Solar & DC

PV String Fuse Rating Calculator (gPV)

Find the allowable gPV string-fuse rating window from the module short-circuit current and the module’s maximum series-fuse rating, and see when string fuses are actually needed. A string fuse protects a module string against reverse current from the other paralleled strings — sized above the normal current but below what the module can withstand.

Module I_sc at STC (A)

Parallel strings

Module max series fuse rating (A)

Sizing rule (lower bound)

ⓘBoth I_sc and the maximum series-fuse rating are on the module datasheet. The fuse rating must fall between the lower bound and the module’s maximum.

Recommended gPV fuse

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Minimum rating

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Maximum rating

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String fuse required?

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The sizing window

Minimum In = 1.5 × I_sc (lower bound, IEC 60364-7-712) Maximum In = min( 2.4 × I_sc , module max series fuse ) Choose a standard gPV rating inside the window

The lower bound keeps the fuse from tripping on normal string current, which can exceed the STC value under high irradiance. The upper bound ensures the fuse still protects the module — it must clear before the reverse current from the other strings exceeds what the module can safely carry, and it can never be rated above the module’s stated maximum series fuse.

When is a string fuse needed?

String fuses guard against reverse current: if one string faults, the healthy paralleled strings feed current back into it. With only a few strings the combined reverse current stays within the module’s withstand and no fuse is required. As the number of parallel strings grows, the reverse current rises until protection becomes necessary — commonly from about three or more strings in parallel, but the exact threshold follows the module’s maximum reverse-current rating per IEC 62548.

⚠Use gPV fuses, not ordinary fuses. gPV fuses (IEC 60269-6) are rated for the full DC string voltage and the low-overcurrent, reverse-current duty of PV. A standard AC or gG fuse may not interrupt a DC PV fault safely.

Frequently asked questions

Why is the minimum 1.5 × Isc and not just Isc?

Irradiance can exceed 1000 W/m² and reflections add current, so the real string current runs above the STC figure. The 1.5 factor keeps the fuse from nuisance-blowing under those conditions.

What if my module max series fuse is below 1.5 × Isc?

Then no standard fuse fits the window, and string fusing by conventional means is not viable — the array must be arranged with fewer parallel strings, or protected by a different method. Re-check the module data.

Do both ends of the string need a fuse?

In a functionally earthed or floating array both polarities may need protection. Follow the array topology and IEC 62548 for whether one or both conductors are fused.

Select the gPV fuse

With the rating window and voltage set, match a gPV fuse and holder rated for the DC string voltage.

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Standards referencedIEC 60364-7-712 (PV systems) · IEC 62548 (PV array design, reverse current) · IEC 60269-6 (gPV fuses).

Estimate for guidance only. The final fuse rating, voltage class, and the need for string protection must be confirmed by a qualified person against the module datasheet, the array topology, and IEC 62548.