PV string fuse protection

Installation3 min readUpdated: 2026-08

String fuses do not protect against a classic overload — they protect against the reverse current a faulty string can receive from other parallel strings during a fault.

With only 1 or 2 strings in parallel, the maximum possible reverse current often stays below the cable's damage threshold — a fuse is usually not required. From 3 parallel strings onward, it becomes necessary.

1.25 × Isc ≤ Fuse_rating ≤ Max_series_fuse_current_from_datasheet

The rating must satisfy both bounds: the panel datasheet gives the maximum allowed series fuse current

Parallel strings	Fuse required?	Note
1	No	No reverse current possible
2	Usually no	Check against short-circuit current and panel datasheet
3 or more	Yes	Each string can potentially receive the combined current of the others

Always use certified DC (gPV) fuses — a standard AC fuse doesn't properly clear a DC arc and can remain conducting even when "blown".

Common mistakes

Installing string fuses when only one string is present (unnecessary, adds a failure point)
Using a standard AC fuse instead of a certified gPV DC fuse
Exceeding the maximum series fuse current listed on the panel datasheet

Sizing breakers and DC/AC protection Solar cable section