Nimesh Katariya An OCPD (overcurrent protective device) is a fuse or circuit breaker that opens a circuit when current exceeds a safe level, protecting conductors and equipment. Under NEC 2023 Section 690.9(B), a PV circuit OCPD is rated at least 125% of the maximum circuit current, itself 125% of Isc, so roughly 1.56 x Isc, rounded up to a standard size.
Standard OCPD sizes and the NEC rules for applying them to solar PV circuits are what this entry covers. It is for solar permit designers, electricians and engineers selecting fuses and breakers, and explains what an OCPD is, how NEC 690.9 sizes it for PV, and which fuse and breaker ratings are standard.
What is an OCPD in electrical systems?
An OCPD is any fuse or circuit breaker that protects a circuit from overloads and short circuits. The PV sizing rule and standard ratings follow.
How does NEC 690.9 size a PV OCPD?
I_max = 1.25 × Isc (NEC 690.8(A))
OCPD_minimum = 1.25 × I_max (NEC 690.9(B)) ≈ 1.56 × Isc, rounded up to a standard sizeWhat are the standard OCPD sizes?
NEC 240.6(A) standard ratings include 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 two 1.25 factors?
The two 125% multipliers account for two separate effects. First, NEC 690.8(A) sets maximum circuit current at 1.25 × Isc because real-world irradiance can push current above the STC nameplate value.
Second, NEC 690.9(B) treats PV current as continuous, and standard fuses and breakers are only rated to carry 80% of their rating continuously, so the device is sized at 125% of that maximum current. The same logic applies to conductor ampacity. The result keeps the device above operating current while still protecting the conductor.
Worked Example
A module datasheet lists Isc = 10.8 A. Maximum current is 1.25 × 10.8 A = 13.5 A, and the OCPD minimum is 1.25 × 13.5 A ≈ 16.9 A, so the design rounds up to a 20 A fuse. The fuse must also not exceed the module’s maximum series fuse rating on the datasheet.
Where three or more strings are paralleled in a combiner box, each string typically gets its own fuse, because the other strings can backfeed fault current into a faulted one.
Key Takeaways
- OCPD = fuse or breaker, NEC 690.9 sized at 1.25 × 1.25 × Isc (about 1.56 × Isc) for PV circuits.
- Source-circuit: typically gPV fuse in combiner.
- Inverter output: AC breaker.
- With three or more parallel strings, each string typically needs its own fuse.
- Round up to a standard NEC 240.6(A) 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.
Disclosure: Qbits Energy is a Heaven Designs group company.
Frequently Asked Questions
4 commonly searched questions about OCPD (Overcurrent Protection Device).
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