Short answer: Wiring above a suspended ceiling is supported by its own support wires or the building structure, not by the ceiling grid or the wires that hold the grid (NEC 300.13). Openings made for wiring through fire-rated walls, floors and ceilings are firestopped with approved methods so the fire rating is kept (NEC 300.23). Ducts used for dust, loose stock or vapor removal get no wiring of any kind. Ducts fabricated to carry environmental air allow only a short list of metal wiring methods, with flexible metal conduit limited to 4 ft. Other air-handling spaces, such as the space above a ceiling used as a return plenum, allow a longer list that still excludes nonmetallic cables (NEC 300.25). In the 2026 edition these sections were renumbered, so 300.22 in older books is now 300.25.
PSI lists Wiring Methods and Materials (10 scored NEC Knowledge and 2 scored Calculations questions) as a subject area of the Texas Journeyman exam. PSI does not name 300.25 or say how many questions cover plenums or firestopping. The Texas Journeyman Practice Pack has 1,000 original questions and 10 timed mock exams in the real two-part format, with step-by-step solutions for every calculation. Try free practice questions without signing up. Get the full pack for US$39 with one payment.
Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC, and follow your supervisor and the local inspector.
Where this fits
The raceway and conduit fill page covers burial depth, protection from damage and how many conductors fit. This page covers the Article 300 rules that decide what wiring method is even allowed in a commercial ceiling: support, fire, and air. The 2026 edition renumbered them: support above ceilings moved from 300.11 to 300.13, and ducts and plenums from 300.22 to 300.25. Older study guides use the old numbers. Low-voltage and fire alarm cables in plenums have their own listings and are on the low-voltage and fire alarm circuits page.
300.13: support above suspended ceilings
- Secured in place. Raceways, cable assemblies, boxes, cabinets and fittings are securely fastened in place.
- Not the ceiling grid. Raceways and cables above a suspended ceiling are generally not secured to or supported by the ceiling grid or its support wires. Use independent support wires or the building structure, and check the section’s narrow exceptions for ceiling systems identified for support.
- Independent support wires. Support wires added only for the wiring, in addition to the ceiling grid wires, are permitted. They are secured at both ends, provide secure support, and are distinguishable from the ceiling support wires by color, tagging or other effective means.
- Raceways as support. A raceway is not used to support other raceways, cables or equipment unless one of the section’s conditions applies, such as a raceway identified for that support, or Class 2 or Class 3 cables used to control the equipment the raceway supplies.
300.22: induced currents in metal enclosures
- Group the circuit. In metal enclosures or raceways, all phase conductors, the grounded conductor where used and the equipment grounding conductors of a circuit are grouped together. Running one phase alone through a steel raceway lets the alternating magnetic field heat the steel.
- Single conductors through metal. Where single conductors of a circuit pass through a ferrous metal plate, the effect is reduced by cutting a slot between the individual holes or by passing all the conductors through one opening in an insulating wall.
300.23: spread of fire or products of combustion
Wiring is installed so it does not substantially increase the possible spread of fire or products of combustion. Openings around electrical penetrations into or through fire-resistant-rated walls, partitions, floors or ceilings are firestopped using approved methods that maintain the fire resistance rating. The method is one the authority having jurisdiction approves. Where the building code or the approved assembly details call for it, that means a listed penetration firestop system installed as tested.
300.25: ducts, plenums and other air-handling spaces
| Space | Wiring allowed |
|---|---|
| (A) Ducts for dust, loose stock or vapor removal | None of any type |
| (B) Ducts specifically fabricated for environmental air | Qualifying Type MI and Type MC cables (impervious metal sheath, no nonmetallic covering), EMT, flexible metallic tubing, IMC and RMC without a nonmetallic covering. Flexible metal conduit up to 4 ft to connect physically adjustable equipment and devices, with connectors that close the openings. Equipment only if needed to act directly on or sense the air. Check the section’s conditions and exceptions. |
| (C) Other spaces used for environmental air, such as above a ceiling used as a return plenum | Qualifying Type MI, Type MC and Type AC cables without a nonmetallic covering, factory-assembled control or power cable specifically listed for air-handling spaces, and other conductors inside EMT, IMC, RMC without a nonmetallic covering, flexible metal conduit, flexible metallic tubing, and where accessible, metal surface raceway or metal wireway with metal covers. Check the section for the other permitted methods and the conditions on each. Nonmetallic cable ties are listed for low smoke and heat release. |
| (D) Information technology rooms | Follow Article 645 |
Dwelling exception. The space between studs or joists in a dwelling, used for air return, may contain wiring that passes through it perpendicular to the long dimension. Check the exact conditions in your code book.
Worked example: a return-air plenum ceiling
An office has a suspended ceiling, and the space above it is used as a return-air plenum. Pick yes or no for each item.
| Item | Allowed? | Why |
|---|---|---|
| Type MC cable without an overall nonmetallic covering | Yes, if the cable meets the 300.25(C) conditions | Confirm the cable construction |
| Type NM cable | No | Not on the list, and nonmetallic sheathed |
| EMT home run | Yes | Listed raceway |
| 6 ft flexible metal conduit whip to a lay-in luminaire | Yes | FMC is allowed in (C) spaces; the 4 ft limit applies inside fabricated ducts (B) |
| MC cable hanging from the grid wires | No | Independent support wires or the structure, not the grid wires |
| A 3 ft FMC connection to a damper motor inside a supply duct | Yes | Within 4 ft, for adjustable equipment acting on the air |
Panels designed to allow access
Cables, raceways and equipment installed behind panels designed to allow access, including suspended ceiling panels, are arranged and secured so the panels can be removed and the area behind them reached.
Check yourself
- Can a raceway be supported by the ceiling grid wires? Generally no. Use independent support wires or the structure.
- What wiring is allowed in a dust removal duct? None.
- Maximum length of flexible metal conduit in a duct fabricated for environmental air? 4 ft.
- What keeps a fire rating at a penetration? An approved firestop method.
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Mark (A), (B) and (C) with three colors
NFPA 70, National Electrical Code, 2026 Edition (softcover). Highlight 300.25(A) “none”, (B) “metal only, FMC 4 ft” and (C) “plenum ceiling list” in three colors so you match the space to the list fast. The Texas exam is open book. PSI allows a soft-bound NEC with highlighting, underlining and notes made in advance. Loose-leaf, spiral-bound and ring-bound copies are not allowed, and only permanent index tabs made or supplied by the NEC publisher are permitted; homemade and third-party tabs are not.
FAQ
Can NM cable be run in a plenum ceiling?
No. Type NM cable is not on the list of wiring methods for spaces used for environmental air.
Can wiring be supported from ceiling grid wires?
Generally no. Wiring uses independent support wires or the building structure; check the narrow exceptions for ceiling systems identified for support.
How long can flexible metal conduit be inside an air duct?
Not more than 4 ft, and only to connect physically adjustable equipment and devices in a duct fabricated for environmental air.
What is required where a conduit passes through a fire-rated wall?
The opening around the penetration is firestopped using an approved method that keeps the wall’s fire resistance rating.
Why are all conductors of a circuit run in the same metal raceway?
So the magnetic fields cancel and the metal is not heated by induced currents.
Sources checked October 2026
- PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- NFPA 70, National Electrical Code, 2026 Edition, Article 300 Part I
- Open Exam Prep, Article 300 study guide (2026 numbering: 300.13 formerly 300.11, 300.25 formerly 300.22): https://open-exam-prep.com/study-guides/mn-electrician/ch11-wiring-methods-general/wiring-support-plenums-fire
