How to size PV combiner box fuses
String fuses in a PV combiner exist to interrupt the backfeed current that healthy strings push into a faulted one. Rate them too low and they open on a bright cold morning; rate them too high and they never protect the module or the home run conductor, so the NEC fixes both ends of the window.
NEC 690.8(A)(1) puts the maximum circuit current at 125% of Isc, and 690.8(B) requires the overcurrent device to be sized for 125% of that continuous current. Multiplied out, the fuse must be at least 156% of Isc, rounded up to a standard rating from 240.6(A). NEC 690.9(B) then caps it at the module maximum series fuse rating printed on the datasheet.
Take a module with 10 A Isc in eight parallel strings and a 20 A maximum series fuse. The minimum is 10 × 1.56 = 15.6 A, which rounds up to a 20 A standard fuse, and 20 A is exactly at the module limit, so it passes. Combiner output is 8 × 10 × 1.25 = 100 A of continuous current, and applying 125% again gives 125 A for the output device. If the required fuse ever lands above the module limit, the tool flags it and the string layout has to change.
Values reflect NEC 2023 practice as of 2026. Conductor ampacity, rooftop temperature adders under 310.15, fuse holder derating and whether the inverter already provides backfeed protection all need their own review. Have a licensed electrical engineer and the authority having jurisdiction confirm the final design.
Frequently asked questions
NEC 690.8(A)(1) sets the maximum circuit current at 125% of rated short circuit current to cover irradiance above test conditions, and 690.8(B) then requires the overcurrent device and conductor to carry 125% of that continuous current. The two factors multiply to 156% of Isc.
Often not. With two parallel source circuits the backfeed a faulted string can see is limited to the output of the single remaining string, which is usually below the module series fuse rating. Confirm against the module datasheet and NEC 690.9(A) before leaving fuses out.