Quick Hits

EPISODE 10 — Disconnect Rating and Conductor Sizing over 1kva (267.39, 267.40)

Slide 1 — Hook "Quick episode — how the disconnect's rating and the feeder/branch-circuit conductors themselves get sized."

Slide 2 — 267.39 Rating of the disconnect

• Transformers only: disconnect rating ≥ sum of the transformers' nameplate ratings.

• Transformers + utilization equipment: disconnect rating ≥ transformer nameplate sum + 100% of the utilization equipment's simultaneous designed load.

• Supervised installations: qualified persons under engineering supervision can determine the rating — but it still can't be less than the size of the feeder/branch circuit supplying that disconnect.

Slide 3 — 267.40 Conductor sizing

• This section doesn't reinvent the wheel — it just points you to the right place: branch-circuit conductor sizing follows 265.19 (the medium-voltage branch circuits article), and feeder conductor sizing follows 266.4 (the medium-voltage feeders article).

Slide 4 — Quiz + CTA Q: "Under a supervised installation, can the disconnect rating ever be smaller than the feeder/branch circuit it's connected to?" A) Yes, if an engineer approves it B) No — it can never be smaller than the supplying feeder/branch circuit C) Only for transformer-only loads D) There's no minimum in supervised installations

✅ Answer: B — 267.39(C): even under engineering supervision, the rating floor is the feeder/branch-circuit size.

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