How do I find the full load current (FLC) of a motor?

The FLC should be taken from the motor nameplate. As a guide for three-phase 400V motors: 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. These are approximate — actual FLC depends on motor efficiency and power factor. Always use nameplate data for design calculations.

Why do motors need Type C or Type D MCBs?

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. Type D 20A MCB trips at 200-400A, so 112A inrush won't cause nuisance tripping.

What overload protection setting should I use?

Overload relays should be set to 100-105% of the motor nameplate FLC per BS 7671 Regulation 435. 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 overload has a thermal trip characteristic that 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.

When should I use a VFD instead of a DOL starter?

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 require Type F RCD protection.

Does voltage drop matter more for motor circuits?

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.

TradeCalculator

Motor Circuit Calculator — BS 7671

Size the complete motor circuit: determine FLC from motor kW, select MCB type and rating, set overload protection, choose cable size, and check voltage drop.

Motor Rating

Three-phase 400V motor

Starter Type

Determines inrush and MCB type

Cable Run Length

MCC/DB to motor

How We Calculate This

This calculator sizes all components of a motor circuit from the motor kW rating, following BS 7671 requirements for motor installations.

Motor Circuit Design

FLC: From motor nameplate or reference tables
MCB: Type D for DOL/Star-Delta, Type C for VFD/soft-start
MCB rating: Next standard size above FLC
Overload: Set to 100-105% of FLC
Cable: Must carry MCB rating, check VD
VD limit: 5% total (20V on 400V three-phase)

Frequently Asked Questions

Related Calculators

Cable Sizing Calculator

Calculate the correct cable size based on BS 7671 18th Edition regulations.

Voltage Drop Calculator

Check voltage drop complies with UK wiring regulations.

Maximum Demand Calculator

Calculate maximum electrical demand for domestic installations.

Circuit Breaker Sizing Calculator

Determine the correct MCB rating for your circuit.

Socket Spacing Calculator

Plan socket layouts with Part P guidance on spacing and positioning.

Last updated: March 2026

All calculations are estimates. Verify with your supplier.