Fuse Size

Free Fuse Size calculator with clear step-by-step results.

About Fuse Size

A fuse must carry the continuous load indefinitely while still clearing a fault, so the rating is set at 125% of the derated operating current and rounded up to a standard size. The inrush energy figure - peak current squared times duration - is what determines whether a fast-acting fuse will nuisance-trip on startup.

Formulas

Minimum rating

Minimum = operating current / (1 - ambient derating) x 1.25

Inrush energy

I2t = (operating current x inrush multiple)^2 x duration in seconds

Worked example

With the default values below, Fuse Size returns Recommended fuse rating of 20.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Normal operating currentminimum 0.01 A: 12 A
Inrush current multipleaccepts 1–20 x: 4 x
Inrush durationaccepts 0.1–5,000 ms: 40 ms
Derating for ambient temperatureaccepts 0–50 %: 10 %

Results

Recommended fuse rating: 20.00 A
Minimum rating after derating: 16.67 A
Inrush energy the fuse must survive: 92.16 A2s
Fuse characteristic: Time-delay (slow-blow)

Step by step

1Values used
2Minimum rating
3Inrush energy
4Recommended fuse rating= 20.00 A
5Minimum rating after derating= 16.67 A
6Inrush energy the fuse must survive= 92.16 A2s
7Fuse characteristic= Time-delay (slow-blow)

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Overcurrent protection must be coordinated with conductor ratings, equipment listings and the applicable electrical code. Selection should be verified by a qualified engineer or electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Overcurrent protection must be coordinated with conductor ratings, equipment listings and the applicable electrical code. Selection should be verified by a qualified engineer or electrician.

Frequently Asked Questions

When do I need a slow-blow fuse?

Whenever inrush exceeds roughly three times the running current for more than about ten milliseconds - typical of motors, transformers and switch-mode supplies charging input capacitors.

Can I just fit a bigger fuse if it keeps blowing?

No. The fuse protects the wiring and the device, and oversizing it removes that protection. A repeatedly blowing fuse means the wrong characteristic or an actual fault.