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Fuse size calculator

Current drawn by an accessory and the minimum fuse rating to protect its wiring.

Units
W
V
%Fuses are usually sized somewhat above the normal current so they do not blow in use.

Result

Minimum fuse rating5.7 A
Normal current4.58 A

How it is calculated, with your numbers
  1. Normal currentamps = w / volts55 / 12 = 4.58 A
  2. Minimum fuse ratingfuse = w / volts × (1 + margin / 100)55 / 12 × (1 + 25 / 100) = 5.7 A

Working uses the metric units the formula is written in.

  • w Accessory power (W)
  • volts System voltage (V)
  • margin Margin above normal current (%)
  • amps Normal current
  • fuse Minimum fuse rating

What it means

Current is power divided by voltage. A fuse protects the wire, not the device, so it should blow well before the wire overheats but not during normal use. Choose the next standard fuse size at or above this figure, and check that the wire is rated for that fuse.

Assumptions and limits

  • Resistive loads. Motors and some LED drivers draw a surge at switch-on.

Sources

  • Ohm's law and power: P = V × I.

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