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Sunlight, Conductors and Raceways on Rooftops

Posted By John Stacey, Monday, September 01, 2008
Updated: Monday, February 04, 2013

Question

When using a 90°C single conductor in an ambient temperature of 81°C – 89°C, what is the correction factor for this application? It is presumed once the temperature reaches 90°C the conductor has met or exceeded its rating. Is there ampacity present in this conductor at 85°C, however minimal it may be?

There is no clarification or note in theNECto address this issue. Since the table stops at 80°C, the 9-degree temperature omission is left up to inspectors and contractors to interpret and to argue this issue in the field. This issue has arisen due to photovoltaic module installation. The modules are sold with factory installed 90°C USE-2 or RHHW-2 single insulated conductors. These modules are sometimes installed close to roof surfaces with little airflow (building integrated photovoltaics), trapping heat under the module where the conductors are located. Therefore, the conductor may be exposed to ambient temperatures in excess of 80°C. Article 690 doesn’t address this issue either. Table 310.16 has the same issue, although it is for cables or conductors installed in raceways. — J. E.

Answer

You are correct, there are no correction factors in 81–90 degree column; this is a good place for a code change.

Let’s take a closer look at theNEC. The issue here is 310.10 that states, "No conductor shall be used in such a manner that its operating temperature exceeds that designed for the type of insulated conductor involved. In no case shall conductors be associated together in such, with respect to the type of circuit, the wiring method employed, or the number of conductors, that the limiting temperature of any conductors, that the limiting temperature of any conductor is exceeded.”

The problem here is that some people want to use the highest temperature the conductor will ever see for the correction factor when you only need to use the average high-ambient temperature depending on what climate you are in. Some average outdoor ambient temperatures for selected cities during the period of June to August are shown below. These temperatures are based on ASHRAEHandbook of Fundamentals2 percent design temperatures and are measured in the shade, not in the sun.

City Average high ambient

Phoenix, AZ 109.5°F
Bakersfield, CA 102.7°F
Los Angeles, CA 78.6° F
Long Beach, CA 86.9°F
Palm Springs, CA 110.9°F

I realize that the solar equipment is in the sun, hopefully. This may add to the actual temperature as it did for raceways on roofs exposed to direct rays of the sun. NEC-2008 had a proposal with two code cycles of testing substantiations that was accepted to have a correction adder for raceways in direct rays of the sun on a roof [310.15(B)(2)(c)]. Panel 6 has seen no testing, or request for conductors, or for solar equipment, or ampacity correction in direct rays of the sun for solar to have an adder to the correction factors. Panel 6 will need true documented testing photos, etc., before the panel will act.
At this time, we electrical inspectors can only require the average high temperature for our calculations.

For example, Palm Springs has an average high temperature of 110.9°F. You stated that you are using single conductors. That allows you to use Table 310.17 for the allowable ampacities. Using a 90 degree, 10 AWG rated at 55 amps x .87 at 110 degrees = 47 allowable amps.

The higher temperatures are not prolonged, they may be for a few hours a day and probably periodic through the year. Section 310.10 FPN No.1 explains. "The temperature rating of a conductor [see Table 310.13 and Table 310.61] is the maximum temperature, at any location along its length, that the conductor can withstand over a prolonged time period without serious degradation. The allowable ampacity tables, the ampacity tables of Article 310 and the ampacity tables of Annex B, the correction factors at the bottom of these tables, and the notes to the tables provide guidance for coordinating conductor sizes, types, allowable ampacities, ampacities, ambient temperatures, and number of associated conductors.”

2008 Change for raceways on roofs

Some examples of the affect on conductor ampacity when temperature adders for conductors in raceways in direct sunlight on rooftops are applied are shown below.

Example 1:
12 AWG THHW-2 in 1/2 in. RNC or EMT installed with the bottom of the RNC or EMT 1/2 in. or less above the rooftop (most severe condition)

Table 310.16 90°C ampacity = 30 amps @ ambient temperature of 30°C.
Temperature adder from Table 310.l5(8)(2)(c) for raceway location = 33°C.
Adjusted ambient temperature = 63°C (30°C + 33°C).
Table 310.16 correction factor for 63°C ambient, 90°C conductor = 0.58.
Adjusted ampacity (30 amps x 0.58) = 17 amps.
240.2 overcurrent protection limitation = 20 amps.
Since the adjusted ampacity is 17 amps, the standard fuse or circuit breaker rating would be 20 amps [see 240.2(8)].

