Generally these days, most new, domestic appliances come with a pre-fitted moulded plug that already incorporates either a 3Amp or 13Amp rated fuse. For example, a table lamp would have a 3A fuse where a washing machine a 13A but rarely will you find anything in between. The reason for this is simply they are the most commonly available sizes and as such are cheaper to buy, whilst meeting the minimum protection required. An incorrect fuse however will continuously blow if under rated or fail to protect your equipment from possible catastrophic meltdown if it is too high.
Firstly, check the fuse you’re replacing. If you can’t tell what the rating is, or it’s not clear, you can check the rating of the appliance. Consult the appliance rating plate (often found on the rear or underside of the appliance) or look at the manufacturer’s instructions to find the wattage. Then use the following as a guide to select the correct fuse: 3 Amp fuse for appliances up to 700 Watts, 5 Amp fuse for appliances ranging from 700 to 1000 Watts, 13 Amp fuse for appliances over 1000 Watts. Remember to never use a fuse which has a higher rating than necessary, as it can be dangerous.
It’s always best to check the exact specifications for your appliance, but the following appliances generally fall into these categories: 3 Amp fuse: Table lamps, radios, DVD players, video recorders, 5 Amp fuse: Irons, toasters, 13 Amp fuse: Kettles, heaters.
If you know the mains voltage (which in the UK is slightly variable but assumed fixed at 240v) and the Wattage gained from the appliance ID plate (not from the plug) we can, using a simple calculation, work out the appliance Amperage and so the fuse size (rating). Wattage divided by Volts = Amps. When a figure is obtained from the calculation it is simply a case of adding approx 10% to the value and choosing the nearest fuse to match. This is a more precise method than just guessing 3, 5 or 13Amp and adds greater protection to your appliance, but you do risk blowing the fuse often if the operating or starting current (Amps) are very close to the fuse rating you have selected. See the examples below.
Example 1: A Kettle - The ID plate on the base of a kettle shows the following information: 220-240v~50Hz 1850-2200W. This tells us that the kettle is suitable for both European 220v (1850W) and UK 240v (2200W). Ignore the 50~ as this is the frequency and not required for the calculation. Our calculation would be 2200W divided by 240v = 9.17A + 10% = 10.087A A good fuse for this would be a 10A. A 13A would be fitted as standard by the manufacturer and will suffice but anything lower than 10A would fail.
Example 2: A table lamp - There is no ID plate but we know the maximum lamp wattage that will be used in the lamp is 100w. Our calculation would be 100W divided by 240v = 0.42A + 10% = 0.46A A good fuse for this would be a 1A. A 3A would be fitted as standard by manufacturers.