Checks a three-phase motor's starting current and transient voltage dip on its supply cable: full-load and starting current for the chosen starting method (DOL, star-delta, soft-start, VSD), and the BS 7671 Appendix 4 mV/A/m dip check against your allowable limit.
Design-stage only. Verify starting method and cable sizing against manufacturer data and BS 7671.
For design guidance only. Always verify with a qualified engineer and manufacturer starting data.
About this motor starting current calculator
This free motor starting current calculator checks a three-phase motor’s full-load and starting current and the transient voltage dip it causes on its supply cable, per BS 7671 Appendix 4. It is aimed at electrical designers and contractors verifying that a motor start will not dip the supply enough to trip contactors or disturb other loads. Enter the motor rating, voltage, efficiency and power factor, the starting method (DOL, star-delta, soft-start or VSD), and the cable mV/A/m figures and length, and it returns the full-load current, starting current, apparent power and the volt-dip check against your limit. Everything runs in your browser. Nothing is uploaded.
How motor starting current is calculated
Full-load current is IFL = P·1000 ÷ (√3·V·η·pf), treating the kW rating as mechanical output (electrical input = P÷η). Starting current is Ist = IFL × k, where k is the starting-method multiplier (DOL ~6–8×, star-delta ~2–3×, soft-start ~2–4×, VSD ~1–1.5×). The transient dip follows BS 7671 Appendix 4 §6: the design mV/A/m at the starting power factor is cosφst·(mV/A/m)r + sinφst·(mV/A/m)x, and the dip is ΔV = mV/A/m × Ist × L ÷ 1000, compared as a percentage against your 10% or 15% limit.
Frequently asked questions
What starting multiplier should I use?
Picking a starting method fills a typical multiplier, but the engine uses the number in the field, so override it with manufacturer data where you have it. DOL is roughly 6–8× full-load current, star-delta about one third of that, soft-starters typically 2–4× and VSDs 1–1.5×. The multiplier drives the starting current and therefore the whole volt-dip result.
Should the dip limit be 10% or 15%?
10% is the classic planning guideline for a comfortable start; 15% is closer to the contactor hold-in floor from IEC 60947-4-1, below which coils may drop out. Pick the one your design or DNO requires, the verdict compares the calculated dip percentage against whichever limit you set, on the raw figures.
When do I need the reactive (x) mV/A/m column?
For small cables (≤16 mm²) reactance is negligible, so leave x at 0 and the resistive figure dominates. For larger cables (≥25 mm²) the reactive component matters, especially at the low starting power factor where sinφ is large. Enter the x column from the BS 7671 tables so the effective mV/A/m reflects it. Always verify against manufacturer starting data.