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EPISODE 5 — Boxes or Fittings and Ampacity for Armored Cable — Part II Finale (320.40, 320.80)
EPISODE 5 — Boxes or Fittings and Ampacity for Armored Cable — Part II Finale (320.40, 320.80)
Slide 1 — Hook "Closing out Part II — the anti-short bushing everyone calls the 'red head,' and how ampacity gets calculated for Type AC cable."
Slide 2 — 320.40 Boxes and fittings — the anti-short bushing
Everywhere the armor terminates, a fitting must protect the wires from abrasion (unless the box/fitting design already provides equivalent protection) — PLUS an insulating bushing (or equivalent) between the conductors and the armor.
That insulating bushing — commonly nicknamed the "red head" — must remain VISIBLE for inspection once the connector/clamp is installed, by design.
Where transitioning from Type AC to a different cable/raceway method, a box/fitting/conduit body is required at that junction point per 300.17.
The internal bonding strip (from 320.100, coming up next episode) often helps physically hold that anti-short bushing in place during rough wiring.
Slide 3 — 320.80 Ampacity
Base ampacity determination follows 310.14 — same framework you learned in Article 310.
(A)(1) Thermal insulation: Type AC installed IN thermal insulation needs 90°C-rated conductors, but the actual USABLE ampacity is capped at what a 60°C-rated conductor would carry — the 90°C rating can still be used for adjustment/correction MATH, just not to exceed that 60°C ceiling.
(A)(2) Where MORE than two Type AC cables (each with 2+ current-carrying conductors) contact thermal insulation/caulk/sealing foam WITHOUT maintained spacing, apply the bundling adjustment from Table 310.15(C)(1) — this specifically overrides the bundled-cable exception you might remember from Article 310's adjustment rules.
(B) Cable tray: ampacity for Type AC in cable tray follows 392.80(A) instead.
Slide 4 — Quiz + CTA Q: "Type AC cable installed in thermal insulation must have conductors rated at 90°C, but the usable ampacity is capped at what rating's equivalent?" A) 60°C B) 75°C C) 90°C — no cap applies D) 105°C
✅ Answer: A — 320.80(A)(1): capped at the 60°C-rated ampacity, even though 90°C conductors are required.
CTA: Follow for more daily quick-hit study notes 📚⚡ — Part II complete! Part III (Construction Specifications) closes out the article.
⚠️ 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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