Fuse and Breaker Sizing Calculator
Size fuses and breakers for solar strings, battery and inverter circuits using the standard continuous-current factors.
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The protection is for the cable
A fuse does not protect the panel or the inverter. It protects the conductor, by opening before the cable can overheat under a fault. That is why the rating sits above normal working current but below what the cable can carry.
Why PV strings use a bigger factor
String protection is calculated from short-circuit current with two allowances stacked: 1.25 for sustained irradiance above test conditions and another 1.25 for continuous operation. The combined 1.56 factor is what keeps a string fuse from nuisance-tripping in July.
Common mistakes
Fitting the largest fuse that fits the holder. Protection sized above the cable rating protects nothing: the conductor becomes the fuse.
Questions people ask
Fuse or breaker? Either, provided the rating and the DC interrupt capacity are right. Breakers are convenient because they double as an isolator; fuses are cheaper and have no moving parts.
Why does DC need special devices? DC has no zero crossing, so an arc does not self-extinguish. An AC breaker used on a battery circuit can fail to break the arc at all.
Where protection goes in a solar system
There are normally three places: on each PV string where more than two strings run in parallel, between the charge controller and the battery, and on the inverter DC feed. Each has its own current and its own reason for existing, which is why one rating cannot cover the whole system.
The battery circuit is the dangerous one
A shorted battery bank can deliver thousands of amps. Protection here needs both the right rating and an interrupt capacity high enough to break that fault, which is what class-T fuses and proper DC breakers are for. An automotive blade fuse in this position is a fire waiting for a loose spanner.