PV Source-Circuit Fuse Sizing (NEC 690.9)
PV source-circuit overcurrent per NEC 690.9.
Example
You enter
- Module Isc (A) 10
- Module max series fuse (A, from label) 20
- Paralleled source circuits 4
You get
- Minimum OCPD (1.56 x Isc) 15.63 A
- Selected standard fuse 20 A
Checked against published worked examples from: NABCEP.
Details, formula, and sources
ScopeFuse >= 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 or tablereq = 1.25 x 1.25 x Isc (1.5625, not rounded); fuse = smallest NEC 240.6(A) standard rating >= req; compliant if fuse <= module max; fuses required when >= 3 paralleled source circuits.
Source and editionNEC 690.9 (PV source-circuit overcurrent protection) and 240.6(A) standard ratings, by name.
Free accessNEC 690.9 and 240.6 are in NFPA 70; many jurisdictions publish the code free to read. The NEC and the AHJ govern.
What governsEstimate. AHJ and licensed professional govern.
API field namesIsc_A, max_fuse_A, n_strings, req_A, fuse_A
- 1.56 x Isc OCPD >= 1.25 x 1.25 x Isc, rounded up to a 240.6(A) standard ratingNEC 690.9(B)
- Module maximum the selected fuse must not exceed the module label's maximum series fusemodule label / NEC 690.9(B)
- Three-string threshold fuses required only with 3+ paralleled source circuits (690.9(A))NEC 690.9(A)