Question 1. NEC Section 370-28(a)(3) states that conduit bodies of smaller dimensions than required in (a)(1) may be used for combinations of conductors less than the values in table 1, chapter 9, up to the maximum that is permanently marked in the conduit body. The contractor intends to pull twelve No. 2/0 conductors through a 3 in. C-condelet, which is stamped "maximum 3 No. 300 MCM XHHW conductors" instead of using properly sized pull boxes. The conductors are oversized for de-rating purposes. Does this violate Section 370-28? — I.N.

Answer 1. Thank you for your question regarding the application of 370-28(a)(3). Because you referenced 370-28(a) (3), I assume you are using the 1999 edition, rather than the 2002, in which Article 370 was relocated as Article 314. Additionally, I am assuming the use of Type THHN insulation and a 3-inch EMT raceway.

Subsection 370-28(a) limits its application to the minimum dimensions of pull or junction boxes installed in a raceway or cable run where the conductors are No. 4 or larger. Parts (1) and (2) deal with the dimensional requirements when straight, angle and U pulls are made. Subsection 370-28(a)(3) permits the use of boxes or conduit bodies with dimensions less than those required in 370-28(a)(1) or (2) when the installation complies with the stated conditions. The stated conditions are:

1. The combination of conductors being used is less than the maximum conduit or tubing fill permitted by Table 1 of chapter 9.

2. The box or conduit body has been approved for and is permanently marked with the maximum number and maximum size of conductors permitted.

Table 5 of chapter 9 gives the approximate area, in square inches, of the No. 2/0 AWG THHN insulated conductors as 0.2223. There are twelve No. 2/0 AWG THHN insulated conductors being installed. This results in a total of 2.6676 square inches [12 x 0.2223 square inches = 2.6676]. Table 1 of chapter 9 requires a maximum 40 percent fill for more than two conductors in a conduit or tubing. Table 4 of chapter 9 gives the 40 percent fill for a 3-inch EMT as 3.538 square inches. The twelve No. 2/0 AWG Type THHN insulated conductors require 2.6676 inches and the 3-inch EMT is permitted to have a maximum of 3.538 square inches at 40 percent fill. This satisfies one of the two conditions stated in 370-28(a)(3).

The C-condelet is stamped with a maximum of three No. 300 MCM XHHW insulated conductors. The use of the term MCM is changed to kcmil in the 1999 NEC. Table 5 of chapter 9 gives the approximate area of the No. 300 kcmil XHHW insulated conductors as 0.4536 square inches. The stamp is for a maximum of three such conductors. This results in a total square inch requirement of 1.3608 [3 x 0.4536 square inches = 1.3608].

The twelve No. 2/0 AWG Type THHN insulated conductors require a minimum of 2.6676 square inches, while the condelet is suitable for no more than 1.3608 square inches. Therefore, this proposed installation is not permitted by the Code. — Wayne A. Lilly, CMP-08

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Question 2. I am a project inspector for a large institutional project in the San Francisco Bay area of California. The electrical contractor has supported his conduits from the drywall utilizing drywall inserts and one-hole straps. In the past, I have always required that the conduits and boxes be supported from framing members (i.e., studs) and not from the drywall only. Can you help me with the requirements on this issue? I have reviewed the Code and it is vague about the adequate support of the conduit and boxes.

Thanks for any help you can give me to convince the contractor to support from the metal framing members and not just into the drywall. — M.W.

Answer 2. Thank you for your question regarding the supporting of conduit. I hope my answer will be of help. You did not state which Code you are using. I will assume that the 2002 edition is applicable in this situation. I will also assume that the voltages involved are less than 600. Your question specifically deals with supports. You did not mention which conduit is being installed. As you are aware, there are several conduits addressed by the NEC. Each conduit article has requirements for supporting that are specific to the conduit addressed by the article. When comparing those requirements, we see there are some differences.

During the last few cycles, the NEC has made an effort to clearly distinguish between securing and supporting of raceways. The intention is to provide more uniform language that is easier to understand and apply. An example is section titles that have been changed from "Supports" to "Securing and Supporting" with separate requirements for "Supports" and "Securely Fastening."

