of the electrical system. The connection is required to be made within the enclosure for each service disconnect.

An example of this is where two or more service disconnecting means in individual enclosures are grouped at one location. This type of installation often is made with a wireway or a short section of busway installed downstream from the metering equipment. In other cases, a wireway or short section of busway is installed ahead of metering and is supplied by a service lateral or service-entrance conductors. Sets of service-entrance conductors supply each of the service disconnecting means. Service disconnecting means are installed from the wireway or auxiliary gutter. (If there are nipples between the disconnecting means and the metal or nonmetallic trough, the trough meets the definition of a wireway from Article 362 rather than an auxiliary gutter from Article 374.) Section 250-28 requires a main bonding jumper be installed in each service disconnect enclosure. As previously mentioned, Section 250-24(b) requires that the grounded service conductor be brought to each service disconnecting means and be bonded to the enclosure. The main bonding jumper is the means to accomplish this requirement.

Figure 5-2. Main bonding jumper—multiple enclosures

Figure 5-2. Main bonding jumper—multiple enclosures

The rules are a little different where more than one service disconnecting means is in a common enclosure. This equipment usually consists of listed switchboards, panelboards or motor control centers.. Where more than one service disconnecting means is located in an assembly listed for use as service equipment, Section 250-28 Exception No. 1 permits the grounded service conductors to be run to a single grounded conductor bus in the enclosure and then be bonded to the assembly enclosure. This means that only one main bonding jumper connection is required to be installed from the common grounded conductor bus to the assembly enclosure. The sections of the assembly are bonded together by means of an equipment grounding conductor bus or by being bolted together.

Exception No. 2 to Section 250-28(b) permits alternate means for bonding of high-impedance grounded neutral systems. See Chapter Four of the IAEI Soares Book on Grounding for methods and requirements for grounding high-impedance grounded neutral systems. Also see NEC® Sections 250-36 and 250-186 for the specific requirements and allowances.

The main bonding jumper is permitted to consist of a wire, bus, screw or other suitable conductor. It must be fabricated of copper or other corrosion-resistant material. Aluminum alloys are permitted where the environment is acceptable. In addition, where the main bonding jumper consists of a screw, it must have a green finish that is visible with the screw installed. This green finish assists in identifying the bonding-jumper screw from the other screws that are on or near the neutral bus. See Sections 250-28(a) and (b).

Figure 5-3. Main bonding jumper for listed assembly

Figure 5-3. Main bonding jumper for listed assembly

Functions of Main Bonding Jumper

The main bonding jumper performs three major functions:

  1. Connecting the grounded service conductor to the equipment grounding bus or conductor and the service enclosure.
  2. Providing the low-impedance path for the return of ground-fault currents to the grounded service conductor. The main bonding jumper completes the ground-fault return circuit from the equipment through the service to the source as is illustrated in Figure 5-4.
  3. Connecting the grounded service conductor to the grounding electrode conductor. Under certain conditions given in Section 250-24(a)(4), it is permitted to connect the grounding electrode conductor to the equipment grounding terminal bar rather than to the terminal bar for the grounded service conductor. This scheme is common on larger switchboard services and is necessary for proper operation of certain types of equipment ground fault protection systems. See Chapter 15 of the IAEI Soares Book on Grounding for additional information on this subject.

Size of main bonding jumper in listed enclosures

Where listed service equipment consisting of a switchboard, panelboard or motor control center is installed, the main bonding jumper that is provided with the equipment is rated for the size of conductors that would normally be used for the service. The method for sizing of the main bonding jumper in listed service equipment is found in Underwriters Laboratories Safety Standard for the equipment under consideration and is verified by the listing agency. Therefore, if a main bonding jumper that is a bus bar, strap, conductor, or screw is furnished by the manufacturer as part of the listed equipment, it may be used without calculating its adequacy. Section 384-3(c) requires the equipment manufacturer to provide the main bonding jumper.

