Overcurrent protection — the fuse or circuit breaker — is the small component that stops a fault from becoming a fire. Size it too small and it trips every time the sun comes out; size it too big and it won't blow when the wire is overloaded. Fortunately the sizing rule is simple and standardized, and this calculator applies it for you.
The rule: current × 1.25, then round up
Solar circuits count as continuous loads (they run for hours), so electrical codes require the protection and conductors to be rated at 125% of the continuous current. The method is:
Required rating = current × 1.25, then round UP to the next standard size
So a circuit carrying 10 A needs 10 × 1.25 = 12.5 A of protection, and since 12.5 A isn't a standard size, you round up to the next one: a 15 A fuse or breaker. You always round up, never down, so the protection is rated at or above the required value.
Why the 1.25 safety factor?
The factor exists so the fuse doesn't nuisance-trip at normal full-load current, while still protecting the circuit from genuine overloads. For solar strings there's an extra wrinkle: the current used is usually the panel's short-circuit current (Isc), because a string can be driven to its Isc under bright sun — and some codes apply the 1.25 factor twice (once for Isc, once for continuous duty). When in doubt, follow your local code and the panel datasheet.
Standard fuse and breaker sizes
Protection comes in fixed standard ratings, not any value you like. Common ones are 1, 2, 3, 6, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, and 200 A. After the multiplication, you pick the next size at or above your result — that's what this calculator returns.
Don't forget the cable
A fuse protects the cable, so the two must match: the cable's current rating (ampacity) must be at least as high as the fuse rating, or the fuse could let through enough current to overheat an undersized wire. Always size the cable and the protection together. Use the cable size calculator to pick the conductor, and the voltage drop calculator to check it over the run.
Related tools
To find the current a circuit carries, size the source first: the charge controller calculator and panel calculator help with array current, and the inverter calculator covers the AC side. For series strings, the string sizing calculator sets the configuration.
Frequently asked questions
What size fuse do I need for my solar panel?
Multiply the circuit's continuous current by 1.25, then round up to the next standard size. A 10 A string needs 12.5 A, so a 15 A fuse. This calculator does it for you.
Why multiply by 1.25 for fuse sizing?
Solar circuits are continuous loads, and code requires protection and conductors rated at 125% of the continuous current — enough margin to avoid nuisance trips while still protecting the circuit. For strings, the current used is usually the panel's Isc.
What are standard fuse and breaker sizes?
Common ratings: 1, 2, 3, 6, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A. Always round up to the next standard size.