Fuse Size Calculator & Simulator

Calculate your required amperage and simulate what happens when you pick the wrong fuse.

1. Circuit Load
Power Draw (Watts) 120 W
System Voltage
Load Duration
Base Current
10.0 A
Minimum Safe Fuse
12.5 A
2. Test a Fuse
Select a fuse above to test the circuit.
12V --

How to Size a Fuse

A fuse is intentionally designed to be the weakest link in your electrical circuit. Its job is to melt and break the connection if too much power flows, protecting your expensive equipment and preventing electrical fires.

  • Step 1: Find the Amperage. Use Watt's Law: Current (Amps) = Power (Watts) รท Voltage (Volts). For example, a 120W lightbar on a 12V battery draws 10 Amps.
  • Step 2: Apply the 125% Rule. Fuses generate heat when running near their maximum capacity. If a circuit runs continuously (usually defined as 3 hours or more), the National Electrical Code standard is to multiply the base current by 1.25. So, our 10A load needs a minimum fuse size of 12.5A.
  • Step 3: Round Up. You can't buy a 12.5A fuse, so you round up to the next standard size available (15A).
Safety Disclaimer: This simulator demonstrates the theoretical relationship between load and fuse size. In real-world applications, your fuse must also be sized to protect the specific gauge (AWG) of wire you are using. A large fuse on a thin wire will cause the wire to melt before the fuse blows. Always consult a licensed electrician or the NEC for actual installations.