8.5 Cable Trays and Underground Wiring

Where These Topics Sit on the Blueprint

Electrical Wiring Methods and Electrical Materials is one of the two heaviest areas on the NEC Knowledge portion (10 scored items) and also carries 2 items on the Calculations portion. The PSI outline names five sub-topics inside it, two of which are covered here: Raceways and Cable Trays and Underground Wiring. Both are almost pure lookup — the exam wants a number or a permitted/not-permitted call, and both sit in parts of Chapter 3 that candidates rarely tab.

Article 392 — Cable Trays

A tray is not a raceway

This is the single most-tested idea in Article 392. A cable tray system is a structural support system for cables, raceways, and conductors. It does not enclose or protect conductors the way a conduit does, so whatever you lay in it must independently be a permitted wiring method. That is why you find Type MC, Type TC, Type MI, metal-clad and tray-rated cables in trays, and why you cannot simply drop loose building wire into one.

Uses permitted and not permitted

• 392.10 permits cable trays to support the wiring methods listed in Table 392.10(A), plus raceways, boxes, and conduit bodies.
• 392.10(B)(1) permits single conductors 1/0 AWG and larger in ladder, ventilated trough, or ventilated channel tray, but only where the tray is used in industrial establishments with qualified maintenance and supervision.
• 392.12 prohibits cable tray in hoistways and where it would be subject to severe physical damage.

Installation rules that get tested

• 392.18(A) — the tray system must be complete before conductors are installed.
• 392.18(G) — trays must be exposed and accessible, and marked so they are not used as walkways or ladders.
• 392.18(H) — covers are required where the tray is exposed to physical damage.
• 392.30(A) — cables must be secured where the tray runs vertically.
• 392.44 — expansion splice plates are required where thermal expansion is a factor.
• 392.56 — splices are permitted inside a tray provided they are accessible and do not project above the side rails. This is a genuine exception to the usual box requirement, and it is a favorite exam item.

Fill (392.22)

For multiconductor cables rated 2000 V or less in a ladder or ventilated trough tray:

• Where all cables are 4/0 AWG or larger, the sum of the cable diameters may not exceed the cable tray width, and the cables must be installed in a single layer.
• Where all cables are smaller than 4/0, the sum of the cable cross-sectional areas may not exceed the maximum fill area in column 1 of Table 392.22(A) for that tray width.
• Mixed installations use the combination formulas in Table 392.22(A).

Ampacity (392.80)

Multiconductor cables in an uncovered tray are sized by the normal Article 310 ampacity rules. Where the tray is covered for more than 6 ft continuously, the allowable ampacity may not exceed 95 percent of the tabulated value, because the cover traps heat.

The tray as an equipment grounding conductor (392.60)

A metallic cable tray may serve as the EGC, but only where all of the following are true:

1. The tray sections and fittings are identified for grounding purposes.
2. The minimum cross-sectional area of the tray conforms to Table 392.60(A) for the largest upstream overcurrent device.
3. All tray sections and fittings are legibly and durably marked with the letters "EGC" or an equivalent.
4. The system is in an industrial establishment with qualified persons providing continuous maintenance and supervision.

In ordinary commercial premises wiring, none of that is satisfied, so an equipment grounding conductor must be run in the tray with the circuit conductors.

NEC 300.5 — Underground Installations

Minimum cover, Table 300.5(A)

Cover is defined as the shortest distance measured between a point on the top surface of any direct-buried conductor, cable, conduit, or raceway and the top surface of finished grade, concrete, or similar cover. Finished paving counts toward cover.

Two rows carry most of the exam weight. The "all locations not specified" row is the default that answers a plain backyard question. The streets and public parking row flattens every column to 24 in. — there is no reduced value for RMC there, which is exactly the contrast the exam likes to draw against the 6 in. figure on the first row.

A 2023 NEC change added electrical metallic tubing to column 3, alongside nonmetallic raceways listed for direct burial. If you are working from a 2020 book, EMT is missing from that column.

The rest of 300.5

• 300.5(B) Wet locations — the interior of an underground raceway is a wet location, so conductors must be listed for wet locations: THWN-2, XHHW-2, RHW-2, or similar. THHN alone does not qualify. Nearly all modern building wire is dual-rated THHN/THWN-2, which is why this passes unnoticed in the field but still gets tested.
• 300.5(C) Under buildings — underground wiring under a building must be in a raceway that extends past the outside walls.
• 300.5(D) Protection from damage — direct-buried conductors emerging from grade must be protected by enclosures or raceways from the required cover depth or 18 in., whichever is less, up to a point 8 ft above finished grade.
• 300.5(E) Splices and taps — direct-buried cable or conductors may be spliced or tapped without a splice box, provided the splice uses methods identified for the purpose.
• 300.5(F) Backfill — backfill may not contain large rocks, paving pieces, or corrosive material that could damage the wiring; granular fill or protective boards are used where needed.
• 300.5(I) — all conductors of the same circuit, including the grounded conductor and any equipment grounding conductors, must be in the same raceway or trench.
• 300.5(J) — where subject to earth movement, direct-buried conductors must be arranged with S loops or expansion fittings to relieve strain.
• 300.5(K) — a bushing or terminal fitting is required where a direct-buried cable enters a raceway from the earth.