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, establish why it operated and fit the correct replacement; do not increase the rating simply to keep the appliance running.
Calculate the load, then check the fuse specification
For the simple 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 for the 700–3,000 W band; actual appliance instructions take precedence.
Worked examples
A small 500 W 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 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 intended to carry its rated current under specified conditions. Its opening time depends on how much overcurrent flows and on the fuse design. It does not necessarily blow as soon as 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. Their availability does not make the next rating above a calculated load the right choice 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.