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Motor Full Load & Starting Current

Full load current from the nameplate rating, then the starting current that the cable, protective device and any generator upstream actually have to cope with. Three-phase usesI = P / (√3 × V × pf × η).

Results

Full load current -

Electrical input power -

Apparent power -

Starting current -

Starting multiple applied -

Starting kVA -

Why the efficiency figure matters. A motor nameplate says what comes out of the shaft, not what goes in at the terminals. A 7.5 kW motor at 90 % efficiency draws about 8.3 kW of electrical power, and it is that larger figure the cable has to carry. Leaving efficiency out undersizes the circuit by roughly a tenth.

Worked example

7.5 kW three-phase motor, 400 V, pf 0.85, efficiency 90 %, DOL start.

Input power = 7.5 / 0.90 = 8.33 kW

FLC = 8330 / (1.732 × 400 × 0.85) = 14.1 A

Starting current = 14.1 × 6.5 = 91.9 A

On star-delta the same motor starts at roughly a third of that, about 30.6 A - which is the whole reason star-delta exists.

Starting multiples are typical values, not standard figures. Where the starting current is load-bearing - generator sizing, discrimination, or a marginal supply - get the actual figure from the motor data sheet. Verify protective device selection against BS 7671.

Regulation reference: BS 7671 Section 552 covers rotating machines, including the requirement that circuits supplying motors are suitable for both starting and running conditions.