Convert between power, current and kVA for DC, single-phase and three-phase circuits — with power factor. Add a cable length to get the voltage drop and a suitable BS 7671 cable size. Free, no sign-up.
This converter gives the steady-state design current for a stated power and power factor. Motor starting currents, harmonics, diversity and a compliant cable design to BS 7671 are separate questions — see our ProDesign cable calculation service.
For a three-phase AC load: I = P × 1000 / (√3 × V × cos φ). A 30 kW motor on a 400 V three-phase supply at 0.85 power factor draws 30,000 / (1.732 × 400 × 0.85) ≈ 51 A. The same 30 kW on a single-phase 230 V supply would draw about 153 A — which is why larger loads are three-phase.
Cables and protective devices see current, not useful power. At 0.8 power factor a load draws 25% more current than the same kW at unity — so an optimistic power factor assumption undersizes everything downstream. If you don't know the power factor, 0.95 is a reasonable assumption for modern equipment with power factor correction; motors without correction are typically 0.8–0.85 at full load.
Real power (kW) does the work; apparent power (kVA) is what the network must carry; reactive power (kVAr) is the difference that circulates without doing work: kVA = kW / cos φ, and kVAr = √(kVA² − kW²). Transformers and generators are rated in kVA — another reason the power factor of your load schedule matters.
Once you know the design current, add a cable length and the tool sizes a suitable BS 7671 cable. Voltage drop is Vd = (mV/A/m) × I × L ÷ 1000, where the mV/A/m figure comes from Appendix 4 for the cable type and size. BS 7671 limits the drop between the origin and the load to 3% for lighting and 5% for other uses — on a 400 V supply that is about 12 V and 20 V. Long runs are usually decided by voltage drop, not by the current-carrying capacity, so a cable that is fine on ampacity can still need to go up a size or two over distance.
The tool picks the smallest size that satisfies both the current-carrying capacity — after ambient (Ca) and grouping (Cg) derating, plus soil and depth factors for buried runs — and the voltage-drop limit. It covers copper and aluminium conductors in PVC or XLPE single-core, multicore/twin & earth, single- and multicore armoured (SWA), flexible/rubber and mineral-insulated (MICC) cable, across BS 7671 installation reference methods A, B, C, D (buried), E, F and G. A three-phase circuit is sized on the three/four-core column of the Appendix 4 tables and a single-phase one on the two-core column — the same cable is rated roughly 10–15% lower on three phases, which is often a whole size. The 90 °C ratings of XLPE (the 4E tables) may only be used where the connected equipment is rated for 90 °C operation (Reg 512.1.5); otherwise the 70 °C column applies. This is an indicative first pass — it does not run the earth-fault-loop (Zs) or adiabatic checks. For a compliant, submission-ready result use the BS 7671 cable sizing calculator or our ProDesign cable calculation service.
At unity power factor. Divide by your own power factor for an inductive load — at cos φ 0.85 the current is about 18% higher.
| Power | 230 V single-phase | 400 V three-phase |
|---|---|---|
| 1 kW | 4.3 A | 1.4 A |
| 2 kW | 8.7 A | 2.9 A |
| 3 kW | 13.0 A | 4.3 A |
| 4 kW | 17.4 A | 5.8 A |
| 5 kW | 21.7 A | 7.2 A |
| 6 kW | 26.1 A | 8.7 A |
| 7 kW | 30.4 A | 10.1 A |
| 7.5 kW | 32.6 A | 10.8 A |
| 9 kW | 39.1 A | 13.0 A |
| 10 kW | 43.5 A | 14.4 A |
| 11 kW | 47.8 A | 15.9 A |
| 15 kW | 65.2 A | 21.7 A |
| 18.5 kW | 80.4 A | 26.7 A |
| 22 kW | 95.7 A | 31.8 A |
| 30 kW | 130.4 A | 43.3 A |
| 37 kW | 160.9 A | 53.4 A |
| 45 kW | 195.7 A | 65.0 A |
| 55 kW | 239.1 A | 79.4 A |
| 75 kW | 326.1 A | 108.3 A |
| 90 kW | 391.3 A | 129.9 A |
| 110 kW | 478.3 A | 158.8 A |
Divide the power in watts by the line voltage times the square root of three times the power factor: I = kW x 1000 / (1.732 x V x cos phi). At 400 V and a power factor of 0.95, a 30 kW load draws about 45.6 A. On single-phase the root-three disappears and you divide by the voltage alone, so the same 30 kW at 230 V would draw about 137 A. On DC there is no power factor at all and the sum is simply watts divided by volts.
Power factor is the ratio of real power (kW, the part doing work) to apparent power (kVA, the part the cable and the supply actually have to carry). At a power factor of 0.8 the current is 25% higher than it would be at unity for the same useful output, because the reactive component still flows in the conductors even though it does no work. That extra current is what heats the cable and drops the voltage, so it has to be included when sizing anything.
No, and this catches people out. A motor nameplate states SHAFT power - the mechanical output - so the electrical input is the shaft rating divided by the efficiency. An 11 kW motor at 90% efficiency draws about 12.2 kW electrically, roughly 10% more current than the nameplate figure suggests. Switch the load type to Motor and enter the efficiency from the nameplate, and the calculator works from the input power instead.
Yes. The BS 7671 Appendix 4 tables print two capacity columns for most cable types: one for a two-core cable on single-phase, and a lower one for a three- or four-core cable on three-phase. The three-phase column runs roughly 10 to 15% lower because three loaded conductors generate more heat than two. The calculator picks the column matching the supply you selected and tells you which one it used, because sizing a three-phase circuit on the single-phase column can put you a whole cable size out.
BS 7671 Appendix 4 Table 4Ab gives 3% for lighting circuits and 5% for everything else, measured from the origin of the installation to the load - so a sub-main and its final circuit share that allowance between them. On a 400 V supply those work out at about 12 V and 20 V. Long runs are usually decided by voltage drop rather than current-carrying capacity, which is why a cable that is comfortable on ampacity can still need to go up a size or two over distance.
Only when every piece of equipment the cable lands in is rated for 90 degree C operation. Regulation 512.1.5 requires the cable to be rated at the maximum operating temperature of the equipment or accessory it connects to, so if that is a 70 degree C terminal then the 70 degree C column applies no matter what the cable itself can stand. Most accessories are 70 degree C. Tick the 90 degree C option only if you have confirmed it; otherwise the calculator sizes on the 70 degree C column, which is the safe assumption.
Treat it as an indicative first pass. It sizes against the load current rather than a protective device rating, and it does not run the earth-fault loop impedance (Zs), adiabatic or thermal-insulation checks that a compliant design needs. The reference data is a subset of BS 7671 and must be verified against your own copy before anything is issued. For a submission-ready result use the full BS 7671 cable sizing calculator on this site, or the ProDesign service.
kW is real power, the part that does useful work. kVA is apparent power, the vector sum of real and reactive power, and it is what determines the current a cable carries and the rating a transformer or generator needs. kVAr is reactive power, which flows back and forth without doing work but still occupies capacity. They form a right-angled triangle: kVA is the hypotenuse, kW the base, kVAr the height, and the angle between kVA and kW is the phase angle whose cosine is the power factor.