Figure 4. The main bonding jumper completes the ground-fault return circuit from the equipment through the service to the source

Figure 4. The main bonding jumper completes the ground-fault return circuit from the equipment through the service to the source

Size of main bonding jumper at single service-disconnect or enclosure

Since the main bonding jumper must carry the full ground-fault current of the system back to the grounded service conductor (which may be a neutral), its size must relate to the rating of the service conductors which supply the service. The minimum size of the main bonding jumper is found in Table 250-66 as required by Section 250-28(d). This relationship is based on the conductor’s ability to carry the expected amount of fault current for the period of time needed for the overcurrent device to open and stop the flow of current.

For example, where 250 kcmil aluminum service-entrance conductors are installed, the main bonding jumper is found to be No. 4 copper or No. 2 aluminum by reference to Table 250-66.

The size of the main bonding jumper does not directly relate to the rating of the service overcurrent device. Do not attempt to use Table 250-122 for this purpose. Table 250-122 gives the minimum size of equipment grounding conductors for feeders and circuits on the load side of the service.

Sizing of main bonding jumper for parallel service conductors

Figure 5-5. Main bonding jumper at single disconnect

Figure 5-5. Main bonding jumper at single disconnect

Where service conductors are installed in parallel, (connected together at each end to form a larger conductor) the total circular mil area of the conductors connected in parallel for one phase are added together to determine the minimum size main bonding jumper required. See Section 250-28(d). For example, where three 250 kcmil conductors are connected in parallel per phase, they are treated as a single 750 kcmil conductor. By reference to Table 250-66 the main bonding jumper, if aluminum service-entrance conductors are used, is 1/0 copper or 3/0 aluminum.

Where the service-entrance conductors are larger than the maximum given in Table 250-66, Section 250-28(d) requires the main bonding jumper to be not less than 12½ percent (0.125) of the area of the largest phase conductors.

This is illustrated by the following example:

Three 500 kcmil copper conductors are installed in parallel as service-entrance conductors.

3 x 500 kcmil = 1500 kcmil.

1500 x .125 = 187,500 circular mils.

Since a 187,500 circular mil conductor is not a standard size, we next refer to Chapter 9, Table 8 to find the area of conductors.

The next conductor exceeding 187,500 circular mils is a No. 4/0 AWG conductor which has an area of 211,600 circular mils. It is always necessary to go to the next larger size conductor since the 12½ percent size is the minimum size permitted.

Follow a similar procedure for determining the minimum size main bonding jumper required for other sizes of parallel service-entrance conductors.

Figure 5-6. Main bonding jumper for parallel runs

Figure 5-6. Main bonding jumper for parallel runs

Bonding of service conductor enclosures

Special rules are provided for bonding enclosures on the line side of the service disconnecting means. This is due to the fact that this equipment does not have overcurrent protection on its line side such as feeders and branch circuits have. Fault current of sufficient magnitude must flow during a short period of time to allow the fuse on the line side of the utility transformer to open. The level of fault current and particularly the duration the current may flow could be much larger than would flow in a feeder or branch circuit as there is not an overcurrent device in series with the conductor.

The basic rule is that all metallic enclosures that contain a service conductor must be bonded together. The bonding ensures that none of the equipment enclosures can become isolated electrically and become a shock hazard should a line-to-ground fault occur. The bonding also provides a low impedance path for fault current to flow in so the fuse or circuit breaker on the line side of the electric utility transformer will open.

Sizing of equipment bonding jumper on line (supply) side of service.
Equipment bonding jumpers on the line side of the service and main bonding jumper must be sized to comply with Table 250-66. This is required by Section 250-102(c). For example, where 250 kcmil copper conductors are installed as service-entrance conductors, Table 250-66 requires a No. 2 copper or 1/0 aluminum bonding jumper.

Where the sum of the circular mil area of the service-entrance phase conductors exceeds 1100 kcmil copper or 1750 kcmil aluminum, the equipment bonding conductor must be not less than 12½ percent (0.125) of the area of the ungrounded phase conductors.

