Watts to amps, and what a 13 A plug or a 15 A circuit can take

The three formulas

For a household supply, power in watts is the voltage times the current: W = V × A. Turn it round for the other two: A = W ÷ V and V = W ÷ A. For heaters, kettles, toasters and old-style bulbs, which simply turn electricity into heat, that is all there is to it. Motors and electronics can draw a little more current than watts ÷ volts suggests, so when an appliance’s rating label gives amps, use the label’s amps rather than working them out.

Which voltage do you have?

In the US, homes are fed two 120 V wires and a neutral; ordinary circuits use one of them, and big appliances such as water heaters, electric ranges and clothes dryers use both for 240 V, as the Electrical Safety Foundation International explains. The Red Cross puts it simply: most household devices run on 120 volts. In Great Britain the declared supply is 230 V, allowed to vary from 10% above to 6% below (about 216 to 253 V) under the Electricity Safety, Quality and Continuity Regulations 2002. Use the nominal figure: 120, 240 or 230 V.

UK plugs, fuses and leads

Electrical Safety First, the UK electrical safety charity, says a standard plug should meet BS 1363 (marked on the back) and its fuse BS 1362. Plug fuses are now standardised at 3 A or 13 A, with 5 A still found in some older equipment. As a general guide, an appliance rated from 700 to 3,000 W should have a 13 A fuse; its 3 A examples are table lamps, televisions and computers. Replace a fuse only with one of the same rating.

Most extension leads are rated 13 A, so the whole lead can carry no more than a single wall socket: about 3,000 W in total, however many sockets it has. Some older leads are rated 10 A, and a cable reel left wound up may be rated as low as 3 or 4 A, which is why Electrical Safety First says to unwind it fully. The extension lead calculator adds up what’s on one lead for you.

US circuits and cords

The number on a breaker is the circuit’s rating in amps. A 15 A circuit at 120 V carries 1,800 W at its rating, a 20 A circuit 2,400 W. The catch is that one circuit usually feeds several outlets and lights, and the breaker sees all of them together, so the check is on everything running on that circuit, not on one outlet. Electricians also leave a margin when a heavy load runs for hours, under the National Electrical Code (NFPA 70). The rules depend on the edition your state has adopted and we couldn’t check the current wording on a free public page, so the calculator doesn’t build that margin in: if a heater or another heavy load will run for hours on one circuit, ask a licensed electrician what that circuit can take.

Extension cords have their own ratings, marked on the cord. The US Consumer Product Safety Commission says a cord’s wattage rating should exceed the total wattage of everything plugged into it, and the Electrical Safety Foundation International says extension cords should only be used on a temporary basis and should meet or exceed the power needs of what is plugged in.

Two worked examples

• US: a 1,500 W space heater and a hair dryer that Honda lists at 1,800 W running, on one 15 A circuit: 3,300 W ÷ 120 V = 27.5 A, nearly twice the rating. The breaker should trip; run them on different circuits, or not at the same time.
• UK: a 3,000 W kettle and a 2,000 W toaster on one 13 A extension lead: 5,000 W ÷ 230 V = 21.7 A, well over 13 A. Plug the kettle into its own wall socket.

When to call an electrician

This page does arithmetic; it isn’t a substitute for an electrician who can see your wiring. Electrical Safety First’s danger signs are a good list: a smell of hot plastic or burning near a socket or plug, sparks or smoke, scorch marks, damaged or frayed cables, and fuses that blow or breakers that trip for no obvious reason. Unplug, and get it checked. If you rely on extension leads and adaptors every day, Electrical Safety First suggests having more wall sockets fitted by a registered electrician instead.