BS 7671 Regulation 552 Compliant Motor Circuit Design
Calculate FLC, size cables, and select protection devices for DOL, Star-Delta, and Soft Start motor installations. Professional tool for electricians and engineers designing motor control panels and MCCs.
Accurate FLC calculation for single-phase and three-phase motors with automatic power factor and efficiency selection based on motor size.
Automatic cable sizing meeting 125% FLC requirement with voltage drop verification. Complete protection device recommendations (overload, MCB, fuse, contactor).
Compare DOL, Star-Delta, and Soft Start methods with starting current, cost, and suitability analysis. Get recommendations based on motor size.
Default: Auto (based on motor size)
Default: Auto (based on motor size)
BS 7671:2018+A2:2022 Regulation 552 covers the specific requirements for circuits supplying motors. Key requirements include:
The simplest and most common starting method for small motors. The motor is connected directly to full supply voltage via a contactor. Starting current is typically 6-8× FLC (use 7× for calculations).
Best for: Motors up to 11kW with infrequent starts and robust electrical supply. Common applications include workshop machinery, small pumps, and compressors with unloaders.
Protection sizing: Overload relay at 1.0-1.25× FLC, Type D MCB ≤400% FLC, gG fuses ≤200% FLC.
The motor starts in star configuration (reduced voltage: 57.7% of line voltage), then switches to delta for running. Starting current is reduced to approximately 1/3 of DOL (2-3× FLC, use 2.3× for calculations). Starting torque is also reduced to 33% of DOL torque.
Best for: Motors 5.5-75kW where starting current must be limited but full starting torque isn't critical. Ideal for centrifugal pumps, fans, blowers, and machine tools.
Protection sizing: Overload relay at 0.58× FLC (in-line with delta winding), Type D MCB ≤250% FLC, gG fuses ≤175% FLC. Requires 6-terminal motor with both ends of all three windings brought out.
Electronic control of voltage ramp-up using thyristors or solid-state switches. Starting current is adjustable (typically 1.5-3× FLC, use 2× for calculations) with smooth, controlled acceleration.
Best for: Any motor size, but especially motors over 30kW, applications with frequent starts, or where mechanical stress must be minimized. Suitable for conveyors, pumps with water hammer risk, and high-value equipment.
Protection sizing: Overload relay at 1.0-1.25× FLC, Type D MCB ≤300% FLC, gG fuses ≤180% FLC. Often includes integrated overload and short circuit protection.
Thermal or electronic device protecting the motor from prolonged overcurrent. Must trip within a time defined by the motor's thermal withstand curve. Class 10A (trips in 2-10 seconds at 7.2× setting) is standard for most motor applications.
Motor circuits require Type D MCBs due to high starting currents. Type D magnetic trip operates at 10-14× rating, preventing nuisance tripping during motor starts while providing short circuit protection. Must be coordinated with the overload relay.
AC3 is the standard utilization category for normal motor starting and stopping. Rated for making 8× rated current and breaking 8× rated current. The contactor rating must equal or exceed the motor FLC. For frequent starts, inching, or reversing, AC4 duty contactors may be required.
Motor cables must satisfy multiple requirements simultaneously:
| Power (kW) | Typical FLC (A) | Recommended Method | Typical Applications |
|---|---|---|---|
| 3.0 | 5-6 | DOL | Workshop machinery, small pumps |
| 7.5 | 13-15 | DOL or Star-Delta | Pumps, fans, compressors |
| 15 | 25-28 | Star-Delta | HVAC fans, industrial pumps |
| 22 | 36-40 | Star-Delta | Large fans, conveyor systems |
| 45 | 75-80 | Soft Start | Industrial conveyors, crushers |
| 132 | 220-240 | Soft Start or VSD | Large compressors, industrial processes |
Note: FLC values assume 400V three-phase supply, 0.85 power factor, and 0.90 efficiency. Actual values vary by manufacturer and motor design.
For three-phase motors: FLC = (Power in kW × 1000) / (√3 × Voltage × Power Factor × Efficiency). For single-phase: FLC = (Power in kW × 1000) / (Voltage × Power Factor × Efficiency). A typical 11kW three-phase motor at 400V with 0.85 PF and 0.90 efficiency draws approximately 21A FLC.
DOL (Direct-On-Line) is best for motors up to 11kW with infrequent starts. Star-Delta is ideal for 5.5-75kW motors where reduced starting current (2-3× FLC) is needed. Soft Start is recommended for motors over 30kW, frequent starts, or applications requiring smooth acceleration. Consider supply capacity, motor size, and starting torque requirements.
BS 7671 Regulation 552.1.1 requires motor cables to handle at least 125% of the motor Full Load Current. This accounts for motors running above nameplate rating, starting inrush, and ensures cables don't overheat during prolonged operation. The 25% margin provides a safety buffer for thermal protection.
For Star-Delta starting, the overload relay is sized at 0.58× FLC when placed in-line with the motor winding (delta configuration). This is because the line current in delta is √3 times the phase current. For a 22kW motor with 35A FLC, set the overload relay at approximately 20.3A (35 × 0.58). For DOL and Soft Start, use 1.0-1.25× FLC.
Motor circuits require Type D MCBs to withstand high starting currents without nuisance tripping. Type D trips at 10-14× rating, compared to Type C (5-7×) and Type B (3-5×). For DOL starting, size the MCB ≤400% of FLC. For Star-Delta, ≤250% FLC. For Soft Start, ≤300% FLC. The magnetic trip must exceed starting current.
AC3 is the standard duty rating for contactors used in normal motor starting and stopping. It accounts for breaking current during motor operation and making current during starts (typically 8× rated current). The contactor rating must equal or exceed the motor FLC. Higher duty ratings (AC4) are used for inching, plugging, or reversing applications with frequent starts.