Fuse Size Calculator
Calculate a recommended fuse size from full load current using the standard 125% continuous-load safety margin.
A
Results
Recommended Standard Fuse Size
15A
Raw Value at 125%
15.00 A
How to Use the Fuse Size Calculator
Enter the full load current (in amps) that a circuit will draw. Electrical codes generally recommend sizing a fuse or breaker at 125% of the continuous full load current, which builds in a safety margin for continuous loads rather than sizing the fuse right at the load's exact current draw. If you only know an appliance's wattage and voltage, use a separate wattage/amperage calculator first (current = watts / volts) to get the amp figure to enter here.
Example
A circuit with a full load current of 12 amps needs 12 × 1.25 = 15 amps at minimum. Since 15 amps is already a standard fuse size, the recommended fuse is 15A. A load of 16 amps would need 16 × 1.25 = 20 amps, which also happens to land exactly on the next standard size, 20A.
Common Use Cases
- Getting a starting estimate for fuse or breaker sizing on a new circuit.
- Checking whether an existing fuse is undersized for a load it's protecting.
- Sizing fuses for automotive, marine, or off-grid solar electrical projects.
FAQs
- Is this a substitute for a licensed electrician? No — this is a general estimate based on the common 125% continuous-load rule of thumb. Actual fuse and breaker sizing must comply with local electrical code, wire gauge ampacity ratings, and equipment-specific requirements, so always confirm with a qualified electrician for real installations.
- Why 125% and not the exact load current? Continuous loads (running 3 hours or more) generate heat over time, and electrical code typically requires a 25% safety margin above the load's rated current to prevent nuisance tripping and reduce fire risk from sustained near-capacity operation.
- What if my exact calculated value falls between two standard sizes? Always round up to the next larger standard fuse size, never down — an undersized fuse can trip under normal operation, while rounding up keeps you within the safety margin the 125% rule is designed to provide.