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.
The fuse protects the wire, not the appliance
The fuse sits between the sized cable and the live circuit it feeds. Its job is to stop the conductor overheating if something goes wrong downstream. The order a circuit comes together in:
- Size the conductor. Load current, run length and voltage drop set the minimum cross-section. The wire gauge calculator checks ampacity and voltage drop together.
- Confirm voltage drop on the run. A cable that passes ampacity can still lose too much voltage over distance. The voltage drop calculator checks the run against the 3% IEC limit.
- You are here: protect the cable with a fuse or breaker. The fuse rating is set by the conductor it protects, not by the appliance. A fuse that is too large lets the cable overheat before it ever blows; a fuse that is too small causes nuisance tripping at normal load. The calculator above gives the correct rating for the cable you have sized.
- Fit cables in conduit. Bundled cables derate. The conduit fill calculator checks cross-section against fill limits.
- Size the supply behind the circuit. Add up the connected load with the power supply calculator; for reactive loads the power factor calculator converts watts to the VA a supply must deliver.
- Know what the circuit costs to run. Once appliances are connected, the appliance running cost and electricity cost calculators convert the load into a monthly figure.
The rule that catches people out: the fuse rating follows the wire, not the load. When I wired the 20 A workshop circuit, the protective device was sized to the 2.5 mm2 cable's 25 A ampacity, not to the tools plugged in downstream. A tool that draws 18 A is fine on that cable; a fuse sized for the tool at 20 A would leave the conductor unprotected on the last 5 A of headroom.
Frequently Asked Questions
Why is the fuse rated to the cable, not to the appliance it feeds?
The fuse's only job is to stop the cable overheating. If excess current flows, the fuse must open before the conductor reaches its thermal limit. Sizing the fuse to the appliance instead of the cable leaves a gap: a cable rated for 16 A protected by a 20 A fuse will carry up to 20 A before the fuse blows, which is above the cable's safe limit. The appliance is protected by its own internal fuse or thermal cutout. The wiring fuse protects the conductor and the structure around it.
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.
Methodology and sources
This tool recommends a fuse rating from the load current and circuit type by applying the relevant derating factor and then rounding up to the next standard IEC fuse size, so the device protects the cable rather than the load.
- Method: Non-continuous load uses the load current directly; continuous load uses load current × 1.25 (the 125% rule); motor circuits size the wire at FLA × 1.25 and the fuse between FLA × 1.75 (recommended time-delay) and FLA × 2.5 (ceiling for startup surge). The result is rounded up to the next value in the standard IEC set (2, 4, 6, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200 A); rounding down is never allowed.
- Standards and sources: IEC 60364-4-43 / NEC 210.19 (125% continuous-load sizing); IEC 60947-4-1 and NEC 430.52 (motor branch-circuit overcurrent protection and inrush allowance); standard IEC fuse rating series (gG/gM types).
- Assumptions and limits: The recommendation assumes the cable ampacity already meets or exceeds the chosen fuse rating; it does not account for ambient temperature, grouping, conductor material, or run-length derating, and it does not size the cable itself. Motor figures assume a direct-on-line start with a time-delay fuse. Always verify the cable rating, the fuse let-through class, and discrimination with upstream devices.
Reviewed and maintained by Rick Oosterling, who builds and wires 12 V, solar and EV systems hands-on. Last reviewed: June 2026. This calculator is a planning aid, not a substitute for a qualified electrician or your local wiring and building code; verify every fuse and cable rating against the installation conditions before relying on it.
Next step in this workflow
Breaker sized: now check that your panel or power supply can handle the total load.