kW to amps calculator, single and three phase (UK)
To convert kW to amps on a single-phase 230 V supply, divide the watts by 230 × the power factor: a 9.5 kW shower draws 41.3 A. On a three-phase 400 V supply, divide by √3 × 400 × the power factor, and the answer is the current in each line.
The shower is 9.5 kW: what does it draw, and what breaker does that need? Type the one you know and read the other, at any voltage and power factor.
P = V × I × pf on one phase and √3 × V × I × pf across three, with the UK's 230 V and 400 V nominal voltages. Checked against BS 7671:2018+A4:2026, updated 3 September 2026. Free, and nothing to sign in to.
The rating on the appliance plate. 9.5 kW shower, 3 kW immersion, 7 kW EV charger.
230 V line to neutral; 400 V line to line on three phase.
Current
41.3 A
At the voltage and power factor set.
- Real power
- 9.50 kW
- Current
- 41.3 A
- Apparent power What the supply has to deliver.
- 9.50 kVA
- Next standard device rating A starting point for the device, never a cable size.
- 45 A
How do you convert kW to amps?
On a single-phase circuit, power is voltage times current times power factor: P = V × I × pf. Turn it round and the current a load draws is I = P ÷ (V × pf). At the UK's nominal 230 V a 3 kW kettle draws 3,000 ÷ 230 = 13 A, which is why 13 A is the plug fuse.
Across a balanced three-phase load the same power is shared by three lines, so P = √3 × V × I × pf with V the line-to-line voltage, 400 V, and I the current in each line. The √3 is the geometry of three phases 120° apart; it is why a three-phase load draws less per line than the same load on one phase.
Power factor
Anything that heats, from showers to cookers to immersion heaters, has a power factor of 1: all the current does work. Motors, drives and some lighting draw more current than their power alone needs, and the power factor on the nameplate, typically 0.8 to 0.9, says how much more. The current the supply must carry is the apparent power in kVA; the work done is the real power in kW; and kVA × pf = kW.
The device rating suggested is the smallest standard rating at or above the current. It is where the design starts, not where it ends: the cable then has to carry that rating after the correction factors for how it is installed, pass the voltage drop, and meet the Zs limit.
Worked example
A 9.5 kW electric shower on a single-phase 230 V supply.
- I = 9,500 W ÷ (230 V × 1.0) = 41.3 A.
- The next standard device rating is 45 A: a B45, or a B50 where 45 is not stocked.
- The cable is then sized for the device: 10 mm² is the usual answer on a typical domestic run, checked for volt drop and Zs.
A 7.5 kW three-phase motor at a power factor of 0.85: I = 7,500 ÷ (1.732 × 400 × 0.85) = 12.7 A in each line, drawing 8.8 kVA from the supply.
Questions the trade asks
On a single-phase circuit, amps = watts ÷ (volts × power factor). A 3 kW kettle at 230 V with a power factor of 1 draws 3,000 ÷ 230 = 13 A. A 9.5 kW shower draws 41.3 A.
Amps = watts ÷ (√3 × line voltage × power factor), with the line-to-line voltage of 400 V, and the answer is the current in each of the three lines. A 7.5 kW motor at a power factor of 0.85 draws about 12.7 A per line.
1 for anything that heats: showers, cookers, immersion heaters, most lighting. 0.8 to 0.9 for motors and drives without correction; the nameplate will say. A lower power factor means more current for the same power.
kW is the real power doing work; kVA is the apparent power the supply has to deliver, which is larger when the power factor is below 1. kVA × power factor = kW. Supplies and transformers are rated in kVA because it is the current, not the work, that heats them.
Not on its own. It gives the design current, Ib, which is the first step: the protective device is chosen to be at least that, and the cable then has to carry the device's rating after the correction factors for how it is installed, pass the voltage drop check, and meet the Zs limit. The suggested rating here is a starting point for the device, never a cable size.
Next door
- Voltage drop calculator to BS 7671Voltage drop on a copper circuit to BS 7671: mV/A/m, volts and percent against the 3% or 5% limit, and the longest run that passes. Free, no sign-up.
- Maximum demand calculator with diversityAssess the maximum demand of a house after diversity: lighting, sockets, cooker, shower, EV charger, against a 60, 80 or 100 A supply. Free, no sign-up.
- Max Zs calculator: maximum Zs for MCBs, RCBOs and fusesMaximum earth fault loop impedance for any MCB, RCBO or fuse to BS 7671, with the 80% figure a site reading is judged against. Free, no sign-up.
The figures go straight onto the certificate
In Pascal the maximum Zs is filled in for every circuit as you pick the device, the schedule of test results carries all 32 columns, and the observation library suggests the wording, code and regulation as you type. EICs, EICRs and minor works, from £25 a month.
See the certificate softwareA calculator applies a method to the numbers you give it. It does not know the installation in front of you, and it is no substitute for BS 7671, the guidance or the judgement of the person signing the certificate. Check anything you rely on.