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PV Circuit Maximum Current and Ampacity (NEC 690.8, the 156% Rule)
PV circuit maximum current and the minimum conductor ampacity it requires.
Example
You enter
- Module Isc per source circuit (A) 10
- Parallel source circuits (strings) 2
You get
- Maximum current (690.8(A)) 25
- Min conductor ampacity (690.8(B)) 31.25 A
- Stacked factor on Isc 1.5625 (156%)
Checked against published worked examples from: NEC 2023 (NFPA 70).
Details, formula, and sources
ScopeSizes the PV source/output-circuit maximum current = module Isc x paralleled strings x 125% (690.8(A)(1)) and the minimum conductor ampacity = that maximum x another 125% (690.8(B)(1)), the two factors stacking to 156% of Isc, before any conditions-of-use derate. The conductor must also satisfy 690.8(B)(2) and the greater governs; the engineer governs.
Formula or tablemax_current = Isc x strings x 1.25 (690.8(A)(1)); min_ampacity = max_current x 1.25 (690.8(B)(1)); the factors stack to 1.5625 (156%) of Isc, before conditions-of-use derate; also size for 690.8(B)(2) and take the greater.
Source and editionPV source/output-circuit current and conductor ampacity, NEC 2023 690.8(A) and 690.8(B), by name.
Free accessNEC is free to read at nfpa.org/freeaccess. The two 125% factors stack to 156% of Isc; the conductor must also satisfy 690.8(B)(2) after temperature/conduit conditions of use, and the greater of (B)(1) and (B)(2) governs.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesmodule_isc_a, parallel_strings, max_current_a, min_ampacity_a, stacked_factor
- Maximum current rated Isc times the number of paralleled source circuits times 125%NEC 690.8(A)(1)
- Minimum ampacity the maximum current times another 125% before any conditions-of-use adjustmentNEC 690.8(B)(1)