Example 2:

3/C 500 kcmil THHW-2 in 2 1/2 in. RNC or EMT (NEC Table C.I) installed with the bottom of the RNC or EMT 3 1/2 in. to 12 in. above the rooftop.

Table 310.16 90°C ampacity = 430 amps @ ambient temperature of 30°C.
Temperature adder from Table 310.15(8)(2)(c) for raceway location = 17°C.
Adjusted ambient temperature = 47°C (30°C + l7°C).
Table 310.16 correction factor for 47°C ambient, 90°C conductor = 0.82.
Adjusted ampacity (430 amps x 0.82) = 353 amps.
Comparison: Table 310.16 75°C ampacity = 380 amps @ ambient temperature of 30°C

As you see, the further from the roof, the more circulation is available. However, this Code change was for raceways and there were no proposals for solar conductors, unless they are in a raceway on the roof. — John Stacey, CMP-6

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Tags:  Focus on the Code  September-October 2008 

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Would this be classified as a raceway?

Posted By David G. Humphrey, Tuesday, July 01, 2008
Updated: Monday, February 04, 2013

Question

Would not this (see photos 1 and 2) be classified as a raceway?
 

Photo 1 indicates ten each 4/0, 3-phase bundled conductors in parallel with ground, in cable tray, mounted in floor, void of a Mobile 2 MW, 480-volt, 3-phase generator trailer. There is a removable cover above the cable tray compartment. Cable tray is totally enclosed with the exception of removing top cover plate. Cover is to be removed (if someone remembers) during operation of the generator.
 
 

Photo 2 indicates ten each 4/0, 3-phase bundled conductors in parallel with one separate 500 MCM ground in cable tray, mounted in floor, void of a Mobile 2 MW, 480-volt, 3-phase generator trailer. There is a removable cover above the cable tray compartment. Cable tray is totally enclosed with the exception of removing top cover plate. Cover is to be removed (if someone remembers) during operation of the generator.

Can the parallel conductor amperage be calculated according to 392.11(B)(4) Table 310.20? I would presume that 392.11(B)(4) pertains to a cable tray installation in open air and not one that is enclosed.

Is each parallel bundled conductor arrangement required to have its own grounding conductor and sized in accordance with Table 250.122? Or arranged as indicated in photo 2 with only one grounding conductor for the 10 each 4/0, 3-phase bundled conductors in parallel per Table 250.122?

Would the bundled paralleled conductors be classified as a cable and have to adhere to Article 250.122(F)(1) as noted in the NEC Handbook commentary?

Your prompt reply would greatly be appreciated. — B.G.

Answer

The multiple questions concerning the installation as described by the submitter involve a number of considerations that make this installation very interesting (see photos 1-3). The submitter does not reference which NEC is being used, so we will base our answers upon the 2005 edition.

The first question describes a cable tray mounted in the floor of a generator trailer where the cable tray is totally enclosed with a removable top cover. Instructions on the cover require the cover to be removed during operation. The question is, Would this not be classified as a raceway? To answer, we must turn to Article 100 and look at the definition of a raceway, which is defined as "an enclosed channel of metal or nonmetallic materials designed expressly for holding wires, cables or busbars, with additional functions as permitted in the Code.” Next, we turn to 392.2 that defines a cable tray system as "a unit or an assembly of units or sections and associated fittings forming a structural system used to securely fasten or support cables and raceways.” In short, a cable tray is a support system that may be used to support raceways and cables; however, it is not itself a raceway.

This raises the question, Is this wiring method as described by the submitter a cable tray, and does it comply with the installation requirements for cable tray? Section 392.6(H) requires that a cable tray "be exposed and accessible except as permitted by 392.6(G).” Article 100 defines exposed (as applied to wiring methods) as "on or attached to the surface or behind panels designed to allow access.” As the referenced cable tray is installed in the floor cavity, compliance with this requirement seems questionable. Cable trays are permitted only to pass vertically through floors as per 392.6(G). The cable tray referenced by the submitter apparently travels horizontally in the floor cavity, which does not appear to meet the requirement of 392.6(G).

