Enter the device power, pick the supply type, and choose whether the load runs continuously to get the nominal current and the recommended fuse or breaker rating.
A continuous load should not exceed 80% of the fuse or breaker rating, which is the same as sizing the protection at 125% of the nominal current. Intermittent loads can be protected closer to the nominal current.
The required rating exceeds the standard sizes listed below. Consult a qualified electrician for a suitable protective device.
Standard ratings available for this supply type:
| Rating (A) |
|---|
This calculation is indicative and educational. It does not replace a qualified electrician or the manufacturer instructions for any real installation. An undersized fuse nuisance-trips; an oversized one fails to protect the circuit and wiring, which is a fire risk.
Fuse and breaker ratings under BS 1362 and BS 60898 in the UK
UK 13A plugs (BS 1363) each carry a small cartridge fuse to BS 1362, colour coded and sold in a handful of standard ratings: 1A, 2A, 3A (red), 5A (black), and 13A (brown), chosen for the appliance rather than the plug itself. Fixed wiring is protected further back at the consumer unit by miniature circuit breakers to BS 60898, almost always curve type B in a domestic setting, in ratings such as 6A, 10A, 16A, 20A, and 32A. This calculator works out the nominal current a device draws from its power and supply voltage, then rounds up to the next standard rating with a safety margin appropriate for continuous or intermittent use.
Common UK fuse and breaker ratings
| Rating | Typical use | Standard |
|---|---|---|
| 3A (red) | Lamps, small appliances under 700W | BS 1362 plug fuse |
| 5A (black) | Appliances between 700W and 1200W | BS 1362 plug fuse |
| 13A (brown) | Kettles, heaters, most appliances over 1200W | BS 1362 plug fuse |
| 6A | Lighting circuits | BS 60898 MCB (curve B) |
| 32A | Ring or radial socket circuits | BS 60898 MCB (curve B) |
What this calculator cannot do
The result is a starting point based on nominal current only. It does not check cable current-carrying capacity, ambient temperature, cable grouping, or discrimination between an upstream and downstream device, all of which a full circuit design must cover. Motor loads, transformers, and other devices with a high inrush current need a fuse or breaker rated and characterised for that surge, not just the steady running current used here. Always confirm the final rating against the appliance nameplate and, for fixed wiring, have it checked by a qualified electrician.
A worked example
A 1800W kettle on a 230V UK ring main draws I = 1800 / 230 = 7.8A nominal. As a short, intermittent load, no extra margin is strictly needed, so the nearest BS 1362 plug fuse rating at or above 7.8A is 13A, which is exactly why UK kettles are fitted with a 13A fuse as standard. For a continuous 150W device on the same 230V supply, the nominal current is 0.65A, but sized at 125% for continuous duty that becomes 0.82A, still comfortably covered by a 3A plug fuse. To work through the underlying voltage, current, and resistance relationship, the Ohm's Law Calculator covers the basics, and the Wire Gauge Calculator checks that the cable feeding the circuit is large enough for that current.