Solar Combiner Box & String Fuse Sizer

When Are Solar String Fuses Required?

Calculation Assumptions

• Maximum string short-circuit current (Isc) multiplied by 1.25 for insolation and 1.25 for continuous duty (1.56 × Isc).
• Required whenever 3 or more strings are connected in parallel to prevent backfeed damage.

Engineering Limitations

• Series strings of 1 or 2 arrays do not require individual string fuses under NEC 690.9(A).

The requirement for series fusing in solar photovoltaic arrays is governed by physical backfeed currents under short-circuit conditions:

• 1 or 2 Strings in Parallel: Fuses NOT Required. If one string experiences a short-circuit fault, only one remaining string can backfeed current into it. Because that backfeed current ($1 \times I_{sc}$) is less than the panel's maximum series fuse rating, the wiring and diodes cannot overheat.
• 3 or More Strings in Parallel: Fuses MANDATORY. If one string faults, all remaining parallel strings $(N - 1)$ will feed their short-circuit current into the damaged string. Because $(N - 1) \times I_{sc} > I_{fuse\_max}$, excessive current will melt the panel bypass diodes and ignite rooftop wiring without series fuses.
• The 1.56x Safety Multiplier: NEC 690.8 requires a $1.25$ factor for continuous load multiplied by another $1.25$ factor for peak edge-of-cloud solar irradiance, resulting in a minimum sizing factor of $1.56 \times I_{sc}$.