Where single paralleled conductors are tripled or quadrupled together into circuit groups, they may be installed only in uncovered cable trays as per 392.11(B)(4). Removal of a cover during operation does not change the type of cable tray from the original design type. One might also show concern that the conductors contained in the fully enclosed cable tray installed in the floor cavity may be exposed to physical damage if the cover is removed during operation of the generator. The enclosure as described seems to be defined more accurately by 390.3(C) as a trench-type raceway also known as trench duct, which is indeed a raceway.

The second question was, Can the parallel conductor amperage be calculated according to 392.11(B)(4), Table 310.20? The information provided in the question leads me to conclude the answer is no, based on the premise that we are actually dealing with trench duct referenced in 390.3(C). We, therefore, may not use Article 392 or any sections therein for this application. The ampacity of the conductors in a trench-type raceway would be based on Table 310.16. Furthermore, 390.17 reminds us that the ampacity adjustment factors, in 310.15(B)(2) apply to conductors installed in underfloor raceways. Accordingly, the maximum ampacity of each of the ten 4/0 conductors per phase (10 sets) is 117 amperes per conductor based on 110.14(1)(b), using 90ºC conductors, assuming that the neutral carries only the unbalanced load, and applying the ampacity adjustment factors of 310.15(B)(2) of 45 percent for 21–30 current-carrying conductors in a raceway or cable.

When utilizing cable trays, 392.11(B)(4) permits the use of Table 310.20 where single conductors are installed in triangular or square circuit groups in uncovered cable trays. Section 392.6(A) requires that cable trays shall be installed as a complete system. Disassembly of a portion(s) of the cable tray assembly during times of use does not change the cable tray type with respect to whether the tray is the covered or uncovered type. Section 392.11(B) permits the application of the adjustment factors of 310.15(B)(2) to be omitted when using uncovered cable trays. Keep in mind that the question references the use of what is described as a covered cable tray, therefore we would not be able to omit the adjustment factors. It is also important to note that 392.11(B)(3) directs that solid bottom cable trays shall have the ampacity of single conductors and single conductor cables determined by 310.15(C).

The third question, "Is each parallel bundled conductor arrangement required to have its own equipment grounding conductor sized in accordance with Table 250.122, or would only one equipment grounding conductor sized per Table 250.122 for the 10 4/0 AWG per phase paralleled bundled conductors be necessary?” Section 300.3(B) requires that all conductors of the same circuit, including the equipment grounding conductor, be contained within the same raceway, auxiliary gutter, cable tray, etc. If using an underfloor raceway (trench duct), a single equipment grounding conductor sized as per 250.122 is permitted. The equipment grounding conductor is sized per Table 250.122, based on the size of the overcurrent device protecting the circuit. In this case, the submitter indicates that a 3200-ampere breaker is being used; therefore, a 500-kcmil copper or an 800-kcmil aluminum conductor would be required. Section 250.118(13) permits "listed electrically continuous metal raceways” to serve as an equipment grounding conductor. Article 390 does not require this type of raceway to be listed, so the suitability of the raceway to serve as the equipment grounding conductor is dependent on whether the raceway is listed. The metal enclosure must be grounded in either event, as per 250.86.

With respect to cable tray installations, we must remember that 250.118(11) allows metal cable tray as permitted in 392.3(C) and 392.7 to be used as an equipment grounding conductor. Section 392.7 requires that "metallic cable trays that support electrical conductors shall be grounded as required for conductor enclosures in accordance with section 250.96.” Accordingly, the tray may qualify as an equipment grounding conductor in its own right and may negate the need for additional equipment grounding conductor installation. When the conductors are all contained within the same metal cable tray as described in 300.3(B), a single conductor would be permitted. If paralleled circuit conductors are installed in multiple cable trays then 300.3(B)(1) would apply the requirement that all conductors be installed in the same raceway, auxiliary gutter, cable tray, etc., to each portion of the paralleled installation. Accordingly, the equipment grounding conductors for each cable tray shall comply with the provisions of 250.122.

