Fuse Size Calculator
Find the correct fuse rating for any electrical circuit. Enter the load current and select the circuit type to get the recommended fuse size, minimum wire ampacity, and IEC 60364 compliance guidance.
Last updated: May 2026
Enter load current and select circuit type to get a fuse recommendation.
Standard IEC fuse sizes: 2, 4, 6, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100 A
How to select the correct fuse size
A fuse protects the wiring, not the load. The rule is to match the fuse to the cable's current-carrying capacity, not to the device's rated power. Using a fuse that is too large defeats the protection; using one that is too small causes nuisance tripping under normal conditions.
For most circuits, the calculation is: fuse rating = next standard size at or above the load current, respecting the derating factor for the circuit type. A 13 A circuit feeding a 10 A load works fine with a 13 A fuse (next standard size above 10 A), but a 16 A fuse would also be acceptable if the cable is rated for 16 A.
Derating rules by circuit type
| Circuit type | Derating factor | Example: 10 A load |
|---|---|---|
| Non-continuous load (less than 3 hours) | 100%: fuse at load current | Next size above 10 A = 13 A fuse |
| Continuous load (3 or more hours, heating, lighting) | 125%: cable and fuse sized at 1.25x load | 10 A x 1.25 = 12.5 A, next size = 13 A fuse |
| EV home charger (Mode 3, dedicated circuit) | 125% minimum: always a continuous load | 16 A charger x 1.25 = 20 A fuse, 2.5 mm2 minimum |
| Motor: direct-on-line (DOL) start | Wire at 125% FLA; fuse up to 250% FLA (time-delay) | 10 A FLA: wire for 12.5 A, fuse up to 25 A (gM type) |
Standard fuse sizes (IEC)
IEC standard fuse sizes in amps: 2, 4, 6, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200. The calculator always selects the next size at or above the required ampacity. Selecting a size below the required figure is not permitted.
Frequently Asked Questions
What is the difference between a fuse and a circuit breaker?
Both protect wiring from overcurrent, but they operate differently. A fuse is a single-use device: when it blows, the element melts and must be replaced. A circuit breaker is a mechanical switch that trips and can be reset. For residential circuits, circuit breakers (MCBs) are now standard in most countries because they allow safe resetting after a fault. Fuses are still common in older consumer units, automotive wiring, and industrial panel boards where specific fault-current characteristics (gG, gM fuse types) are required.
Why does a motor need a larger fuse than its nameplate current suggests?
Electric motors draw 5 to 8 times their full-load amperage (FLA) for the first few cycles at startup. This inrush current lasts only a fraction of a second but is enough to blow a fuse sized exactly at the running current. IEC 60947-4-1 and NEC 430.52 both allow motor branch-circuit fuses to be rated up to 250% of FLA (using time-delay or gM type fuses) to ride through the inrush without false tripping. The wiring must still be sized for 125% of FLA, independent of the fuse rating.
What does the 125% rule for continuous loads mean?
A load is classified as continuous when it runs at more than 80% of its rated current for 3 hours or more. Heating elements, EV chargers, lighting circuits, and pumps typically qualify. For continuous loads, IEC 60364 and NEC 210.19 both require that the wire ampacity and the fuse (or circuit breaker) be sized at 125% of the actual load current, not 100%. This accounts for heat buildup in wiring over sustained operation. A 16 A EV charger is a continuous load: the minimum cable ampacity is 16 x 1.25 = 20 A, and the fuse must be at least 20 A.
Can I use a higher-rated fuse if the smaller size keeps tripping?
Only if the cable is also rated for the higher fuse size. Fitting a 20 A fuse on a cable rated for 13 A removes the protection for the cable. If a fuse trips repeatedly under normal operating conditions, investigate the actual load current, check for loose connections that increase resistance and heat, verify the cable is not undersized for the run length, and confirm the load is not drawing more than expected. Upsizing the fuse without also upsizing the cable is a fire risk. If the nuisance tripping is caused by motor inrush, switch to a time-delay (gM or type D) fuse of the same rating rather than a larger fast-acting fuse.
Next step in this workflow
Breaker sized: now check that your panel or power supply can handle the total load.