Fuse Size Calculator
Calculate the correct fuse or circuit breaker size from load current and wire gauge. Enter values for instant results with step-by-step formulas.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Fuse Size >= Load Current x Continuous Factor (1.25 if continuous)
The fuse must be at least 125% of continuous load current per NEC 210.20. It must also not exceed the derated ampacity of the conductor, which accounts for temperature correction and conduit fill derating factors.
Worked Examples
Example 1: Residential Kitchen Circuit
Problem:A kitchen circuit draws 16 amps continuously on 12 AWG wire at 120V, 30C ambient, 3 conductors in conduit.
Solution:Load current = 16A\nContinuous load factor = 16 x 1.25 = 20A minimum\n12 AWG ampacity = 25A (75C THWN)\nTemp derating (30C) = 1.00, Conduit derating (3 wires) = 1.00\nDerated ampacity = 25 x 1.00 x 1.00 = 25A\nNext standard fuse size >= 20A = 20A\n20A fuse does not exceed 25A wire rating
Result:Use a 20-amp fuse or breaker with 12 AWG wire
Example 2: Workshop Motor Circuit in Hot Garage
Problem:A table saw motor draws 12 amps (non-continuous) on 14 AWG wire at 120V in a 45C garage, 3 conductors.
Solution:Load current = 12A\nNon-continuous factor = 12 x 1.00 = 12A minimum\n14 AWG ampacity = 20A (75C THWN)\nTemp derating (45C) = 0.82\nDerated ampacity = 20 x 0.82 x 1.00 = 16.4A\nNext standard fuse >= 12A = 15A\n15A fuse does not exceed 16.4A derated rating
Result:Use a 15-amp time-delay fuse with 14 AWG wire
Frequently Asked Questions
How do I determine the correct fuse size for my circuit?
The correct fuse size must be equal to or greater than the adjusted circuit load but must not exceed the ampacity of the conductor protecting the circuit. For continuous loads running three hours or more, the National Electrical Code requires you to multiply the load current by 1.25 to get the minimum fuse rating. For non-continuous loads, the fuse simply needs to meet or exceed the actual current draw. Standard fuse sizes come in specific increments such as 15, 20, 25, 30, 40, and 50 amps, so you round up to the next standard size above your calculated minimum.
What is the difference between a fuse and a circuit breaker?
A fuse contains a metal element that melts and permanently breaks the circuit when current exceeds its rating, requiring replacement after each trip. A circuit breaker uses an electromagnetic or thermal mechanism that can be reset after tripping without replacement. Fuses generally react faster to overcurrent conditions and have no moving parts to wear out, making them preferred in some industrial and motor protection applications. Circuit breakers offer more convenience for residential use since they can be reset by flipping a switch. Both devices serve the same fundamental purpose of protecting wiring from overheating and preventing fires caused by excessive current flow.
How does ambient temperature affect fuse and wire sizing?
Higher ambient temperatures reduce the amount of current a wire can safely carry because the insulation is already partially heated before any electrical load is applied. The NEC provides temperature correction factors that derate wire ampacity based on the surrounding air temperature. At 40 degrees Celsius the ampacity drops to about 88 percent of its 30-degree rating, and at 50 degrees Celsius it falls to about 75 percent. This means a 12 AWG wire rated for 25 amps at 30 degrees Celsius can only safely carry about 19 amps at 50 degrees Celsius. Always check conditions in attics, boiler rooms, and outdoor enclosures where temperatures regularly exceed the standard 30-degree baseline.
What wire gauge should I use with each fuse size?
The NEC specifies minimum wire sizes based on the overcurrent protection device rating. A 15-amp fuse requires minimum 14 AWG copper wire, a 20-amp fuse requires 12 AWG, a 30-amp fuse requires 10 AWG, a 40-amp fuse requires 8 AWG, and a 50-amp fuse requires 6 AWG when using 75-degree-rated copper conductors. You can always use a larger wire gauge than the minimum requirement, which reduces voltage drop over long runs. However, you must never use a fuse larger than the wire ampacity rating, because the wire would overheat and potentially start a fire before the oversized fuse trips to protect it.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy