Motor Circuit Calculator — BS 7671

The FLC must be taken from the motor nameplate. As a conservative design guide for three-phase 400V motors (figures pitched slightly above typical 415V chart values so the protection errs safe): 0.75kW ≈ 2.1A, 1.5kW ≈ 3.7A, 3kW ≈ 6.9A, 5.5kW ≈ 12.2A, 7.5kW ≈ 16A, 11kW ≈ 23A, 15kW ≈ 30A, 22kW ≈ 43A, 30kW ≈ 57A, 37kW ≈ 70A, 45kW ≈ 85A, 55kW ≈ 103A, 75kW ≈ 139A. Actual FLC depends on the motor's efficiency and power factor, so these are approximate only — always use nameplate data for the final design.

Motors draw a high inrush current at start-up — typically 6-8 times FLC for DOL starters. A Type B MCB (3-5× trip) would nuisance-trip on every motor start. Type C (5-10×) handles moderate inrush from soft starters and VFDs. Type D (10-20×) is needed for DOL and star-delta starters. For a 7.5kW motor (16A FLC) with DOL starter: inrush = 112A and the next standard MCB rating is 16A. A 16A Type D MCB trips instantaneously at 10-20× = 160-320A, so the 112A inrush stays below the trip threshold and won't cause nuisance tripping. Use our Circuit Breaker Sizing Calculator for general MCB sizing guidance.

Overload relays should be set to 100-105% of the motor nameplate FLC. BS 7671 covers overload protection in Section 433 (coordination of conductor and protective device, Reg 433.1.1) and motor circuits in Section 552 — there is no 'Regulation 435'; always follow the relay manufacturer's instructions and the overload class (IEC 60947-4-1). The overload protects against sustained overcurrent (not short-circuit — that is the MCB's job). For a 16A FLC motor: set overload to 16.0-16.8A. The thermal trip characteristic allows brief overloads (starting) but trips on sustained overcurrent. Class 10 overloads trip in under 10 seconds at 7.2× setting, suitable for most applications.

VFDs (Variable Frequency Drives) are preferred when: variable speed is needed (fans, pumps), soft starting is required (reduce mechanical stress), energy savings are important (fan/pump laws — halving speed reduces power by 87.5%), the supply cannot handle high inrush currents, or the motor needs precise speed/torque control. VFDs also eliminate the need for star-delta starters and provide built-in motor protection. Downside: VFDs generate harmonics, and a three-phase drive can produce smooth DC residual current that blinds Type A/AC/F devices — so where RCD protection is required, a three-phase VFD typically needs a Type B RCD (BS 7671 Reg 531.3.3); always follow the drive manufacturer's instructions. See our Power Factor Calculator to assess the impact on your installation.

Yes — motors are particularly sensitive to low voltage. A 5% voltage drop at the motor terminals causes approximately 10% reduction in starting torque (torque is proportional to voltage squared). BS 7671 limits total voltage drop to 5% (20V on 400V three-phase). For long cable runs to motors (e.g., pump houses, outbuildings), voltage drop often determines the cable size rather than current capacity. Always check VD for motor circuits over 30m.