Conduits are to be securely fastened so that strain and stress will not be transmitted to their termination points. For instance, 342.30 requires intermediate metal conduit (IMC) to be "securely fastened within 900 mm (3 ft) of each outlet box, junction box, device box, cabinet, conduit body, or other conduit termination. Fastening shall be permitted to be increased to a distance of 1.5 m (5 ft) where structural members do not readily permit fastening within 900 mm (3 ft). Where approved, conduit shall not be required to be securely fastened within 900 mm (3 ft) of the service head for above-the-roof termination of a mast."

Supports are installed to aid in preventing damage between points where the conduit is securely fastened, such as might occur at a conduit coupling. Each conduit article has requirements for supporting the conduit. For instance, the support requirements for flexible metal conduit (FMC), as found in 348.30, are "Horizontal runs of flexible metal conduit FMC supported by openings through framing members at intervals not greater than 1.4 m (4½ ft) and securely fastened within 300 mm (12 in.) of termination points shall be permitted."

The NEC does not provide for the methods that are to be used in meeting the requirements for securely fastening and supporting conduits. Because of weight differences, the methods for securely fastening and supporting a run of metric designator 103 (trade size 4) rigid metal conduit (RMC) are different than those for metric designator 12 (trade size 3/8) flexible metal conduit (FMC). Use of wording such as "securely fastened" is meant to convey that the conduit is to be fastened in such a manner that the conduit will not move. This will prevent damage at the conduit termination. Support requirements are meant to permit some movement, but not movement that will damage the conduit or conduit couplings. It is the responsibility of the authority having jurisdiction (AHJ) to determine what methods are best suited for the installation circumstance. The use of one-hole versus two-hole straps and toggle bolts versus screws directly into wood are decisions that the AHJ must make. There may be additional requirements specific to local conditions such as seismic considerations.

Your question also mentioned boxes. Support of boxes is covered in 314.23. This section does provide some specific guidance relative to the support of boxes. The AHJ must determine what methods are appropriate for the installation circumstance. — Wayne A. Lilly, CMP-8

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Question 3. I have a question concerning the minimum and maximum mounting height of a main breaker in a 125a, NEMA3 panel installed at a single family dwelling. The inspector says that the Code states that the main breaker handle must be between 5' 6"and 6' 6". Is this true? If so, where is it stated in the Code? The only thing I have found is in 230.70(a)(1), (2), (3) and 404.8(A), which states 6' 7" max to center of breaker or disconnect handle, but nothing on minimum height requirements. The main breaker in the panel is currently at 4' 6" and the meter can is at 6' to the center, mounted directly above the panel with an offset nipple. — F.M.

Answer 3. In 2002 NEC 230.70(A)(1), (2), and (3), and 404.8(A) state, a maximum of 6' 7" to center of breaker or disconnect handle. The only articles to mention a minimum height are 550.11(A) and 551.77(D) for mobile homes and recreational vehicles with a minimum of 24" above the ground. On a single-family dwelling, I can find no requirement to minimum height. — Joe Bolesina, CMP-9

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Question 4. This question concerns the use of expansion joints as discussed in NEC 1999, Section 300-7(b). The text states, "raceways shall be provided with expansion joints where necessary to compensate for thermal expansion and contraction." The FPN then discusses the 0.20 co-efficient for steel conduit, which could be applied based on the values in Table 347-9(A) for rigid nonmetallic conduit.

According to 347-9, Expansion Fittings, under rigid nonmetallic conduit, provisions must be made for thermal expansion and contraction when the conditions are such that there could be a 1/4" movement. Does this requirement for thermal expansion and contraction apply to rigid metallic conduit?

Are expansion fittings required for rigid metallic conduit? For example, if 100 feet of rigid metallic conduit is subject to 100ºF temperature change, then the pipe would expand/contract 4.1" x 0.20 = 0.82".

I don't see anything in the NEC that addresses provisions for the expansion/contraction of rigid metallic conduit, other than the FPN under 300-7(b). I realize that the FPN is for clarification or explanation; but, nonetheless, it seems that 1/4" movement is the same regardless what the conduit material is. Provisions must be made for rigid nonmetallic conduit when the movement is 1/4".

