NEC Questions and Answers based on 2017 NEC^® - July 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. Underlined text denotes a change in the 2017 NEC.

Q1. What is the Code rule regarding support of raceways and boxes by a cable tray?
A1. In industrial facilities where conditions of maintenance and supervision ensure only qualified persons will service the installation, and if the cable tray system is designed and installed to support the load, cable tray systems can support raceways, cables, boxes, and conduit bodies [392.18(G)].

For raceways terminating at the tray, a listed cable tray clamp or adapter must be used to securely fasten the raceway to the cable tray system. The raceway must be supported in accordance with the appropriate raceway article.

Raceways or cables running parallel to the cable tray system can be attached to the bottom or side of a cable tray system. The raceway or cable must be fastened and supported in accordance with the appropriate raceway or cable's Code article.

Boxes and conduit bodies attached to the bottom or side of a cable tray system must be fastened and supported in accordance with 314.23.

Q2. What are the Code rules for the installation of single or parallel conductors in cable trays?
A2. To prevent unbalanced current in the parallel conductors due to inductive reactance, all circuit conductors of a parallel set [310.10(H)] must be bundled together and secured to prevent excessive movement due to fault-current magnetic forces [392.20 (C).

Single conductors of a circuit not connected in parallel must be installed in a single layer, unless the conductors are bound together [392.20(D)].

Q3. How many conductors or cables are permitted in a cable tray by the NEC?
A3. The number of multiconductor cables, rated 2,000V or less, permitted in a single cable tray must not exceed the requirements of 392.22(A). The conductor sizes herein apply to both aluminum and copper conductors. Where dividers are used, fill calculations apply to each divided section of the cable tray.

If ladder or ventilated trough cable trays contain multiconductor power or lighting cables, the maximum number of cables must conform to the following [392.22(A)(1)]:
If all of the cables are 4/0 AWG and larger, the sum of the diameters of all cables must not exceed the cable tray width, and the cables must be installed in a single layer [392.22(A)(1)(a)].

Cable trays must be supported in accordance with the manufacturer's installation instructions [392.30(A)].

Cables and conductors must be secured to and supported by the cable tray system in accordance with 392.30(B)(1), (2), and (3) as applicable.

Cables installed vertically must be securely fastened to transverse members of the cable tray [392.30(B)(1)].

Supports must be provided to prevent stress on cables where they enter raceways or other enclosures from cable tray systems [392.30(B)(2)].

A box isn't required where cables or conductors exit a bushed raceway used for the support or protection of the conductors [392.46].

Q4. What is the Code rule regarding cable splices in a cable tray?
A4. Splices are permitted in a cable tray if the splice is accessible and insulated by a method approved by the authority having jurisdiction. Splices can project above the side rails of the cable tray if not subject to physical damage [392.56].

Q5. Does the NEC allow a cable tray to be used as an equipment grounding conductor?
A5. Metal cable trays can be used as equipment grounding conductors where continuous maintenance and supervision ensure that only qualified persons service the cable tray system [392.60(A)].

Metal cable trays containing single conductors must be bonded together to ensure they have the capacity to conduct safely any fault current likely to be imposed in accordance with 250.96(A).

Metal cable trays containing communications, data, and signaling conductors and cables must be electrically continuous through approved connections or the use of a bonding jumper.

Metal cable trays can serve as equipment grounding conductors where the following requirements have been met [392.10(B) and (C)]:
(1) Metal cable trays and fittings are identified as an equipment grounding conductor.
(4) Cable tray sections, fittings, and connected raceways are effectively bonded to each other to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed on them [250.96(A)]. This is accomplished by using bolted mechanical connectors or bonding jumpers sized in accordance with 250.102. Figure 01