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Fuse Rating Calculator

Work out fuse rating instantly with clear inputs, formula shown and shareable results.

About Fuse Rating Calculator

A fuse must carry the load continuously without ageing, so the rating is at least 125% of continuous current, more for high-inrush loads. It must also be no larger than the cable can withstand, since the fuse is what protects the cable rather than the load.

Formula

Fuse rating

In >= 1.25 x load current (1.5 x for high inrush) and In <= cable ampacity

Worked example

With the default values below, Fuse Rating Calculator returns Recommended fuse rating of 25. Change any input above and every figure here is recalculated in your browser.

Inputs used

Continuous load current
18 A
Load typeone of Resistive / lighting, Motor (inductive), Transformer / capacitive inrush
Motor (inductive)
Cable ampacity
25 A
Ambient temperature
35 °C

Results

Recommended fuse rating
25 A
Minimum rating for the load
23.06 A
Maximum allowed by the cable
25.00 A
Fuse characteristic
Time delay (gG or motor rated)

Step by step

  1. 1Values used
  2. 2Fuse rating
  3. 3Recommended fuse rating= 25 A
  4. 4Minimum rating for the load= 23.06 A
  5. 5Maximum allowed by the cable= 25.00 A
  6. 6Fuse characteristic= Time delay (gG or motor rated)

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.
  • Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

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

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

Why 125% of load current?

Fuse elements age under sustained current near their rating. The 25% margin keeps the element cool enough for a long service life.

What is the fuse actually protecting?

The cable. Equipment usually has its own internal protection, but the circuit conductors rely entirely on the upstream device.

Related Calculators

Fuse Rating Calculator – Calculate Recommended fuse rating