Figure 5-7. Size of equipment bonding jumper on line side of service

Figure 5-7. Size of equipment bonding jumper on line side of service

Sizing of equipment bonding jumper for parallel conductors

Two methods are provided for bonding service raceways that are installed in parallel. The first method is to add the circular mill area of the service-entrance conductors per phase together and treat them as a single conductor. The bonding jumper size is determined from Table 250-66 and is connected to each conduit bonding bushing in a "daisy-chain fashion.” This method often results in an equipment bonding jumper that is quite large and difficult to work with.

For example, if five 250 kcmil copper conductors are installed in parallel for a phase, the equipment bonding jumper for bonding the metal raceways must not be smaller than 3/0 copper.

This is determined as follows:

Five x 250 kcmil = 1250 kcmil.

1250 kcmil x .125 = 156,250 circular mils.

Figure 5-8. Size of equipment bonding jumper on line side of service

Figure 5-8. Size of equipment bonding jumper on line side of service


The next larger conductor found in Chapter 9, Table 8 is 3/0 with an area of 167,800 circular mils.

In this case, a 3/0 copper equipment bonding conductor must be connected from the grounded service conductor or equipment grounding bus to each metal raceway in series (daisy-chain fashion from one raceway to another).

A more practical method of performing the bonding for services supplied by multiple raceways may be to connect an individual bonding conductor between each raceway and the grounded service conductor terminal bar or equipment grounding bus. This is permitted by Section 250-102(c). This will usually result in a smaller equipment bonding conductor which is easier to install.

Again, using the example above and referring to Table 250-66, the minimum size equipment bonding conductor for the individual raceways containing 250 kcmil copper service-entrance conductors is No. 2 copper or 1/0 aluminum. A properly sized equipment bonding jumper is installed from the terminal bar for the grounded service conductor or from the equipment grounding terminal bar to each conduit individually.

Different conductor material

Section 250-28(d) provides instructions on sizing the main bonding jumper or equipment bonding jumper on the supply side of the service where different conductor materials are used for the service-entrance conductors and the bonding jumper. The procedure involves assuming the phase conductors are of the same material (copper or aluminum) as the bonding jumper and that they have an equivalent ampacity to the conductors that are installed. This is illustrated as follows:

Assume aluminum phase conductors and a copper bonding jumper are installed.

Three 750 kcmil Type THW aluminum conductors are installed.

From Table 310-16, 385 amperes x 3 = 1155 amperes. The smallest type THW copper conductor that has an equivalent rating is 600 kcmil with an ampacity of 420.

Next, determine the total circular mil area of the copper conductors.

Three x 600 kcmil = 1800 kcmil.

1800 kcmil x .125 = 225 kcmil.

The next standard size is 250 kcmil copper which is the minimum size bonding jumper permitted to bond equipment at or ahead of the service equipment in this example.

Bonding service equipment enclosures

The Code requires that electrical continuity of service equipment and enclosures that contain service conductors be established and maintained by bonding. The items required to be bonded together are stated as follows in Section 250-92(a):

(1) The service raceways, cable-trays, cablebus framework or service cable armor or sheath.

(2) All service equipment enclosures containing service conductors, including meter fittings, boxes or the like, interposed in the service raceway or armor.

(3) Any metallic raceway or armor which encloses the grounding electrode conductor. (This subject is covered in detail in Chapter 7 of this text.)

An exception to this requirement for bonding at service equipment is mentioned in Section 250-92(a)(1). It refers to Section 250-84 which has rules on underground service cables that are metallically connected to the underground service conduit. The Code points out that if a service cable contains a metal armor, and if the service cable also contains an uninsulated grounded service conductor which is in continuous electrical contact with its metallic armor, then the metal covering of the cable is considered to be adequately grounded.

Figure 5-9. Bonding service equipment enclosures

Figure 5-9. Bonding service equipment enclosures


Use of neutral for bonding on line side of service

Section 250-94(1) permits the use of the grounded service conductor (may be the neutral) for grounding and bonding equipment on the line side of the service disconnecting means. This is also permitted by Section 250-142(a)(1). (Two other applications of this bonding are explored in later chapters of the IAEI Soares Book on Grounding.) Often, connecting the grounded service conductor to equipment such as meter bases, current transformer enclosures, wireways and auxiliary gutters is the most practical method of bonding these enclosures.

