The fuse size calculator determines the correct fuse or circuit breaker rating for wire gauge and load current. Proper fuse sizing prevents wire fires while avoiding nuisance tripping.
Calculate Fuse Size
How to Calculate Fuse Size for Electrical Circuits
Fuses protect wire, not loads. The fuse must be sized to blow before the wire reaches its melting temperature. The fuse rating must never exceed the wire's ampacity. For branch circuits, fuse = 125% of load current (or next standard size), but never larger than wire ampacity.
Standard Fuse/Breaker Sizes
Standard sizes: 1, 3, 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400A. Always select the next standard size up from your calculated minimum, without exceeding wire ampacity.
Frequently Asked Questions
How do I choose the right fuse size?
Fuses protect wire from overheating. The fuse rating must be: (1) at least 125% of the load current for branch circuits, (2) never more than the wire's ampacity, and (3) equal to the next standard size up from your calculated minimum. For a 15A load on 12 AWG wire (20A rated): 15A × 1.25 = 18.75A → use a 20A fuse.
What happens if a fuse is too large for the wire?
An oversized fuse allows current that can overheat and burn the wire before the fuse blows. This is a serious fire hazard. The fuse must NEVER exceed the wire's ampacity — the fuse is there to protect the wire, not just the load.
What is a time-delay fuse used for?
Time-delay (dual-element) fuses allow brief current surges, like motor starting current, without blowing. Motors can draw 5-7× their running current for the first second of starting. Standard fast-blow fuses would trip during this surge. Time-delay fuses handle the surge and still protect against sustained overloads.
What are continuous loads?
A continuous load is one that operates for 3 hours or more without interruption (per NEC). The NEC requires circuit breakers for continuous loads to be rated at 125% of the load current — so a 16A continuous load needs a 20A breaker minimum.
Is this fuse calculator free?
Yes, completely free with no signup needed.