Quick Hits

Episode 9 — Ungrounded Conductors and Interrupting Ratings

Covers: 240.15, 240.16

Hook: Every hot conductor needs its own overcurrent device — but can you use single-pole breakers tied together instead of a true multipole? Sometimes.

Slide 2: A fuse or breaker trip unit has to sit in series with every ungrounded conductor. Breakers generally have to open ALL ungrounded conductors of a circuit together — but handle-tied single-pole breakers are allowed for multiwire branch circuits on line-to-neutral loads, for 120/240V line-to-line loads on grounded single-phase systems, for certain 3-phase/2-phase systems with a grounded neutral at ≤120V to ground, and for 125/250V 3-wire DC circuits under similar grounded conditions.

Slide 3: All OCPDs listed under 240.7 need a minimum interrupting rating of 5000A — meaning they have to be able to safely clear at least that much fault current without failing catastrophically.

Quiz: What's the minimum interrupting rating required for OCPDs under 240.16?
Answer: 5000 amperes.

CTA: Follow — Part II starts next: where OCPDs actually have to be located in the circuit.

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