Quick Hits

EPISODE 15 — Plenums, Access Panels & Exit Enclosures — Article 300 FINALE (300.25(C)–(D), 300.26, 300.27)

Slide 1 — Hook "Final episode of Article 300! Wrapping up with plenum spaces, access-panel wiring, and stairwell exit enclosures."

Slide 2 — 300.25(C) Other spaces used for environmental air (plenums)

• Applies to spaces NOT specifically built for air handling but used that way anyway — classic example: the space above a hung ceiling used to return air. Doesn't apply to habitable rooms/areas whose primary purpose isn't air handling, or to dwelling unit joist/stud spaces where wiring runs perpendicular through them.

• Approved wiring methods (a longer list than fabricated ducts): totally enclosed nonventilated insulated busway (no plug-ins), RMC, IMC, EMT, flexible metallic tubing, flexible metal conduit, Type MI cable, Type MC cable, Type AC cable, listed flexible bus systems, and other specifically listed factory-assembled control/power cable or metallic manufactured wiring systems — plus other cable/conductor/raceway types IF installed inside one of those listed methods, or in accessible surface metal raceway/metal wireway with metal covers.

• Nonmetallic cable ties/accessories used in these spaces must be listed for LOW smoke and heat release properties.

• Metal cable tray systems can support the approved wiring methods; solid-side/bottom metal trays with solid metal covers can enclose other cable types per 392.10(A)/(B).

• Equipment: metal-enclosed, OR nonmetallic-enclosure equipment specifically listed for air-handling spaces with low smoke/heat release properties, is permitted (integral fan systems need their own specific listing for the space).

Slide 3 — 300.25(D), 300.26, 300.27

• 300.25(D): information technology equipment wiring in raised-floor air-handling areas follows 645.6(E), where the installation meets 645.5's special requirements.

• 300.26: cables/raceways/equipment behind access panels (including suspended ceiling panels) must be arranged/secured so the panels can still be removed and the equipment stays accessible.

• 300.27: in fire-resistance-rated exit enclosures (stair towers), only wiring serving equipment the AHJ has approved for that enclosure is allowed inside — exception for egress lighting fixtures on exterior doorways off the enclosure, which can be supplied from inside it.

Slide 4 — Quiz + CTA Q: "Nonmetallic cable ties used to secure cables in an air-handling plenum space must be listed for what specific property?" A) High tensile strength only B) Low smoke and heat release properties C) UV resistance only D) No special listing is required

✅ Answer: B — 300.25(C)(1)(c): listed for low smoke and heat release properties in these spaces.

CTA: 🎉 That's a wrap on ALL of Article 300 — General Requirements for Wiring Methods and Materials! Follow for more daily quick-hit study notes as we move into Article 305 (the medium-voltage version) and then the cable-type articles next.

← View Chapter 3 Home

← Back To Article 300 Home

← Back To Quick Hits Home