Usually, self-contained meter sockets and meter-main combination equipment are produced with the grounded conductor terminals or bus (often a neutral) bonded directly to the enclosure. The enclosure is then effectively bonded by the connection of the grounded circuit conductor to these terminals. No additional bonding conductor connection to the meter enclosure is required. Current from a ground fault to the meter or meter-main enclosure will return to the source by the grounded service conductor (may be a neutral) and, hopefully, will allow enough current to flow in the circuit to operate the overcurrent protection on the line side of the utility or other transformer.

Figure 5-10. Use of neutral for bonding on line side of service

Figure 5-10. Use of neutral for bonding on line side of service

In addition, meter enclosures installed on the load side of the service disconnecting means are permitted to be grounded (bonded) to the grounded service conductor provided that:

(a) Service ground-fault protection is not installed; and

(b) The meter enclosures are located near the service disconnecting means. (No distance is used to clarify what is meant by the word "near.”), and

(c) The size of the grounded circuit conductor is not smaller than the size specified in Table 250-122 for equipment grounding conductors. See Section 250-142(b) Exception No. 2.

Means of bonding at service equipment

The methods for bonding at service equipment are outlined in Section 250-94. These requirements for bonding are more restrictive at services than downstream from the service. The reason this is so important is service equipment and enclosures may be called upon to carry heavy fault currents in the event of a line-to-ground fault. The service conductors in these enclosures have only short-circuit protection provided by the overcurrent device on the line side of the utility transformer. Only overload protection is provided at the load end of the service conductor by the overcurrent device. This is one of the reasons the Code limits the length of service conductors inside of a building.

Figure 5-11. Methods of bonding service equipment

Figure 5-11. Methods of bonding service equipment


Bonding of these enclosures is to be done by one or more of the following methods from Section 250-94:

(1) Bonding to the grounded service conductor through the use of exothermic welding, listed pressure connectors such as lugs, listed clamps, or other listed means. These connections cannot depend solely upon solder.

(2) Threaded couplings and threaded bosses in a rigid or intermediate metal conduit system where the joints are made up wrench-tight. Threaded bosses include hubs that are either formed as a part of the enclosure or are supplied as an accessory and installed according to the manufacturer’s instructions.

(3) Threadless couplings and connectors are permitted where they are made up tight for rigid and intermediate metal conduit and electrical metallic tubing and metal-clad cables.

(4) Other approved devices such as bonding-type locknuts and bushings.

Bonding jumpers are required to be used around concentric or eccentric knockouts that are punched or otherwise formed so as to impair an adequate electrical path for ground-fault current. It is important to recognize that concentric and eccentric knockouts in enclosures such as panelboards, wireways and auxiliary gutters have not been investigated for their ability to carry fault current. Where any of these knockout rings remain at the conduit connection to the enclosure, they must always be bonded around to ensure an adequate fault-current path.

Figure 5-12. Bonding fittings

Figure 5-12. Bonding fittings


The Code states here that "Standard locknuts or bushings shall not be the sole means for the bonding required by this section.” This statement does not intend to prevent the use of "standard” locknuts and bushings, it is just that they cannot be relied upon as the sole means for the bonding that is required by this section. "Standard” locknuts are commonly used outside the enclosure on conduit that is bonded with a bonding bushing or bonding locknut inside the enclosure. Standard locknuts are used to make a good, reliable mechanical connection as required by Section 300-10.

Parallel bonding conductors

Section 250-102(c) requires that where service-entrance conductors are paralleled in two or more raceways or cables and the equipment bonding jumper is routed with the raceways or cables, the equipment bonding jumper must be run in parallel.

In this case again, the size of the bonding jumper for each raceway is based upon the size of the service-entrance conductor in the raceway by referring to Table 250-66.

Grounding and bonding of remote metering

Figure 5-13. Parallel bonding conductors

Figure 5-13. Parallel bonding conductors

As mentioned before, Section 250-92(a) requires all equipment containing service conductors to be bonded together and to the grounded service conductor. This includes remote (from the service equipment) meter cabinets and meter sockets.

