2026-08-11 · Technical Guide
Under the mainstream safety standards — NEC in North America and IEC internationally — selecting the correct inline fuse for a PV string comes down to one core formula:
Minimum fuse current rating >= module short-circuit current (Isc) x 1.56
The 1.56 factor is the product of two 1.25 coefficients. It exists because a PV system generates continuously and because irradiance enhancement (cloud edge effect, strong reflection) can briefly raise string current above nameplate Isc.
Worked example
Take a module with an Isc of 10A:
Apply the formula: 10A x 1.56 = 15.6A — this is the calculated minimum rating.
Pick the next standard size above it: a 16A or 20A DC fuse. Common gPV ratings are 6A, 8A, 10A, 12A, 15A, 16A, 20A, 25A and 30A.
Remember the basis: 1.56 = 1.25 (continuous current factor) x 1.25 (overcurrent device factor). The margin prevents nuisance operation on a system that runs all day.
Three golden rules
Never exceed the ceiling. The selected rating must never exceed the module datasheet value for maximum series fuse rating — that number is the module's own safe limit.
Use a DC-dedicated fuse. AC fuses cannot extinguish a DC arc and are a serious fire hazard. Only use fuses certified to IEC 60269-6 (gPV) or UL 2579.
Match the voltage. The fuse DC voltage rating must be greater than or equal to the array maximum open-circuit voltage (Voc) corrected for the lowest expected ambient temperature.
When is a string fuse actually required?
One or two strings in parallel: in most cases the conductors and connectors can carry the fault current, so per-string fuses are usually not required.
Three or more strings in parallel: fuses are mandatory. When one string faults, every healthy string back-feeds the fault point. Unprotected, that combined reverse current easily burns equipment or starts a fire.
Selection checklist
Core data needed: module Isc, maximum series fuse rating, system Voc max.
Current calculation: 1.56 x Isc = minimum required rating.
Rating choice: equal to or slightly above the calculated value, and never above the module maximum series fuse rating.
Voltage match: fuse DC rating >= array Voc at the lowest site temperature.
Type: a PV-dedicated DC fuse — gPV per IEC 60269-6 or a UL 2579 listed type.
Sizing a string fuse is about following the standard, not estimating from experience. Calculate 1.56 x Isc, run the checklist, and the string is safe, compliant and reliable.