The fourth question, "Would the bundled paralleled conductors be classified as a cable and have to adhere to Article 250.122(F)(1) as noted in the NEC Handbook commentary?” The installation as described by the submitter would be one of bundling conductors into circuit groups in the underfloor raceway. Section 390.17 reiterates the general requirement that 310.15(B)(2) is applied to current-carrying conductors installed within the raceway whether or not the conductors are bundled into circuit groups.

Section 392.11 differentiates between multiconductor cables in part (A) and single conductor cables in part (B). Single conductors or single conductor cables installed in a triangular or square configuration as per 392.11(B)(4) would not change the definition of each phase set to that of a multiconductor cable. It would constitute a conductor or cable configuration within the cable tray. In short, conductors configured or bundled together into circuit groups within either trench duct or cable tray would not re-classify these bundled circuit groups as cables and, accordingly, would not trigger requirements of 250.122(F), as all of the conductors are contained in the same raceway or cable tray.

The fifth question, "Does the short 3600-amp 90ºC bus attached to the 75ºC 3200-amp breaker allow the use of 90ºC rated conductors calculated at their 90ºC current rating?” (See photo 3).


As indicated by photo 3, does the short 3600-amp 90°C bus attached to the 75°C 3200-amp breaker allow the use of 90°C rated conductors calculated at their 90°C current ratings?

We first turn to 110.14(C)(1)(b) which limits "termination provisions of equipment for circuits rated over 100 amperes, or marked for conductors larger than 1 AWG,” to 75ºC, or up to the ampacity of the conductor "if the equipment is listed and identified for use with such conductors.” The limiting factor here is the 75ºC rating of the 3200 ampere breaker. We may, accordingly, use these conductors only at their 75ºC rating; however, we are given permission to use the actual 90ºC conductor rating for purposes of applying adjustment factors due to high ambient temperatures, or to meet the requirements of Table 310.15(B)(2).

The answers to the five questions presented are based on the installation of the conductors in what appears to be an underfloor raceway as described by Article 390. In an attempt to answer more thoroughly the submitter’s questions, Article 392, Cable Trays, is referenced. However, the installation as described by the submitter does not appear to conform to the installation requirements of Article 392, Cable Trays. Refer to NEC 445 for additional information concerning generator applications.

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Tags:  Focus on the Code  July-August 2008 

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Question: Can you daisy chain connections to fixtures?

Posted By Michael Johnston, Tuesday, July 01, 2008
Updated: Monday, February 04, 2013

Question

I am connecting 2 x 4 lay-in fluorescent fixtures in a suspended ceiling, and I am being told that you cannot daisy chain the connections to fixtures. I am using type MC cable with a ground and the lengths in some cases do exceed 6′. Am I in code violation and what code would reference this? — G. M.

Answer

This response is based on the information provided in the question. 2 x 4 lay-in style fluorescent luminaires are permitted to be wired with any branch circuit wiring method in chapter 3 of the NEC, depending on the type of building construction it is being installed in. You indicated that the luminaires are being wired using MC cable that is run from one luminaire to the next, using the ballast compartment as a junction box. As long as the luminaire is listed for this connection to the branch circuit, there is no Code requirement that would prohibit this installation. There is no length limitation in the Code for MC cable that is installed as a branch circuit.

Let’s go a step further for clarification. If we are talking about ⅜ in. fixture whips that include 18 AWG fixture wire, these are limited for use in 6 foot lengths to connect to a full size branch circuit in junction box typically located above the luminaire in a ceiling access space. The section that permits the use of these fixture whips (fixture tap conductors) is 210.19(A)(4), Exception 1(b) that refers to 410.117(C). This section permits the tap conductors in lengths not less than 18 inches and not longer than 6 feet, to connect to the branch circuit conductors. That being said, if the MC cable you refer to in your question is the factory whips that contain only tap conductors (18 AWG), then the installation described in the question is in violation of the NEC.

The next 6-foot restriction is in 250.118(5) and restricts flexible metal conduit from being used as an equipment grounding conductor in lengths exceeding 6 feet. Where an equipment grounding conductor is provided in flexible metal conduit, this length restriction is not applicable. There is no length limitation for flexible metal conduit unless it is ⅜ in. size and used for fixture tap conductor connections, once again the reference in 348.20(A)(2)(c) points to 410.117(C). However, the question does not deal with flexible metal conduit; it deals with MC cable installed as a branch circuit.