Grounding and bonding of equipment such as meters, current transformer cabinets and raceways to the grounded service conductor at locations on the line side of and remote from the service disconnecting means increases safety.

This equipment should never be grounded only to a grounding electrode such as a ground rod. Figures 5-14 and 5-15 show why. If a ground-fault occurred at this line-side equipment, and it is not bonded as required, the only means for clearing a ground fault would be through the grounding electrodes and earth. Given the relatively high impedance and low current-carrying capacity of this path through the earth and high resistance of grounding electrodes such as rods, little current will flow in this path. This leaves the equipment enclosure(s) at a dangerous voltage above ground potential just waiting to shock or possibly electrocute a person or animal that may contact it. The voltage drop across this portion of the circuit can easily be calculated by using Ohms Law. (Resistance times the current gives the voltage.) There are many records of livestock being electrocuted while contacting electrical equipment that was improperly grounded. Sections 250-2 and 250-54 require that the earth not be used as the sole equipment grounding conductor or fault-current path.

The most practical method for grounding and bonding this line-side equipment is to bond the grounded service conductor to it. As can also be seen in Figures 5-14 and 5-15, a ground fault to the equipment will have a low impedance path back to the source through the grounded service conductor. This will allow a large current to flow in the circuit to cause the overcurrent protection on the line side of the transformer to clear the fault.

Supplementary grounding electrodes

Figure 14

Figure 14

In accordance with Section 250-54, it is permissible to install a grounding electrode at the remote meter location shown in Figures 5-14 and 5-15 to supplement the grounded service conductor. This Code section refers specifically to grounding electrodes supplementing the equipment grounding conductors. Some electric utilities require a grounding electrode at meter equipment installed remote from service equipment such as on poles. The Code in Section 230-66 makes it clear that individual meter socket enclosures are not to be considered service equipment. The same is true for metering equipment installed in remote current-transformer enclosures. As mentioned earlier, it is critically important that these meter enclosures be properly bonded as they contain service conductors.

This additional grounding electrode will attempt to keep the equipment at the earth potential that exists at the meter location. In addition, the electrodes at the remote meter and at the service location are bonded together by the grounded service conductor installed between the metering and service equipment. This brings the installation into compliance with Section 250-58 which requires a common grounding electrode or where two or more electrodes are installed, they must be bonded together.

Figure 15

Figure 15

As previously stated, these grounding electrodes should never be used as the only means for grounding or bonding these enclosures or to carry fault current.

More extensive discussion of this subject is found in Chapter Six of the IAEI Soares Book on Grounding.

Bonding of multiple service disconnecting means

Installation of multiple services as permitted by Section 230-2(a) through (d) and installations of services that have multiple disconnecting means can take several forms. Additional services are permitted by Section 230-2 for:

(a) Fire pumps, emergency, legally required, standby, optional standby or parallel power production systems.

(b) By special permission, for multiple occupancy buildings where there is no available space for service equipment that is accessible to all occupants, or, for a single building or structure that is large enough to make two or more services necessary.

(c) Capacity requirements; where the service capacity requirements exceed 2,000 amperes at 600 volts or less, where load requirements of a single-phase installation is greater than the serving utility normally provides through a single service, or by special permission (related to capacity requirements).

(d) Different characteristics of the services such as different voltages, frequencies, or phases, or for different uses, such as for different rate schedules.

The basic rule for sizing of the equipment bonding jumper for bonding these various configurations is found in Section 250-102(c). This section requires that the bonding jumpers on the line side of each service the main bonding jumper be sized from Table 250-66. Also, the size of the bonding jumper for each raceway is based on the size of service-entrance conductors in each raceway. As discussed earlier, conductors larger than given in Table 250-66 are required for larger services. Since different sizes of service-entrance conductors may be installed at various locations, the minimum size of the equipment bonding conductor and main bonding jumper is based on the size of the service-entrance conductors at each location.

