Why is the multiplier 1.56?

NEC 690.8 requires the maximum circuit current to be 1.25 times Isc to account for irradiance above standard test conditions. NEC 690.9 then requires the overcurrent device to be at least 1.25 times that current, giving 1.25 times 1.25, which equals 1.56 times Isc.

When do I actually need string fuses?

Series overcurrent protection is required when there are enough parallel strings that the back-feed from the other strings into a faulted string can exceed the module maximum series fuse rating. With three or more parallel strings this usually applies, but always check the module rating against the available fault current.

What if 1.56 times Isc exceeds the module max series fuse?

Then the standard module cannot be protected by a series fuse in that configuration, or you must reduce the number of parallel strings. The fuse can never exceed the module maximum series fuse rating printed on the label, so the configuration must change instead.

Why size conductors at 1.56 times Isc too?

The source-circuit conductor must carry the maximum circuit current continuously. NEC requires the conductor ampacity, after temperature and conduit-fill derating, to be at least 1.56 times Isc, and also at least the overcurrent device rating, whichever is larger.

Does this replace a full design review?

No. This sizes the basic source-circuit fuse and conductor. A complete design must also address voltage drop, temperature-corrected conductor ampacity, terminal ratings, and the inverter or combiner listing. Treat the result as a starting point for the stamped design.

Solar PV String Fuse Sizing Calculator (NEC 690.9)

Get the string fuse wrong and you either nuisance-trip in full sun or fail to protect a faulted module from back-feed. This calculator applies the NEC 690.9 and 690.8 overcurrent rules: it takes module short-circuit current and the module’s maximum series fuse rating, then recommends a compliant string fuse and the minimum source-circuit conductor ampacity.

How it works

The chain of code multipliers produces the familiar 1.56 factor, then the result is rounded up to a standard fuse and checked against the module’s printed limit:

max circuit current = 1.25 × Isc            (690.8, irradiance margin)
min OCPD rating      = 1.25 × max current    (690.9, continuous duty)
                     = 1.25 × 1.25 × Isc = 1.56 × Isc
string fuse          = next standard ≥ 1.56 × Isc,  but ≤ module max series fuse
conductor ampacity   ≥ max(1.56 × Isc, fuse rating)  after derating

Standard fuse sizes used here are the common PV values: 10, 15, 20, 25, 30, 35, 40, 45, 50, 60 A. If the required fuse would exceed the module maximum series fuse, the configuration itself must change.

Example and notes

A module with Isc of 9.5 A needs at least 1.56 × 9.5 = 14.8 A, so the next standard fuse is 15 A. If the module datasheet lists a maximum series fuse of 20 A, the 15 A fuse is fine and the source-circuit conductor must carry at least 14.8 A after temperature derating. Always confirm the number of parallel strings: with two strings, back-feed is limited and series fuses may not be required at all, but three or more parallel strings almost always need them.