Article 330 covers MC cable and 330.10(A) provides the general uses for MC cable. Section 330.10(A)(1) indicates that MC cable can be used for branch circuits (without length limitations). Section 330.12, Uses Not Permitted, does not indicate that MC cable cannot be used in lengths longer than 6 feet. Type MC cable is recognized as providing equipment grounding conductor path when it meets the provisions in 250.118(10). There is no length limitation here either.

Summary: As long as the 2 x 4 lay-in luminaires are connected to the full size branch circuit (MC cable) conductors (usually 12 AWG with equipment grounding conductor), and the luminaire includes a junction box (ballast compartment) that is recognized for use as a junction point for branch circuit conductors (most lay-in luminaires are), then the installation as described would meet the minimum requirements in the NEC. If the question is, Can I daisy chain the luminaires with the ⅜ in. flexible metal conduit whips that typically are provided with the luminaires, the answer is no because these conductors are only tap conductors (usually 18 AWG) and not full size branch circuit conductors. See the information and Code sections provided in the first paragraph. I hope this effectively answers your question. As always check with the local authority having jurisdiction for any local code requirements that might modify or be in addition to the minimum requirements in the NEC. — Michael J. Johnston, CMP-5

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Static Electricity and UL 971

Posted By Michael Johnston, Tuesday, July 01, 2008
Updated: Monday, February 04, 2013

Question

Your web site gives information regarding the close relationship IAEI has with UL, with both having focus on safety issues related to electrical issues. I wonder how and what is your input and influence when UL is submitting standards and test procedures such as UL 971 (plastic pipes for fuel transport). In this standard there is no clause and no requirement for static protection and grounding at all. Even though UL 971 should be in harmony with the NFPA 77, the static electricity is forgotten. — M.C.

Answer

This message is in response to your request for information about static electricity issues when plastic piping is used for transport. This subject is beyond the scope of NEC requirements. Often UL Standards writing has input from industry professionals that have expertise in a given field to attain the most meaningful input and to work on developing comprehensive product safety standards. IAEI does not have a representative on the UL 971 Standards Technical Panel (STP). IAEI does not have a representative on NFPA 77 technical committee. It is not cost effective for IAEI to provide these representatives because of the limited situations where these types of problems arise, and the lack of coverage of this subject in the NEC.

The information provided in the article I wrote for IAEI News (May-June 2004) was an effort to raise awareness levels of static electricity hazards, specifically in hazardous (classified) locations that are covered by the NEC. I used related information with NFPA 77 to support the concepts I was providing readers in that article. For additional information about static electricity, I suggest referring to National Fire Protection Association directly. Perhaps NFPA could put you in touch with a member of the NFPA 77 technical committee or other resources who might be able to assist you with your specific questions and concerns. I apologize for my delay in this response. — Michael Johnston, CMP-5

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Insulated bushing required when large conductor installed in EMT?

Posted By Richard Owen, Tuesday, July 01, 2008
Updated: Monday, February 04, 2013

Question

Would you please help me in understanding NEC-2002 300.4(F)? If a 4 AWG or larger conductor is being installed in EMT with a insulated throated connector, would a insulated bushing be required? — R. F.

Answer

NEC-2002, Section 300.4(F) says: "…the conductors shall be protected by a substantial fitting providing a smoothly rounded insulating surface…” and does not specifically require a bushing. The bushing is only mentioned in the second paragraph in regard to a different requirement of 300.4(F)

The idea is to have an insulated surface contacting the conductor, and an insulated throat connector accomplishes this. — Richard Owen, CMP-3

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Does equipment need a second UL listing once it has been incorporated into final product?

Posted By Lanny McMahill, Tuesday, July 01, 2008
Updated: Monday, February 04, 2013

Question

There seems to be a mindset in the opinions of many inspectors (that I work with), that the use of UL-listed equipment is subject to the equipment receiving a second UL listing once the equipment has been incorporated into a final product. I would like to test this opinion and determine where the cut-off is (if any) and where it is specified in the Code.