Does this same requirement apply to other conduits (specifically, RMC)? Section 300-7(b) seems to require it. But I can't see anything in the Code that spells it out as clearly as 347-9 does for rigid nonmetallic conduit.

What are the requirement, authority and procedure for RMC?—K.C.

Answer 4. Thank you for your interesting questions regarding the use of expansion joints in rigid metal conduit (RMC) and other metallic conduits and tubing.

You are correct in your assessment that a 1/4-inch or more movement of rigid nonmetallic conduit requires the application of expansions fittings. There are other specific requirements for expansion fittings. For example, nonmetallic auxiliary gutters as required in 374-9(e)(2)(a)(4) and (b)(2). You are also correct that 300-7(b) of NEC 1999 requires the use of expansion fittings for raceways where necessary to compensate for expansion and contraction. This requirement applies to metallic as well as nonmetallic raceways.

FPNs are informational. They are not enforceable. However, the FPN in 300-7(b) does provide information on how to determine the expansion for RMC, IMC and EMT. The illustration you gave is approximately correct. There will be roughly an expansion of 0.82-inches per 100 feet for the 100ºF temperature change in your example. However, there are no provisions, as there are for RNC, for establishing an entry threshold, such as 1/4-inch, at which expansion fittings are required.

The language in 300-7(b) requires the use of expansion fittings "where necessary." A clear need has been established for the use of expansion fittings with RNC. Panel 8 has taken the necessary action by requiring their use in 347-9. However, a need for expansion fittings has not been demonstrated for RMC, IMC, or EMT. The Code does not require the use of expansion fittings for RMC, IMC, or EMT. If, in the future, a need for the fittings can be demonstrated a Code proposal should be made to change the present requirements.—Wayne A. Lilly, CMP-08

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Question 5. The following is a description of a situation followed by a two-part question.

 A NEMA 3R enclosure is connected to a NEMA 3R disconnect via a horizontal run of 2" EMT.  Fittings assumed to have the UL listed continuous ring in the throat to meet UL requirement for raintight EMT fittings.

 The first question is whether or not the EMT fittings should have a gasket at the enclosure and disconnect in order to retain its raintight listing.

 The second question is if the EMT fittings do not have a gasket at the entry to these enclosures, does that affect the raintight listing of the enclosures?

 I have been told that 3R will allow the enclosure to leak as long as it does not create a short or ground-fault condition.  Thus, the lack of gaskets on the EMT fittings.  I think that the gaskets are required both to retain the raintight features of the conduit fittings and the enclosures. — R. D.

Answer 5. Thank you for your question regarding the use of electrical metallic tubing (EMT) raintight type fittings and NEMA 3R equipment. Your question did not specify which Code you are working under. I will assume you are using the 2002 edition. I will also assume that the NEMA 3R equipment is the correct choice for the application and that the equipment is installed in a wet location. The following is based on operational voltages of less than 600 volts.

We will begin by reviewing some of the more pertinent information and Code requirements. Part 1 of the UL Directory General Information for Electrical Equipment, commonly referred to as the UL White Book, under the heading "Enclosure Types" lists the enclosure type numbers and the degree of protection provided by each within specific environmental conditions. This information states that a NEMA 3R enclosure type is suitable for outdoor use and will be "undamaged by the formation of ice on the enclosure." NEMA 3R equipment is not suitable for use in areas where there is splashing or hose-directed water. These applications would require a NEMA 4 enclosure.

Additional information provided in the White Book under the heading "Enclosure Types" states "A Type 3R enclosure may be marked ‘Rainproof.’" We are further told that a "Type 3 enclosure may be marked ‘Raintight.’" Article 100 of the NEC defines rainproof as "constructed, protected, or treated so as to prevent rain from interfering with the successful operation of the apparatus under specified test conditions." The word raintight is defined by Article 100 as "constructed or protected so that exposure to a beating rain will not result in the entrance of water under specified test conditions." Although these are subtle differences, it is important to use these terms correctly.

