Electrical Load Calculator
Add up the appliances on a circuit to see the total current drawn, and check it against the breaker rating before something trips or overheats.
Formula
Total current = total watts ÷ supply voltage
- UK supply is 230 V
- Daily kWh = total watts × hours used ÷ 1,000
- A UK ring final circuit is normally protected at 32 A
7,000 W at 230 V draws 30.4 A, close to the limit of a 32 A circuit
Adding Up the Load
Current (A) = Total watts ÷ Supply voltage
On a UK 230 V supply, 4,000 W of appliances draws 4,000 ÷ 230 = 17.4 A.
That already exceeds a 16 A breaker and sits close to a 20 A one. Add up everything that might run simultaneously, not everything plugged in — a kettle and a toaster together matter, a rarely used lamp does not.
Leave Headroom
Common practice is to load a circuit to no more than 80% of its rating continuously. A 20 A circuit should therefore carry around 16 A of sustained load.
UK plug fuses top out at 13 A, about 3,000 W at 230 V, which is why kettles, heaters and irons sit near the limit individually and must never share an extension lead. Extension leads themselves carry a rating, often 13 A total across all sockets regardless of how many there are.
Know Where the Limit Is
Breakers protect the cable, not the appliance. Fitting a larger breaker to stop nuisance tripping removes that protection and is genuinely dangerous, because the cable can then overheat before anything disconnects.
Motors also draw a large inrush current at startup, several times their running figure, which is why a circuit can trip when a compressor or pump kicks in despite handling the steady load fine.
Anything involving fixed wiring, consumer units or new circuits is notifiable work under Part P in England and Wales. Use a qualified electrician; this calculator is for estimating only.
Quick Reference
| Load | Current | Suitable circuit |
|---|---|---|
| 1,000 W | 4.3 A | Any lighting or socket circuit |
| 3,000 W | 13.0 A | Single 13 A plug at its limit |
| 7,000 W | 30.4 A | Near the limit of a 32 A ring |
| 10,000 W | 43.5 A | Needs its own higher-rated circuit |
Frequently Asked Questions
- How do I calculate electrical load?
- Add the wattage of everything that may run at once and divide by the supply voltage. 4,000 W at 230 V draws about 17.4 amps.
- How much can a 13 A plug handle?
- About 3,000 W at 230 V. Appliances near that figure should each have their own socket rather than sharing an extension lead.
- Why does my breaker trip when a motor starts?
- Motors draw a large inrush current at startup, several times their running current, which can exceed the breaker threshold briefly.
- Can I fit a bigger breaker to stop tripping?
- No. Breakers protect the cable, so a larger one lets the wiring overheat before disconnecting. Investigate the cause or consult an electrician.