Example 1. The use of a UL-approved NEMA 3R junction box that is sold without knockouts. To install this unit, knockouts are placed where needed and proper connection materials are used (sealing washers or sealing hubs). Is this still an NEC-approved set-up? Consider all components UL-approved.

Example 2. As example 1, now add a lighting contactor operated by a photo-eye. Is this now an NEC-approved job?

Example 3. As example 1, now add a UL-approved NEMA 3R control switch. Consider the hole for the switch punched with a button punch and notched with a notching punch. Is this NEC-approved?

Example 4. As example 1, now add a UL-approved switching mechanism, UL-approved fused switch, with UL-approved fuses. Is this now NEC-approved?

I know that it may seem trite to build on one example but I fail to see any real difference between these uses. All components are UL-approved, used in the manner they were meant to be used, and installed in a good workman-like manner. I realize that individual jobs may mandate UL approval of multi-component equipment, but I fail to see where it is mandated in the NEC. If the customer mandated the combined UL approval I would be fine with supplying it, but I’m not sure the inspector has a right to mandate it for the customer. By the way, there is no written documentation at the AHJ level, only the impression of the inspector that somehow it is mandated by NEC.

Am I missing something? — C. D.

Answer

First, the National Electrical Code defines approved as "acceptable to the authority having jurisdiction (AHJ).” Therefore, electrical products, equipment and installation approvals are the responsibility of the AHJ. See NEC Article 100 definition of approved and NEC 10.2 for approval requirements.

Second, listed and labeled generally means that a qualified testing laboratory, such as a nationally recognized testing laboratories (NRTLs), has evaluated products and equipment for safety. Listing and labeling provides the AHJ with reasonable assurance that such products and equipment are safe for installation and use. An AHJ commonly relies on NRTLs to evaluate products and equipment. The NRTLs do not approve products, equipment or installations. See NEC Article 100 definitions of listed and labeled.

Third, the NRTLs listing mark, as a rule, is only valid when applied in the product or equipment manufacturer’s factory. The NRTLs generally do not authorize the application of a listing mark in the field. A NRTL safety assessment of a product or equipment in the field is typically a field evaluation. If the product or equipment complies with the appropriate safety standard, a marking stating as such is field applied. Such marking is not a listing mark, and the product or equipment is still subject to the AHJs approval.

Fourth, keep in mind that the NEC is an installation code as opposed to a product or equipment safety standard. An installation code addresses the installation and use of products and equipment; a product or equipment safety standard addresses the production aspects of the products and equipment. Installation codes and product and equipment safety standards are both concerned with safety of persons and property, and generally work hand-in-hand.

Regarding the examples:

Example 1. Generally, a NEMA 3R listed and labeled enclosure (junction box) is acceptable to the AHJ. Knockouts for the installation of raceways and cables are typically acceptable too. This is assuming that wire bending space and deflection of conductors is in compliance with NEC requirements.

Example 2. Although a bit challenging, if the equipment is not too complex, the AHJ could approve of the installation of a lighting contactor and photo sensor in the enclosure. The AHJ may require that the installation comply with the requirements of NEC Article 409 – Industrial Control Panels. Article 409 contains specific requirements and makes reference to UL 508 safety standard for industrial control panels.

Example 3. Again, the AHJ could approve the installation of a control switch in a NEMA 3R enclosure. The integrity of the enclosure and NEMA rating must be maintained.

Example 4. The AHJ could approve the installation of a switching mechanism and fused switch too.

So where does this lead us? It is difficult to give definitive responses to the questions. Generally, the questions posed are an enforcement and consistency issue for the AHJ. Enforcement in that the AHJ may require that all electrical products and equipment be listed and labeled; consistency in that the AHJ may prefer not to pick and choose what products and equipment must be listed and labeled.

In addition, it may be difficult for an AHJ to have the necessary time and resources to evaluate products and equipment that are field-assembled. Therefore, as a generally rule the AHJ may simply require the installation and use of listed and labeled electrical products and equipment. This practice mirrors the NEC requirement that "The conductors and equipment required or permitted by this Code shall be acceptable only if approved.”

