Motor Circuit Calculator
Cable sizing and protection for motor circuits
Reference Info & Formulas
DOL: 6× FLC, simple
Star-Delta: 2× FLC, reduced torque
Soft Starter: 3× FLC, smooth
VFD: 1.5× FLC, best control
• Cable sized for 1.25× FLC
• Type D MCB for DOL start
• Overload at 1.05× FLC
BS 7671 Section 552
Data: BS 7671:2018+A2:2022 — Section 552, Appendix 4
For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.
How This Calculator Works
Motor circuit design sizes cable to full load current, selects a starter type, and coordinates overload and short-circuit protection.
Unlike most circuits where cable is sized to the protective device rating, motor circuits use a split protection approach. The cable is sized to the motor's full load current (FLC), and a thermal overload relay set at or near FLC protects against sustained overcurrent. The MCB or fuses provide short-circuit protection only.
Starting current for DOL motors is typically 6-8 times FLC, lasting 2-10 seconds depending on motor size and load inertia. The protective devices must be coordinated so the starting transient does not cause nuisance tripping, while genuine faults are still cleared rapidly.
Quick Reference — Starting Methods
Motor starting methods and their characteristics
BS 7671 Section 552| Starting Method | Starting Current | Starting Torque | Notes |
|---|---|---|---|
| DOL (Direct On Line) | 6-8× FLC | 100% rated | Simple, high inrush, up to ~5.5kW typical |
| Star-Delta | 2-3× FLC | 33% rated | Reduced torque, requires 6 cables to motor |
| Soft Starter | 2-4× FLC | Variable | Smooth ramp, reduced mechanical stress |
| VFD (Variable Frequency Drive) | 1-1.5× FLC | Full at all speeds | Full speed control, generates harmonics |
Starting current multiples are approximate and depend on motor design and connected load.
Protection Coordination
Motor circuit protection requires coordination between two devices: the short-circuit protective device (MCB or fuses at the distribution board) and the thermal overload relay (at the motor starter). The MCB is rated above the starting current to avoid nuisance tripping, while the overload relay is set at or near FLC to protect against sustained overcurrent.
This split approach means the cable only needs to be sized for the motor FLC rather than the MCB rating, reducing cable costs on larger motor circuits. The overload relay acts as the cable's running protection.
Practical Notes
Type D MCBs for Motor Circuits
Star-Delta Overload Setting
VFD Harmonic Considerations
Frequently Asked Questions
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