PV Source-Circuit Fuse Sizing (NEC 690.9)
PV source-circuit overcurrent per NEC 690.9: fuse >= 1.56 x Isc, rounded up to a 240.6(A) standard rating and checked against the module label maximum; fuses required with 3+ paralleled strings. Isc 10 A -> a 20 A fuse; a 25 A requirement on a 20 A module max is non-compliant. The NEC and AHJ govern.
Formula and source
req = 1.56 x Isc (1.25 x 1.25); fuse = smallest NEC 240.6(A) standard rating >= req; compliant if fuse <= module max; fuses required when >= 3 paralleled source circuits.
NEC 690.9 (PV source-circuit overcurrent protection) and 240.6(A) standard ratings, by name.
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