Quick Hits

Episode 12 — Transformer Secondary Conductor Protection

Covers: 240.21(C)

Hook: Transformer secondaries get their own whole set of tap rules, separate from feeder taps — here's the rundown.

Slide 2: The round-up rule never applies to transformer secondary conductors. Protection by the primary OCPD only works for single-phase 2-wire or delta-delta 3-wire secondaries, sized against the voltage ratio. For secondaries up to 10 ft: ampacity has to cover the load and match the downstream OCPD, stay within the enclosure they feed (or meet a 1/10-of-primary-OCPD ratio times the voltage ratio if they leave it), and run in raceway.

Slide 3: Industrial-only installations get a 25-ft secondary tap allowance for switchgear/switchboards under qualified supervision, where ampacity meets the transformer's secondary rating and combined downstream OCPDs don't exceed it, all OCPDs are grouped, and conductors are protected from damage. Outdoor secondary conductors follow the same unlimited-length outdoor tap rules as feeder taps. And the general 25-ft secondary rule requires ampacity at least 1/3 of the primary OCPD times the voltage ratio, single-OCPD termination, and physical protection.

Quiz: Does the "round up to next standard OCPD size" rule ever apply to transformer secondary conductors?
Answer: No — never permitted for secondary conductors.

CTA: Follow — services, busways, motors, generators, and batteries wrap up 240.21 next.

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