Fuse Size Calculator
Calculate the correct fuse size for any electrical circuit based on wattage, voltage, and wire gauge.
Prevent fires and damage.
A fuse protects a circuit by melting (blowing) when current exceeds a safe level. Choosing the correct fuse size is critical for fire prevention and equipment protection.
The basic formula:
Current (Amps) = Power (Watts) / Voltage (Volts)
Fuse sizing rule: The fuse should be rated at 125% of the normal operating current. This provides a safety margin while still protecting against overloads.
Fuse rating = Operating current × 1.25
Then round up to the next standard fuse size.
Standard fuse sizes (Amps): 1, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 100
Wire gauge must match the fuse: The wire must be rated to safely carry the current the fuse allows. If the fuse is too large for the wire, the wire can overheat before the fuse blows.
Common wire ampacity ratings (copper):
- 18 AWG: 5-7 amps
- 16 AWG: 10 amps
- 14 AWG: 15 amps
- 12 AWG: 20 amps
- 10 AWG: 30 amps
- 8 AWG: 40-50 amps
- 6 AWG: 55-65 amps
Fuse types:
- Fast-blow: Opens quickly on overload. Use for sensitive electronics.
- Slow-blow (time-delay): Tolerates brief surges. Use for motors, compressors, and inductive loads.
- Automotive blade fuses: Color-coded by amperage for easy identification.
Important: The fuse protects the wire, not the device. Always size the fuse to the wire capacity, not the device rating.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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