12.3 Busways & Cable Tray Systems (Articles 368 & 392)
Key Takeaways
Busways must be supported at intervals not exceeding 5 ft unless otherwise designed and marked (368.30).
Overcurrent protection is required where a busway is reduced in ampacity, except in industrial establishments where the smaller busway is not over 50 ft long and has an ampacity of at least one-third of the upstream OCPD rating (368.17(B)).
A vertical busway passing through a dry floor must be totally enclosed (unventilated) where it passes through and for at least 6 ft above the floor, and ventilated risers need a curb around the floor opening to keep liquids out (368.10).
Cable trays may not be installed in hoistways or where subject to severe physical damage (392.12).
The 2026 NEC 392.18(F) requires at least 12 in. of access space above a cable tray, with four exceptions.
Busways (Article 368)
A busway is a factory-assembled raceway containing busbars, used as a feeder or to distribute power through plug-in units to machines in industrial plants.
Uses and installation
| Rule | Requirement |
|---|---|
| Uses permitted (368.10) | Located in the open and visible; behind access panels if the busway is totally enclosed and nonventilated with accessible joints, and either the space is not used for air handling or, if it is, the busway has no plug-in provisions and its conductors are insulated |
| Uses not permitted (368.12) | Where subject to severe physical damage or corrosive vapors; in hoistways; in hazardous locations unless specifically approved; outdoors or in wet or damp locations unless identified for the use |
| Through walls | Permitted through dry walls in unbroken lengths |
| Through floors | Through dry floors and platforms only if totally enclosed (unventilated) where it passes through and for at least 6 ft above the floor; riser busways with ventilating openings need a curb around the floor opening, within 12 in. of it, to keep liquids out |
| Support (368.30) | At intervals not exceeding 5 ft, unless otherwise designed and marked |
| Dead ends (368.58) | Closed |
| Grounding (368.60) | Busway enclosures are connected to an EGC as required by Article 250 |
Overcurrent protection (368.17)
- Rating: the OCPD protecting a busway is selected using the busway ampacity, under 240.4.
- Reduction in size (368.17(B)): overcurrent protection is required where a busway is reduced in ampacity. The exception applies only in industrial establishments: a smaller busway may continue without additional protection if it is not over 50 ft long and its ampacity is at least one-third of the rating or setting of the OCPD next back on the line, and it is free from contact with combustible material.
- Branches (368.17(C) and 368.56): where a busway is used as a feeder, plug-in devices or connections that tap off feeders or branch circuits must contain the required OCPD for those circuits, with limited exceptions such as cord-and-plug drops and fixed or semifixed luminaires within specific limits.
Busway feeders and plug-in devices must have interrupting ratings and short-circuit current ratings at least equal to the available fault current, which can be high near large transformers (110.9 and 110.10).
Cable trays (Article 392)
A cable tray system is a unit or assembly of units forming a rigid structural system used to securely fasten or support cables and raceways. It is not a raceway.
What may be installed (392.10)
Table 392.10(A) lists the wiring methods permitted in trays, including Types AC, MC, MI, MV, TC, ITC, PLTC, UF, USE, SE, NM (where permitted), power-limited and fire alarm cables, optical fiber cables, and raceways. In industrial establishments where conditions of maintenance and supervision ensure only qualified persons service the system, single-conductor cables 1/0 AWG and larger may be installed in ladder or ventilated trough trays, with smaller single conductors permitted in specific cases. Cable trays are also permitted in hazardous locations with the wiring methods allowed there, and nonmetallic trays in corrosive areas.
Not permitted (392.12): in hoistways or where subject to severe physical damage.
Installation
| Rule | Requirement |
|---|---|
| Complete system (392.18(A)) | Installed as a complete system; field bends or modifications must keep electrical continuity and support |
| Covers | Permitted where protection is needed; solid unventilated covers affect ampacity |
| Access (392.18(F), new in 2026) | At least 12 in. of access space above the tray, with four exceptions |
| Through partitions and walls | Permitted, with penetrations firestopped as required for fire-rated assemblies |
| Securing cables (392.30) | Cables in other than horizontal runs must be securely fastened to transverse members |
| Over 1000 V | Cables rated over 1000 V and those rated 1000 V or less may not share a tray unless separated by a solid fixed barrier or the higher-voltage cables are Type MC |
Cable fill and ampacity (392.22 and 392.80)
- Multiconductor cables 4/0 AWG and larger in ladder or ventilated trough trays: the sum of the cable diameters may not exceed the tray's inside width, and the cables are installed in a single layer.
- Multiconductor cables smaller than 4/0 AWG: the total cross-sectional area may not exceed the allowable fill area in Table 392.22(A) for the tray width.
- Ampacity of multiconductor cables: the Table 310.15(C)(1) adjustment factors apply only to the number of current-carrying conductors within each cable, not to the total number of conductors in the tray. Where a tray is covered for more than 6 ft with solid unventilated covers, ampacity is reduced to 95%.
- Single-conductor cables have their own ampacity rules in 392.80(A)(2), based on free-air ampacities with reduction factors for uncovered and covered trays.
Cable tray as an equipment grounding conductor (392.60)
Metal cable trays may serve as EGCs where continuous maintenance and supervision ensure qualified persons service the system, the tray sections and fittings are identified for grounding, the tray has the cross-sectional area required by Table 392.60(A) for the largest OCPD of the circuits in the tray, and all sections are bonded with listed connectors or bonding jumpers. Otherwise, a separate EGC is run in or with the tray.
Worked example: a busway tap
An industrial plant has a 1600 A busway protected by a 1600 A breaker. A 600 A busway section continues 40 ft without its own OCPD.
- Is it industrial? Yes.
- Is the smaller busway 50 ft or less? Yes, 40 ft.
- Is its ampacity at least one-third of the upstream OCPD? A, and 600 A is larger, so yes.
The reduced busway may continue without additional overcurrent protection. In a commercial office building, the exception would not apply, and an OCPD would be required at the size reduction.
What is the maximum support interval for a busway that is not otherwise designed and marked?
3 ft
10 ft
5 ft
15 ft
In an industrial plant, a 2000 A busway protected at 2000 A is reduced to a smaller busway without additional overcurrent protection. Which smaller busway meets 368.17(B)?
A 700 A busway, 45 ft long
An 800 A busway, 60 ft long
A 600 A busway, 30 ft long
A 500 A busway, 20 ft long
Where are cable trays prohibited under 392.12?
In industrial establishments without qualified staff
In any hazardous (classified) location
Above suspended ceilings in offices
In hoistways or where subject to severe damage
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