Your question did not specify if the enclosure and disconnect fall within Article 312 or Article 314. Section 312.2(A) states, in part, "In damp or wet locations, surface-type enclosures within the scope of this article shall be placed or equipped so as to prevent moisture or water from entering and accumulating within the cabinet or cutout box and shall be mounted so there is at least 6 mm (1/4 in.) airspace between the enclosure and the wall or other supporting surface. Enclosures installed in wet locations shall be weatherproof." Section 314.15(A) states, "In damp or wet locations, boxes, conduit bodies, and fittings shall be placed or equipped so as to prevent moisture from entering or accumulating within the box, conduit body, or fitting. Boxes, conduit bodies, and fittings installed in wet locations shall be listed for use in wet locations."

Section 358.6 requires EMT, factory elbows, and associated fittings to be listed. Section 358.42 states, "Couplings and connectors used with EMT shall be made up tight. Where buried in masonry or concrete, they shall be concrete tight type. Where installed in wet locations, they shall be of the raintight type." Section 110.3(B) states, "Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling."

In light of the foregoing information, I will try to answer your questions. If the equipment is installed in a wet location per 312.2(A) or 314.15(A), water must not be permitted to enter the enclosure or box. Therefore, fittings that are connected to the enclosure or box must be installed so as to prevent the entrance of water. Further, Sections 358.6 and 358.42 require the fitting to be listed as raintight type. The use of sealing hubs and sealing locknuts would be examples of installation methods that prevent water entry.

Sealing locknuts, as found under the heading Conduit Fittings (DWTT) in the UL White Book, "are intended for use with threaded rigid metal conduit and intermediate metal conduit." The information found under this heading also permits sealing locknuts for use with listed wet location fittings "where so marked on the fitting carton." If a sealing hub or fitting is used gaskets, or O-rings, must be installed to maintain the integrity of the box or enclosure against moisture entry. I would agree that enclosures or boxes located in wet locations that are installed so that water may enter are not code-compliant. — Wayne A. Lilly, CMP-08

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Question 6. I am located at the U.S. Naval Base in Greece. A U.S. contractor is installing 3/4" flex conduit liquidtight in lengths greater than 6 ft for a branch circuit from a power panel to a wiring cabinet utilized for electronic communication devices. My understanding from the 1999 NEC is that flex conduit in any form is not allowable in lengths greater than 6 ft. The contractor states new code changes have deleted this requirement. Is this true? Thanks for your help. — E.F.

Answer 6. I very much appreciate this opportunity to address your question regarding the use of liquidtight flexible conduit. I hope my response will be helpful. As I understand your question, my answer will be based on the changes between the 1999 and 2002 NEC editions. The question did not specify the use of liquidtight flexible nonmetallic conduit (LFNC) or the use of liquidtight flexible metallic conduit (LFMC). I will answer the question for both types of liquidtight flexible conduit.

First we will take a look at LFNC. A definition for LFNC can be found in 351-22 of the 1999 NEC and in 356.2 in the 2002 NEC. Although there are some minor changes in these sections, the definitions are the same for both. Looking at these sections reveals that there are actually three definitions, one for each type of LFNC. The three types are LFNC-A, LFNC-B, and LFNC-C.

The uses permitted and not permitted for LFNC are found in 351-23 of 1999 NEC and 356.10 and 356.12 of 2002 NEC. Sections 351-23(b)(3) and 356.12(3) provide one of the uses not permitted for LFNC. These sections stipulate that LFNC lengths longer than 6-feet are not permitted to be used. However, the language in both sections goes on to permit longer lengths under certain circumstances.

The first of these circumstances permits a longer length if "a longer length is approved as essential for a required degree of flexibility." This requires the AHJ to determine if a length longer than 6 feet is essential. If the AHJ determines that a longer length is "essential for a required degree of flexibility" any of the three types of LFNC may be used. The other circumstance permits LFNC to be installed in any length if it is Type LFNC-B. In either circumstance, the applicable installation requirements of the Code must be met.

