How the Workshop Power Calculator Works
Every machine draws a current that depends on its power and the supply voltage. Add up everything that runs at the same time and compare it with the smallest rating in the chain: the plug and its fuse, the extension lead's cable, and the circuit breaker. Then check the start-up surge and how much voltage the lead loses on the way.
In the UK a 13 A plug (BS 1363) is the limit for anything plugged into a normal socket, whatever the circuit behind it. At 230 V that is about 3 kW. A 4-way extension lead does not give you four times as much: all four sockets share its one 13 A plug.
The Formulas
1. Running current
amps = watts ÷ (volts × power factor)
total = sum of machines that are on
Heaters and kettles have a power factor of 1. Motors draw more current than their watts suggest; the presets use about 0.7–0.8 for induction motors and 0.95 for brushed motors.
2. Start-up surge
peak = total running + (start × − 1) × biggest starter
Induction motors and compressors can draw several times their running current for a moment. A type B breaker trips instantly somewhere between 3 and 5 times its rating, so a big surge on a small circuit can trip it.
3. Voltage drop in the lead
drop (V) = 2 × length × Ω per metre × 1.2 × amps
max length = 5 % × volts ÷ (2 × Ω/m × 1.2 × amps)
The current goes out on the live and back on the neutral, hence × 2. The × 1.2 allows for a warm cable (about 70 °C). Ω per metre: 1.0 mm² 0.0195, 1.25 mm² 0.0156, 1.5 mm² 0.0133, 2.5 mm² 0.00798.
4. Welders
output V = 14 + 0.05 × A (MIG), 20 + 0.04 × A (stick)
supply A ≈ output V × A ÷ (0.8 × 0.7 × supply V)
I1eff ≈ I1 × √duty cycle
The load voltages are the IEC 60974-1 conventional values. The 0.8 efficiency and 0.7 power factor are typical assumptions for a hobby inverter; the I1max and I1eff on your rating plate are better.
Worked Example
1800 W site table saw + 1400 W shop vac + 100 W of LED lights on a 25 m, 1.25 mm² lead from a 32 A ring
- Saw: 1800 ÷ (230 × 0.95) = 8.2 A. Vac: 1400 ÷ (230 × 0.95) = 6.4 A. Lights: 100 ÷ (230 × 0.9) = 0.5 A.
- Total running: 15.1 A. That is more than one 13 A plug or a 13 A lead can carry, even though the 32 A ring could. Run the vac from a different socket on its own lead, or use a vac with a power take-off socket and keep the total under 13 A.
- Peak when the saw starts: 15.1 + (2.5 − 1) × 8.2 = 27.5 A for a moment, under 3 × 32 A, so the breaker should not trip.
- Voltage drop at 15.1 A: 2 × 25 × 0.0156 × 1.2 × 15.1 = 14.2 V, about 6.2 %. With the saw alone (8.2 A) it is 7.7 V, 3.4 %, which is fine.
Practical Tips
- Unwind cable reels completely for heaters, compressors, saws and welders. A wound reel traps heat; its coiled rating is printed on it and is far lower than the uncoiled one. Reels made to BS EN 61242 have a thermal cut-out, but do not rely on it.
- Do not daisy-chain extension leads or plug a 4-way into another 4-way. The first plug still carries everything.
- Use the shortest, thickest lead you have. 2.5 mm² cable loses about half the voltage of 1.25 mm² over the same length.
- Compressors that trip breakers often start better on a shorter, thicker lead, or with a longer air hose instead of a longer extension lead.
- Welders: check your machine's duty cycle at the amps you weld at with the duty cycle calculator, and look up starting settings in the hobby welder settings guide.
- Running cost: the electricity cost calculator turns these watts into pence per job. Three-phase machines on a single-phase supply are covered by the phase converter sizer.
- New sockets or a garage supply should be designed and fitted by a registered electrician. In England and Wales a new circuit is notifiable work under Part P of the Building Regulations.
Frequently Asked Questions
About 3,000 W at 230 V (13 A × 230 V = 2,990 W), shared between all of its sockets, and only if its cable is rated for 13 A. Leads with 1.0 mm² cable are usually rated 10 A (about 2,300 W). Check the label, and unwind a reel fully before using it near its limit.
Many small inverter welders are sold with a 13 A plug, because their average current (I1eff on the rating plate) is under 13 A even though they draw more while the arc is on. At low and medium amps that is fine. Near the machine's maximum, long welds can blow the plug fuse, and bigger machines usually need a 16 A commando socket on its own circuit. Follow the maker's instructions and plate.
Its motor starts against air pressure and can draw several times its running current for a moment. A type B breaker trips instantly between 3 and 5 times its rating, so on a 16 A circuit a surge over about 48 A can trip it. A long or thin extension lead makes it worse by lowering the voltage, so the motor takes longer to start. Try a short, thick lead first; if it still trips, ask an electrician.
For a 13 A load, 1.25 mm² is fine for short runs, 1.5 mm² is better up to about 25 m, and 2.5 mm² is the choice for 50 m or for compressors and saws that struggle to start. Aim to lose no more than about 5 % of the voltage at full load; the calculator gives the longest length for your cable and load.
UK site transformers are centre-tapped to earth, so each conductor is only about 55 V from earth, which reduces the shock risk on wet, rough sites. The trade-off is current: a 110 V tool draws a little over twice the current of a 230 V tool of the same power, so leads need thicker cable for the same length. Check the transformer's continuous kVA against your tools.
You can, but the 13 A plug and fuse are then the limit, so a machine that really needs 16 A will blow the fuse or overheat the plug. A machine that is fitted with a 16 A plug by its maker should go on a proper 16 A socket.
Machine figures, power factors and start-up multipliers are typical values, not data for your machine. Cable resistances and ratings are approximate for copper flexible cable. This page is a guide to whether a load is reasonable, not a substitute for electrical design or an installation check by a qualified electrician.