Fuse Size Calculator

How to Use MGI’s Fuse Size Calculator

First, determine whether the fuse / breaker will be installed in a feeder circuit or in a branch circuit. As their names suggest, the “feeder” circuit (or main conductor) feeds power to smaller sub-circuits called “branches”.

A “feeder” circuit splits off toward multiple “branches”.

Then, simply enter the rated current of your load and indicate whether it is a motor. The calculator will output an optimal fuse / breaker size, along with a couple product recommendations from MGI.

Branch Circuit

Calculating the appropriate fuse or breaker size for a branch circuit is straightforward. For most applications, an overcurrent device should be sized about 125% of the load’s rated amperage. This way, your load will be protected from electrical short-circuit.

For electric motors, a fuse should be sized at 250% to allow for some initial short-lived cranking amperage that typically exceeds the motor’s rated current.

	Electric motors (variable torque – pumps & fans)	fuse size = ( motor rated current x 2.5 )
	Resistive loads (standard lighting)	fuse size = (load rated current x 1.25)
	Spotlights or high-current resistive loads such as amplifiers	follow only the wire and fuse size recommendations of the load manufacturer

Main Feeder

While branch fuses protect individual load circuits, a main fuse needs to protect the main feeder conductor. Since the feeder circuit splits into branch circuits, calculating the correct main fuse size is a bit more complicated. You’ll need to input the largest rated branch load (or largest motor) and also the sum of each branch load.

The recommended main fuse size is based off the sum of all amperages + 125% overcurrent for the largest branch (or 250% for the largest motor).