The answer for LFNC is that there are no differences in the length requirements between the 1999 and 2002 Codes. There was a change between the 1996 and 1999 editions, which permitted Type LFNC-B to be used in lengths longer than 6 feet.

Section 351-4 of 1999 NEC and Sections 350.10 and 350.12 of 2002 NEC provide the uses permitted and not permitted for LFMC. LFMC is not restricted in length by either Code in these sections. However, 250-118(6) and (7) in both editions limit the length of LFMC where it is used as an equipment grounding conductor.

I hope I have addressed your question. If not, please feel free to contact IAEI again. — Wayne A. Lilly, CMP-8

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Question 7. Reviewing Article 318 on cable trays, I do not find a provision that allows use of solid bottom cable tray for single-conductor power cables. Specifically, 318-9 allows the use of solid bottom tray for multiconductor control and/or signal cables. However, I see no mention of solid bottom tray in 318-10. Is solid bottom tray suitable for use with single-conductor power cables less than 2000 V? If so, what derating factors or ampacity tables are used? — J. A.

Answer 7. Thank you for your question regarding the use of single-conductor cables with cable tray. You did not specify which Code you are using. However, you did refer to Article 318. The article on cable tray was relocated from 318 to 392 in the 2002 edition. For that reason, I will assume you are using NEC-1999.

Because your question deals with single-conductor power cables, I will assume you are installing the cables in an industrial establishment as required and permitted in 318-3(b). Section 318-3(b) states, in part, "The wiring methods in Section 318-3(a) shall be permitted to be used in any industrial establishment under the conditions described in their respective articles. In industrial establishments only, where conditions of maintenance and supervision ensure that only qualified persons will service the installed cable tray system, any of the cables in (1) and (2) shall be permitted to be installed in ladder, ventilated trough, or ventilated channel cable trays." You are correct in your assertion that single-conductor power cables in solid bottom tray are not permitted. The 1999 Code permits only single-conductor cables "to be installed in ladder, ventilated trough, or ventilated channel cable trays."

This limitation changed with the 2002 edition. Proposal 8-21 in the Report on Proposals for the 2002 Code was accepted. The substantiation for the proposal was "Cables are installed within solid metal enclosures every day, i.e., conduit and wireway. There is no reason to exclude solid bottom tray from containing these cables as long as they are being installed by qualified individuals in industrial establishments and the fill requirements are consistent with the conservative numbers used for conduit and wireway." Also, Article 318 became Article 392. With the acceptance of these revisions, Section 392.3(b) states, in part, "In industrial establishments only, where conditions of maintenance and supervision ensure that only qualified persons service the installed cable tray system, any of the cables in 392.3(B)(1) and (2) shall be permitted to be installed in ladder, ventilated trough, solid bottom, or ventilated channel cable trays." The 2002 edition does permit single-conductor power cables in solid bottom tray.

Section 392.11 provides the ampacity requirements for multiconductor cables and single cables rated 2000-volts or less in cable tray. 392.11(B) details the requirements for single-conductor cables. Your question did not specify a cable size. You will need to read all of 392.11(B) to determine which of the items meets your specific needs.

A new exception is added in the 2005 edition in Section 392.11(B)(3). Code-making panel 8 accepted Proposal 8-291. The new exception is, "Exception: For solid bottom cable trays the ampacity of single-conductor cables shall be determined by 310.15(C)." The substantiation for this proposal was: "The issue is the ampacity of single-conductor cables in solid bottom cable trays where the ventilation is restricted relative to ladder and ventilated trough cable trays. The situation is somewhat analogous to the condition of solid unventilated covers. Section 392.11(B)(1) indicates a reduction in ampacity, as given in tables 310.17 and 310.19 from 75 percent to 70 percent. These ampacity tables apply to single conductors in free air. Subsections (2) and (4) of 392.11(B) derate the free air ratings in 310.17 and 310.19 by 75 percent and are already derated for solid bottom cable trays. Item (3) however, allows the use of 100 percent of the free air ampacity ratings in tables 310.17 and 310.19 if the cables are spaced a cable diameter apart. Solid bottom cable trays will restrict the free air flow to these conductors; therefore, the ampacity should be determined under engineering supervision per 310.15(C)." As a result of this exception, the ampacity of single-conductor cables in solid bottom cable tray may only be determined by engineering supervision that utilizes the formula in 310.15(C).

