Code reference · NEC 690.8(A)(1), 690.8(B); 240.6(A)
NEC 690.8: sizing PV circuits (the 125% × 125% chain)
Two separate 125% factors stack: maximum circuit current = 125% of Isc (690.8(A)(1) — solar can exceed STC irradiance), then conductors/OCPD at 125% of that for continuous duty (690.8(B)). Net: 156.25% of Isc for simple cases.
Example: a string with Isc 10 A → 12.5 A maximum circuit current → 15.63 A conductor minimum → next standard OCPD 20 A. Two parallel 9.6 A strings → 24 A → 30 A conductor → 30 A OCPD.
Rejections cluster here: NEC 690.8 sizing errors are among the most common solar permit correction items — run the numbers before submitting, not after the rejection letter.
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More references: NEC Table 310.16 ampacity, explained · The NEC 705.12 120% rule, actually explained · What wire size for a 100 A sub panel? · NEC 625: EV charger circuit sizing · NEC 706: battery energy storage disconnects · NEC 240.4(D): the small-conductor overcurrent caps · all 97 references