Off-Grid Solar Calculators

Wiring and electrical safety

Solar Combiner Box Fuse Sizer

Size PV string and combiner fuses at 1.56 × Isc (NEC 690.8 stacked with 690.9). Three or more equal parallel strings typically need a fuse on every string, never above the module maximum series-fuse label. Two strings often skip fuses if the conductors are already 1.56 × Isc.

Inputs

A
A

Printed on the module label. The string fuse must not exceed this.

Results

String fuse
25.0A
Combiner output fuse
90.0A
String fuses typically required
Yes

Yes at three or more equal parallel strings in the usual 1.56 × Isc conductor case.

Recommended gear

Engineering methodology

NEC 690.8 and 690.9 stack two 125% factors: maximum circuit current is 1.25 × Isc, and the overcurrent device is at least 1.25 × that, which is 1.56 × Isc. String fuse = next standard size ≥ 1.56 × Isc, never above the module maximum series-fuse label. Combiner output fuse ≥ 1.56 × Isc × number of strings. String fuses are typically required at three or more equal parallel strings.

Why PV fuses are 1.56 × Isc

NEC 690.8 takes maximum circuit current as 1.25 × Isc. 690.9 then sizes the overcurrent device at least 125% of that continuous current. 1.25 × 1.25 = 1.56 × Isc. The next standard fuse must not exceed the module maximum series-fuse label.

Two strings vs three or more

Two equal strings often skip string fuses if each conductor is already 1.56 × Isc: backfeed is about 1.25 × Isc. Three strings can push about 2.5 × Isc into a fault, which exceeds typical module withstand, so string fuses are the usual case.

Frequently asked questions

Why 1.56 times Isc for PV fuses?

1.25 × Isc is the maximum circuit current under 690.8. Overcurrent devices for continuous PV current are then at least 125% of that value under 690.9, so 1.25 × 1.25 = 1.56 × Isc.

Do two parallel solar strings need fuses?

Often no, if each conductor is already sized to 1.56 × Isc: a faulted string sees about 1.25 × Isc from the other string, which modules and that wiring can usually withstand. Three or more strings can backfeed ~2.5 × Isc, so string fuses are the usual case.

Can I exceed the module “maximum series fuse” rating?

No. That label is a reverse-current withstand limit (UL 61730 / nameplate). The string fuse must be at least 1.56 × Isc and not larger than that maximum. This tool marks the result invalid if the next standard fuse is above the label.

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