Naturally, safety of persons and property is of critical importance. Although listing and labeling is the preferred method of approving electrical products and equipment, AHJs do their best at being flexible by providing alternative means and methods of approval. However, regardless of the approval method used, be aware that other factors may require that all electrical products and equipment be listed and labeled by an NRTL.

No, you are not missing anything; you have asked some good questions. I just hope that my response helps to clarify your questions on approval of electrical products and equipment. — Lanny McMahill, CMP-1

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When is an electrical vault required?

Posted By Lanny McMahill, Tuesday, July 01, 2008
Updated: Monday, February 04, 2013

Question

Article 110.31 defines enclosures for electrical installations. 1) When is an electrical vault required [section A of code]? and 2) If I am providing a 4160-volt motor control center for an industrial plant [qualified personnel], am I required to meet the fire rating as shown for a vault? Or can I supply this motor control center in a generic modular building? — D. G.

Answer

It is difficult to give a concrete response to "When is an electrical vault required,” as the use of a vault could be a design consideration or a mandatory Code requirement. There are conditions and other criteria that must be considered.

For example, where equipment is operating at 50 volts or more, Section 110.27(A) allows the use of a vault for guarding of live parts against accidental contact. This is a design consideration. The vault could be used for guarding live parts in lieu of an electrical enclosure, such as a typical switchboard or panelboard enclosure. Section 230.6(3) also allows the use of a vault for service conductors to be considered outside the building.

Again, this is design consideration and not a mandatory Code requirement. The Code allows the service conductors to be considered outside the building where installed in a vault. Keep in mind that the service conductors inside the vault could be exposed and insulated or uninsulated.

An example of mandatory requirements is Article 110, Part III, which applies to over 600-volt installations, and it indicates that over 600-volt installations must comply with Part I and the requirements of this section, which supplement or modify the requirements of Part I. As such, Section 110.31 allows the enclosure for over 600 -volt installations to be a vault.

Such vault must comply with the mandatory requirement of 110.31(A), Fire Resistivity of Electrical Vaults. Keep in mind that the over 600-volt conductors inside the vault could be exposed and insulted or uninsulated, too. Again, the vault could be used in place of a typical electrical enclosure, such as metal-enclosed switchgear enclosure.

The 4160-volt motor control center in question is likely constructed as metal-enclosed switchgear. Generally, metal-enclosed switchgear is not required to be installed in a vault. For additional information on construction requirements for metal-enclosed switchgear, see Article 490, Part III, Metal-Enclosed Power Switchgear and Industrial Control Assemblies. The 4160-volt motor control center should meet the requirements of Part III.

Further, if the 4160-volt equipment is metal-enclosed, installed indoors, and accessible to unqualified persons, Sections 110.31(B)(1) and 110.31(D) come into play. Section 110.31(B)(1) provides information on openings in the equipment and signage requirements; Section 110.31(D) provides information on ventilating openings, protection against physical damage, and specific requirements where the general public may have access to the equipment.

To summarize, a vault may or may not be required for over 600-volt installations. The use of a vault is dependent on the system design and the type of equipment installed. Where the electrical installation contains exposed energized parts accessible to qualified persons only, a fire resistive electrical vault or other type of enclosure is likely required.
Where metal-enclosed equipment is installed, and the equipment is accessible to unqualified persons, generally a vault is not required.

I hope this provides some guidance to your questions. If of any reassurance, based on the information provided, and if all other applicable code requirements are considered, I am comfortable in saying that the motor control center could be installed in a "generic modular building!” — Lanny McMahill, CMP-1

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Conduit system under slabs in patient care areas

Posted By Eugene Morgan, Tuesday, July 01, 2008
Updated: Monday, February 04, 2013

Question

This question is about patient care areas, and I am referencing Sections 517.13(A) and (B) and 517.19(D). We are installing a conduit system under the slab and our branch circuits from the panel in the area are in PVC-coated galvanized rigid conduit to patient care devices. It is our understanding that the feeder from the MDP to this panel can be in PVC (rigid polyvinyl chloride conduit type PVC) conduit. We are installing a properly sized equipment-grounding conductor per Table 250.122 in the PVC conduit. Please advise. — R. V.