The NEC is a living document that is constantly changing to reflect changes in the electrical industry. Article 392 is a prime example. Many changes have taken place in 392 during the last three code cycles. That process will continue during the coming cycles. Everyone is encouraged to be an active participant in the process by submitting proposals and comments and by being active in the IAEI. — Wayne A. Lilly, CMP-08

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Question 8. What is the NEC spacing requirement when installing .48 diameter, 4-conductor Belden 29500 variable frequency drive cable in a lay-in wireway? — T.R.

Answer 8. You have brought up an interesting question for the use of the Belden variable frequency drive (VFD) type cable and are asking if this 29500 cable is allowed to be installed in lay-in wireway trough.

I think a couple of items need be clarified about the Belden 29500 cable. This is a 16 AWG cable and is being used to carry power from AC drive systems to AC motors that may be located in harsh locations. When you consider a VFD, the power associated with cable fed from the VFD to the motor is coming off of an internal bussing that may operate at up to 650 volts DC. In this case when we look at the manufacturer’s (Belden) information listing from their catalog, it shows a UL rating of 600 volts when used as tray cable (TC) from Article 336 of NEC-2002, or as open wiring as per Article 398 of NEC-2002, and has a 1000 volts rating UL/CSA for uses other than those listed above.

Wireway has no voltage limitations as some raceway types, but when VFD cable is installed in lay-in wireway, the conditions for the installation requirements for type TC cable has to be considered. The uses permitted in 336.10 are: (1) for power, lighting, control, and signal circuits; and (2) it allows type TC cable to be used in cable trays, or in raceways. There are no restrictions for uses not permitted when installed in a lay-in wireway as per 336.12.

The next consideration will be for the use for a possible over 600-volt applications. The basic requirements of Article 300, Wiring Methods, cover all wiring methods for all wiring installations unless modified by other articles. Additionally, Section 300.3(C)(1) and (2) provides information for conductors of different systems. Section 300.3(C)(1) allows 600 volts, nominal or less. Conductors of circuits rated 600 volts, nominal, or less, ac circuits, and dc circuits shall be permitted to occupy the same equipment wiring enclosure, cable, or raceway. All conductors shall have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the enclosure, cable, or raceway.

Section 300.3(C)(1) makes it clear that it is the maximum circuit voltage in the raceway, not the maximum insulation voltage rating of the conductors in the raceway, that determines the minimum voltage rating required for the insulation of conductors for systems of 600 volts or less. If the installation is a 600 volts or less type of use for the VFD cable and all other conductors contained in the lay-in wireway are rated for the maximum operating circuit voltage in the wireway, no conductor separation would be required.

If the installation has mixed voltages of 600 volts or less and over 600 volts within a raceway or wireway, then 300.3(C)(2) would have to be considered: "Conductors of circuits rated over 600 volts, nominal, shall not occupy the same equipment wiring enclosure, cable, or raceway with conductors of circuits rated 600 volts, nominal, or less unless otherwise permitted in (C)(2)(a) through (C)(2)(e)."

Section 300.3(C)(2)(d) states, "In motors, switchgear and control assemblies, and similar equipment, conductors of different voltage ratings shall be permitted." If these are mixed conductors from the VFD control assembly for motor(s) only for the power conductors and the conductors have shielded insulation (Belden 29500 is a shielded cable) for that system only, then no separation of conductors would be required. If the operating voltage is over 600 volts, then the opening statement of 300.3(C) would not allow the VFD type cable to be installed within a lay-in wireway with other conductors and follow the installation requirements of type TC cable. — James Imlah, CMP-8

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The views of the authors of Focus on the Code and the editor are provided solely as a public service. 

The views expressed are not the official position of NFPA, the NEC Correlating Committee or any of its panels, IAEI, IAEI News, or the author's employers. Nor are they intended to represent a formal or informal interpretation of the NEC. 

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