A worked example of picking the correct plug fuse from an appliance power rating.
The fuse in a UK plug helps protect the appliance flex against excessive current. Use the appliance manufacturer’s specified fuse rating and type. When replacing a blown fuse, find out why it blew and fit the correct replacement; never fit a bigger fuse just to keep the appliance running.
Calculate the load, then check the fuse specification
For the simple resistive loads in these examples, current = power ÷ voltage. The result is the normal running current, not a complete fuse specification. Starting current, the flex and the fuse’s operating characteristic also matter. General plug-fuse guidance commonly uses 3 A for smaller appliances and 13 A, a brown fuse, for the 700–3,000 W band; actual appliance instructions take precedence. That upper figure is also the limit for the socket: Electrical Safety First gives 3,000 W as the maximum rating of a wall socket, roughly what 13 A at 230 V delivers. Everything plugged into one socket, through an adaptor or an extension lead, draws its current through that socket. So a 2.2 kW heater and a 1 kW dehumidifier sharing one are too much for it.
Worked examples
A small 500 W resistive appliance on 230 V draws 500 ÷ 230 ≈ 2.2 A. Under the general guide used in this example, the plug takes a 3 A fuse. Check that this agrees with the appliance’s instructions.
A 2,500 W resistive appliance on 230 V draws 2,500 ÷ 230 ≈ 10.9 A. Its power lies in the general guide’s 13 A band, so the worked selection is a 13 A fuse, subject to the actual appliance specification.
Rated current is not a blowing threshold
A fuse is made to carry its rated current under specified conditions. How quickly it blows depends on how big the overcurrent is and on the fuse's design: it does not blow the moment its marked current is reached. Besides the common 3 A and 13 A plug fuses, other BS 1362 ratings, including 5 A and 10 A, exist. That they exist does not make the next size up from the calculated load right for every appliance.
Applying this to plumbing equipment
Pumps, boilers, controls and immersion heaters need a supply and protection suited to their actual load and installation. A plug-fuse example does not specify the protective device for a fixed circuit. Follow the equipment instructions, check the cable and isolation arrangement, and keep electrical work within your competence.