Answer

NEC Sections 517.13(A) and (B) require the installation of branch circuits to receptacles and fixed equipment in a patient care area to have two separate and different grounding paths. One grounding path is a metal raceway system, or a cable having a metallic armor or sheath assembly that "shall itself qualify as an equipment grounding conductor in accordance with 250.118.” The other grounding path is, of course, the insulated copper grounding conductor.

The intent of Panel 15 to require two separate and different grounding paths for branch circuits was emphasized during consideration of proposals for the 2008 National Electrical Code. There were two proposals that would allow a branch circuit to be run in a nonmetallic raceway with two equipment grounding conductors. Both proposals were rejected by a unanimous vote of the panel.

Section 517.19 gives additional requirements for critical care areas. Section 517.19(D) details grounding requirements "Where a grounded electrical distribution system is used and metal feeder raceway or Type MC or MI cable that qualifies as an equipment grounding conductor in accordance with 250.118 is installed…” This does not require the use of metal raceways, Type MC or MI cables for feeders to panelboards in other areas.

Section 517.12 allows the use of wiring methods in chapters 1 through 4 of the Code to be used, "Except as modified in this article.” However, one must always be aware of the project design specifications, and of locally adopted codes that may affect the wiring methods that can be used. — Eugene Morgan, CMP-15

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Tags:  Focus on the Code  July-August 2008 

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When was it that UL-approved panel shops became better (or the only) designers and fabricators of industrial control panels?

Posted By Robert Fahey, Tuesday, July 01, 2008
Updated: Monday, February 04, 2013

Question

This is becoming a big problem with some local inspectors taking the position that all control panels must be fabricated in UL-approved panel shops.

Many electrical contractors are able to fabricate a safe control panel from design documents stamped by a licensed electrical engineer. Is this not an option now?

When was it that UL-approved panel shops became better (or the only) designers and fabricators of industrial control panels? — R. K.

Answer

The best way to answer your question regarding industrial control panels is by taking each part of the question and answering it separately. Answers are based on NEC-2008, although answers would not be different based on NEC-2005.

Is it now required to have industrial control panels listed? Article 409 does not have any specific requirements that these industrial control panels (ICP) must be listed by a third party independent testing laboratory, although NEC 110.2 requires the conductors and equipment be acceptable only if approved; the definition of approved in Article 100 is "acceptable to the authority having jurisdiction (AHJ).” NEC 110.2 allows the AHJ the option to accept an unlisted ICP, but then requires the AHJ to inspect (approve) the ICP; or the AHJ may require the ICP to be listed in order to gain the AHJ’s approval. An ICP in certain installations will be small and simple (grain elevator at a farm) with 4 motor starters, relays and/or stop-start controls and the local AHJ may be comfortable inspecting this in accordance with the requirements of the NEC, and more specifically NEC 110.3(A), Article 409 and UL 508A (UL Standard for ICPs). In other installations, the ICP can be very large and complex with many sections connected together; in this situation, the AHJ may require the more complex ICPs to be listed by a third party testing agency or be evaluated by a licensed electrical engineer to be assured the ICP complies with the NEC and UL 508A.

Can a licensed electrical engineer design and have a licensed electrician fabricate an ICP? The ICP could be designed by an electrical engineer and fabricated by a licensed electrician, but the AHJ could still require the ICP to be listed by a third party independent testing agency, or in some areas a licensed electrical engineer may be acceptable, so that the AHJ would be assured the ICP meets the requirements in UL 508A.

If an ICP has only control devices installed and has only control power entering the ICP, does 409 apply? The definition for the NEC-2008 has been restructured into three subsections for clarity, subsection (2) states where there are "control circuit components only, such as pushbuttons, pilot lights, selector switches, timers, switches, control relays,” this new restructured language clearly indicates Article 409 applies to those ICP with only control devices.

In summary, ICPs can be required to be listed if the local AHJ requires the listing as part of the approval. Licensed electrical engineers can design an ICP and electrical contractors can fabricate the ICP, but a listing of the ICP may also be required in this situation. Article 409 does apply to an ICP with only control power and control power devices. — Robert Fahey, CMP-11

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Tags:  Focus on the Code  July-August 2008 

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