Quick Hits

EPISODE 13 — Fire Spread or Plenums 1kva , Access Panels & Exit Enclosures (305.31, 305.32, 305.33, 305.35)

Slide 1 — Hook "Today: firestopping, wiring in air-handling ducts/plenums (a shorter, stricter list than Article 300's), access panels, and stairwell exit enclosures."

Slide 2 — 305.31 & 305.32(A)–(B)

• 305.31: same firestopping principle as 300.23 — installations in hollow spaces/shafts/ducts can't substantially increase fire/combustion spread; penetrations through fire-rated assemblies must be firestopped to maintain the rating.

• 305.32(A): dust/loose stock/flammable vapor ducts, and vapor-removal or commercial-cooking-ventilation ducts/shafts: NO wiring of any type, period.

• 305.32(B): ducts specifically fabricated for environmental air: equipment/wiring only if necessary for direct action on/sensing of the air; approved methods limited to Type MI cable, Type MC cable (smooth/corrugated impervious sheath), EMT, flexible metallic tubing, IMC, or RMC — flexible metal conduit up to 4 ft allowed just to connect adjustable equipment.

Slide 3 — 305.32(C), 305.33, 305.35

• 305.32(C) Other plenum spaces: notably STRICTER than Article 300's equivalent — approved wiring methods are limited to just totally enclosed nonventilated insulated busway (no plug-ins) OR Type MC cable (no nonmetallic covering) — a much shorter list than the 11 methods allowed in low-voltage plenums. Nonmetallic cable ties need the same low smoke/heat release listing. Metal cable trays can support these methods; solid-covered trays can enclose other cable types per 392.10.

• 305.33: raceways/equipment behind access panels (including suspended ceiling panels) must stay arranged/secured so panels can be removed and equipment accessed.

• 305.35: in fire-resistance-rated exit enclosures (stair towers), only wiring serving AHJ-approved equipment is allowed inside — this version doesn't carry the low-voltage egress-lighting exception.

Slide 4 — Quiz + CTA Q: "In 'other spaces used for environmental air' (plenums), Article 305 limits wiring methods to which of these?" A) The same 11 methods Article 300 allows B) Totally enclosed nonventilated insulated busway or Type MC cable only C) Any listed cable type D) NM cable only

✅ Answer: B — 305.32(C)(1): a much narrower list than Article 300's plenum wiring methods.

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