The purpose of the fuse within a UK plug is to protect the cable. It is a deliberate weak link designed to melt in the event of a fault, thus cutting the power supply. The fuse's current rating must be lower than that of the cable. This is to ensure that the fuse will fail first to prevent the cable from overheating.
The appropriate fuse size is ultimately determined by the cable's current rating. However, it's generally safe to assume that the manufacturer has fitted a cable and fuse suitable for the appliance's power rating. If a fuse blows and needs replacement, it's important to stick with the manufacturer's recommended fuse rating.
Moulded Plugs
If the appliance has a moulded (non-rewirable) plug fitted, the easiest way is to look for the fuse rating on the plug. All UK moulded plugs (BS1363) must have the fuse rating marked on them.
While there's no universal marking system, you can typically find the fuse size displayed alongside the voltage rating on most plugs. For instance, it might be indicated as 5A/250V.
Rewirable Plugs
Somewhat confusingly, all rewirable type plugs are marked as 13A, but this is the current rating of the plug, not the fuse. In some cases, the manufacturer will put a sticker on the cover to indicate the correct fuse size, but this is not a legal requirement.
If the fuse recommended by the manufacturer keeps blowing, you should not replace it with a larger one to prevent this. Such a situation likely indicates a problem with the appliance itself, causing the fuse to blow. Instead, you should have the appliance inspected by a professional electrician.
700W Rule
If an appliance has a rewirable type plug fitted and the original fuse size fitted by the manufacturer is not known, the recommended method is to use the 700W rule:
1. For an appliance rated at 700W or less, a 3A fuse should be fitted.
2. For appliances above 700W, a 13A fuse should be fitted.
All modern appliances should have the power rating marked on them. This is usually on a sticker or sometimes printed or embossed directly on the appliance.
This example shows an appliance with a power rating of 1200W, so a 13A fuse should be used.
It's important to note that the 700W rule is a guideline, and there may be exceptions for certain appliances. Many appliances experience a temporary surge in current when they are turned on. This surge is often higher than the normal operating current and occurs only for a short duration. This is known as inrush current and mainly affects equipment containing electric motors, such as vacuum cleaners and power tools. To address this, a higher-rated fuse is often used for such appliances. A common approach is to use a 5A or 10A fuse for devices with inrush current, rather than a 3A fuse that the power rating would suggest.
Fuse Rating based on Cable Size
Should the manufacturer's original cable have been replaced, or if there's any uncertainty about the correct fuse size, the fuse rating should be determined by the size of the cable.
Cable size is a measurement of the cross-sectional area (CSA) of the copper wires within the cable and is given in mm2. The CSA should be marked on the cable. This example shows a cable with a CSA of 1.25mm2
| Conductor CSA (mm2) | Rewirable Plug Max Fuse Rating | Moulded Plug Max Fuse Rating |
|---|---|---|
| 0.5 | 3 | 3A |
| 5A Only for appliances with high inrush current | ||
| 0.75 | 7A | 13 7A for extension leads |
| 10A Lead length limited to 2m | ||
| 1 | 10A | 13A 10A for extension leads |
| 1.25 | 13A | 13A |
| 1.5 | 13A | 13A |
Where the power rating of an appliance allows, a smaller fuse than indicated in the table may be used.
Standardised Fuse Ratings
The British Standard for plugs, BS1363, has standardised on just two preferred fuse ratings, 3A and 13A, however, any fuse rating up to 13A can still be used.
3A fuses are marked in red and 13A fuses are brown. Fuses are also available in 1A, 2A, 5A, 7A & 10A sizes and are marked in black. 5A and 10A fuses are the most common non-standard sizes and are often found in detachable leads connected to IT equipment.
Learn more
If you are looking to learn more about electrical safety, including how to select the correct fuse rating, we regularly run courses at various venues across the country.
