The fuse in a plug or junction box is a protection device which is designed to ‘blow’ if excessive current passes through the wire and into the fuse. Current can increase if there is a short-circuit in an appliance. This ‘over-current’ produces heat which can damage the appliance, or in household equipment, potentially start fires.The fuse is essentially a short length of wire situated within a ceramic or glass outer casing. If an over-current occurs, the heat produced will melt the fuse wire and interrupt the circuit, stopping the flow of electricity before too much heat is produced in the appliance

Regular plug fuses are commonly available in the following sizes; 1.5A, 3A, 5A and 13A. The Amp rating (A) is the fuse’s capacity. Different size fuses are designed to be used on electrical equipment according to their wattage rating. To calculate the correct fuse for an appliance you simply divide its wattage rating by the mains voltage (240V).The formula for calculating fuse size is; A = W ÷ V(A is the fuse Amp rating, W is the pump Wattage, and V is mains Voltage)E.g. a pump has a Wattage rating of 700W

Fuse Size (A) = W ÷ V

= 700 ÷ 240

= 2.9

(the result is rounded up to the nearest fuse size)

= 3ATherefore, the pump will require a 3 Amp fuse.When calculating a fuse for a junction box you must use the total sum wattage rating for all the appliances which are running off the fuse. You may have a junction box which has individually fused outlets, the fuse rating for these is calculated in the same way as a plug’s fuse.The table below shows the maximum wattage rating for which the common fuses are designed to handle.