For example, the appropriate size of bonding jumper for the installation in Figure 5-16 with the assumed size of conductors is as follows: (all sizes copper)

Service-Entrance ConductorBonding Jumper
a. 500 kcmil in service mast1/0
b. 1000 kcmil in wireway2/0
c. 300 kcmil to 300 ampere serviceNo. 2
d. 3/0 to 200 ampere serviceNo. 4
e. No. 2 to 125 ampere serviceNo. 8


A practical method for bonding the current transformer enclosure and wireway (sometimes referred to as a "hot gutter”) is to connect the grounded service conductor directly to the current transformer enclosure or wireway. This may be done by bolting a multi-barrel lug directly to the wireway and connecting the neutral or grounded service conductors to the lug. Be sure to remove any nonconductive paint or other coating that might insulate the connector from the enclosure.

As previously discussed, the grounded service conductor must also be extended to each service disconnecting means and be bonded to the enclosure.

Excerpted from Chapter 5 of the IAEISoares Book on Grounding, 7th Edition

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Tags:  Featured  November-December 1998 

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A Vote for the Future

Posted By Philip Cox , Sunday, November 01, 1998
Updated: Monday, August 27, 2012

A hearty thanks to IAEI members. The proposed change in the IAEI Articles of Association that included an increase in membership dues received a favorable vote during the 1998 IAEI Annual Section meetings. Over 90% of those casting votes during the section meetings supported the dues increase. The support expressed by members of the IAEI was and is vital to the existence of the IAEI as an active and effective organization in the electrical industry. With dues making up less than half of the income and covering less than half of the operating expenses, the IAEI needed help from its members. It was a difficult decision to ask members to pay higher membership dues but there was little choice. IAEI International President Tom Trainor identified many services presently being provided for members as well as some obligations that must be met by the IAEI in the future. In order to become really effective in industry affairs, we must become a more active participant. We need to greatly expand our knowledge of world affairs and become more directly involved in them. The IAEI Board of Directors is taking the challenge of preparing the IAEI for the 21st century very seriously. Concerns ranging from local needs to international affairs face the Board and with your support, we should be more able to address them. It is clear that the IAEI must operate in a businesslike manner and it must meet issues that affect its stated goals in a professional manner.



Members Working Hard in Mexico


The new Central Mexico Chapter of the IAEI is active and growing. Under the leadership of Chapter President Manuel Vila, Vice President Javier Velez, and Secretary/Treasurer Antonio Macias, a strong education program is being promoted and focus is placed on achieving safe electrical installations. The photographs included with this article show the Central Mexico Chapter officers being installed during the first official meeting of the chapter. The installation ceremony was preceded by an two-day Code training session. The National Fire Protection Association joined with the IAEI in assisting the Central Mexico Chapter in conducting the training. John Caloggero of the NFPA Engineering Department, who has a good command of the Spanish language, presented a major portion of the material. He is very effective in bridging the language barrier between those who speak little or no English and those of us who speak Spanish with difficulty or not at all. Thanks must also be given to AMERIC and its staff for their effort in making the IAEI educational seminars in Mexico City possible.

Work is also being done to establish additional chapters in Mexico. A petition for a new Sinaloa Chapter is expected to be approved during the 1998 IAEI Board of Directors meeting. It is anticipated that one or two more chapters will be established during 1999. With the quality of leadership being demonstrated by IAEI chapter officers and members in Mexico, it is expected that electrical training involving the electrical code and installation practices will expand rapidly and the interest in the work of the chapters will grow. Raising the level of awareness of the need for safe installation and use of electrical systems is a major goal and will ultimately benefit those who use electricity.

Those who need to make contact with the Central Mexico Chapter can do so by contacting Secretary/Treasurer Antonio Macias. His telephone number in Mexico City is 594 91 93. His e-mail address is He can also provide information on new IAEI chapters being established in Mexico.

Editor’s Note: Photos from the installation ceremony of the new IAEI chapter in Mexico were incorrectly mixed with another article submitted from Mexico in the September/October magazine. The photos properly belong with the above information.

Read more by Philip Cox

Tags:  Editorial  November-December 1998 

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