OCPD (Overcurrent Protection Device)

NEC 690.9 Sizing

OCPD_minimum = 1.25 × Isc (rounded up to standard)

Standard OCPD Ratings

15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400 A.

Why the 1.25× Factor

The 125% multiplier in NEC 690.9 is not an arbitrary safety margin — it accounts for two effects that stack on top of each other. First, a module’s short-circuit current under real-world high-irradiance conditions can run above its STC nameplate Isc, so a fuse sized to the exact datasheet value would be prone to nuisance trips on a clear, cold, high-sun day. Second, NEC 690.8 already limits continuous-duty conductors to 80% of their rated ampacity, and 1 ÷ 0.8 works out to the same 1.25 factor from the wire side of the calculation. Sizing the OCPD at 1.25 × Isc keeps the fuse or breaker comfortably above expected operating current while still tripping fast enough to protect the conductor if a real fault develops.

Worked Example

A module datasheet lists Isc = 10.8 A. Applying NEC 690.9: 1.25 × 10.8 A = 13.5 A. Since 13.5 A isn’t a standard rating, the design rounds up to the next standard OCPD size — 15 A. If two or more strings are paralleled into a combiner box ahead of that point, each string typically needs its own fuse at this same rating, since parallel strings can otherwise backfeed fault current into one another during a short circuit.

Key Takeaways

• OCPD = fuse or breaker, NEC 690.9 sized at 1.25 × Isc.
• Source-circuit: typically gPV fuse in combiner.
• Inverter output: AC breaker.
• Each parallel string > 2 needs individual fuse.
• Round up to standard rating.

Related Reading

OCPD sizing is one line item in a much larger DC electrical package — the fuse or breaker rating, its placement in the combiner box, and its relationship to conductor ampacity all need to show up correctly on the drawing set, which is exactly what our Electrical CEIG Drawings service documents for permit review, alongside the Solar SLD software workflows EPCs increasingly use to auto-generate that same NEC 2023 protection scheme. Overcurrent protection is closely tied to how conductors and strings are sized in the first place; QbitsEnergy’s guide to string sizing and overcurrent protection walks through that upstream calculation in more depth. For the other DC protection device that gets specified alongside OCPD on the same circuits, see Arc Fault — the two are sized and tested independently, but both end up on the same combiner-box schedule.