NEC Questions and Answers based on 2017 NEC^® - April 2019

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For EC&M Magazine
By Mike Holt, NEC^® Consultant
Here's the follow-up to yesterday's newsletter. This includes the answers to the questions sent, so you can see how you did.

Note: The answers to these questions are based on the 2017 NEC.

A1. Raceways must be large enough to permit the installation and removal of conductors without damaging the conductors' insulation. When all conductors within a raceway are the same size and insulation, the number of conductors permitted can be found in Annex C for the raceway type [344.22].

Cables can be installed in a raceway, if the number of cables doesn't exceed the allowable percentage fill specified in Table 1, Chapter 9.

A2. Raceway bends aren't permitted to be made in any manner that would damage the raceway, or significantly change its internal diameter (no kinks). The radius of the curve of the inner edge of any field bend isn't permitted to be less than shown in Table 2, Chapter 9 [344.24].

Author's Comment:
• This usually isn't a problem because benders are made to comply with this table. However, when using a hickey bender (short-radius bender), be careful not to over-bend the raceway.

Q3. What is the NEC requirement for securing and supporting Rigid Metal Conduit?

A3. Rigid metal conduit must be installed as a complete system in accordance with 300.18 [300.10 and 300.12], and it must be securely fastened in place and supported in accordance with 344.30(A) and (B).

(A) Securely Fastened. RMC must be secured in accordance with any of the following:
(1) Fastened within 3 ft of each outlet box, junction box, device box, cabinet, conduit body, or other conduit termination. Š

Author's Comment:
• Fastening is required within 3 ft of terminations, not within 3 ft of each coupling.

(2) When structural members don't permit the raceway to be secured within 3 ft of a box or termination fitting, the raceway must be secured within 5 ft of the termination. Figure 01

(3) Conduits aren't required to be securely fastened within 3 ft of the service head for an above-the-roof termination of a mast. Š

(B) Supports.
(1) General. Rigid metal conduit must be supported at intervals not exceeding 10 ft.

(2) Straight Horizontal Runs. Straight horizontal runs made with threaded couplings can be supported in accordance with the distances contained in Table 344.30(B)(2).

(3) Vertical Risers. Exposed vertical risers for fixed equipment can be supported at intervals not exceeding 20 ft, if the conduit is made up with threaded couplings, firmly supported, securely fastened at the top and bottom of the riser, and if no other means of support is available.

(4) Horizontal Runs. Conduits installed horizontally in bored or punched holes in wood or metal framing members, or notches in wooden members, are considered supported, but the raceway must be secured within 3 ft of termination.

Author's Comment:
• Rigid metal conduit must be provided with expansion fittings if necessary to compensate for thermal expansion and contraction [300.7(B)]. The expansion characteristics for metal raceways are determined by multiplying the values from Table 352.44 by 0.20, and the expansion characteristics for aluminum raceways is determined by multiplying the values from Table 352.44 by 0.40 [300.7 Note].

Q4. What are the Code requirements for couplings and connectors installed in Rigid Metal conduit runs?

A4. (A) Installation. Threadless couplings and connectors must be made up tight to maintain an effective ground-fault current path to safely conduct fault current in accordance with 250.4(A)(5), 250.96(A), and 300.10 [344.42].

Author's Comment:
• Loose locknuts have been found to burn clear before a fault was cleared because loose connections increase the impedance of the ground-fault current path.

If buried in masonry or concrete, threadless fittings must be the concrete-tight type. If installed in wet locations, fittings must be listed for use in wet locations and prevent moisture or water from entering or accumulating within the enclosure in accordance with 314.15. Š

Threadless couplings and connectors aren't permitted to be used on threaded conduit ends, unless listed for the purpose.

(B) Running Threads. Running threads aren't permitted for the connection of couplings, but they're permitted at other locations.

Q5. What is the NEC rule for the installation of bushings on Rigid Metal Conduit?

A5. To protect conductors from abrasion, a metal or plastic bushing must be installed on conduit threads at terminations, regardless of conductor size, unless the box, fitting, or enclosure is designed to provide this protection [344.46].

Informational Note: Conductors 4 AWG and larger that enter an enclosure must be protected from abrasion, during and after installation, by a fitting that provides a smooth, rounded, insulating surface, such as an insulating bushing, unless the design of the box, fitting, or enclosure provides equivalent protection, in accordance with 300.4(G).