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EPISODE 14 — Fire Spread, Bend Limits & Wiring in Air-Handling Ducts (300.23, 300.24, 300.25(A)–(B))
EPISODE 14 — Fire Spread, Bend Limits & Wiring in Air-Handling Ducts (300.23, 300.24, 300.25(A)–(B))
Slide 1 — Hook "Today: firestopping penetrations, the 360° bend rule everyone learns early, and the start of the plenum/duct wiring rules."
Slide 2 — 300.23 & 300.24
300.23: electrical installations in hollow spaces/vertical shafts/air-handling ducts must be arranged so they don't substantially increase the spread of fire or combustion products. Penetrations through fire-resistance-rated walls/partitions/floors/ceilings must be FIRESTOPPED using approved methods that maintain the original fire rating.
300.24: the TOTAL degree of bends in any combination of conduit/tubing for a single run can't exceed 360° between pull points — the classic "four 90s max" rule (though it's really about cumulative degrees, not just counting 90° bends).
Slide 3 — 300.25(A) & (B) — ducts
(A) Ducts for dust/loose stock/flammable vapor removal, and any duct/shaft used for vapor removal or commercial cooking equipment ventilation: NO wiring system of ANY type is allowed in these.
(B) Ducts specifically FABRICATED for environmental air: equipment/wiring only permitted if necessary for directly acting on or sensing the contained air; enclosed gasketed luminaires allowed where illumination is needed for maintenance. Approved wiring methods are limited to: Type MI cable (no nonmetallic covering), Type MC cable with smooth/corrugated impervious metal sheath (no nonmetallic covering), EMT, flexible metallic tubing, IMC, or RMC (no nonmetallic covering) — flexible metal conduit up to 4 ft is allowed just to connect adjustable equipment, with connectors that close any gaps.
Slide 4 — Quiz + CTA Q: "What's the maximum total degrees of bend permitted in a conduit run between pull points?" A) 180° B) 270° C) 360° D) 450°
✅ Answer: C — 300.24: 360° total, across however many individual bends make up that total.
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⚠️ Educational NoticeQuick Hits are educational explanations based on my understanding of the NEC. Always verify requirements in the current adopted NEC, applicable local amendments, manufacturer instructions, and with the AHJ.
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