Every fuse is designated by an ampere rating which has been determined by the manufacturer under specified test circuit conditions. When choosing a fuse ampere rating, consider the type of load and code requirements for the specific application. The ampere rating of the fuse should normally not exceed the current carrying capacity of the circuit to provide reliable overload and short-circuit protection. However, there are some special situations where the fuse ampere rating can be greater than the current carrying capacity of the circuit. A common example is in a motor circuit, where dual-element fuses are sized at 175% and non-time delay fuses are sized at 300% of the motor full load amperes. (These fuses are intended principally for short-circuit protection.)
In general, the fuse ampere rating and switch combination should be selected at 125% of the full load current. This usually corresponds to the circuit capacity which is also designed at 125% of the full load current. There are exceptions such as when the fuse-switch combination is approved for continuous operation at 100% of their rating.
