A motor circuit has more parts than a cable and a breaker. It has a starter, an overload relay and an isolating switch too. This guide sizes all of them for one pump with the motor circuit calculator. The ideas are in Motor starters, contactors and overload relays and Motor circuits and starting.
The pump
- A 15 kW pump, 400 V three‑phase, power factor 0.85, efficiency 0.9.
- The board is 30 m away. The cable runs in enclosed air.
- The breaker is type D. The earth loop impedance at the board, Ze, is 0.35 Ω.
- The pump starts direct on line.
Step 1. Find the running and starting current
The full load current is 28.3 A. The maker gives no locked-rotor figure, so the tool takes 8 times full load current. That is 226.4 A (AS/NZS 3000:2018 Cl 4.13.1.2(i)). A direct on line start draws all of it.
Step 2. Pick the circuit-breaker and the cable
The smallest standard rating at or above 28.3 A is 32 A. A type D breaker does not trip on a start below about 10 times its rating. For 32 A, the tool takes that as 320 A. The 226.4 A start is below it, so the breaker rides through.
The cable size tool then sizes the cable for the 32 A breaker. The result is 16 mm² on 32 A type D. The voltage drop is 0.45 %.
Step 3. Pick the contactor and the relay
The tool gives a 32 A AC-3 contactor. The overload relay is set to 28.3 A, in the 23 to 32 A range. The isolating switch must be 28.3 A or more at AC-23. It can then break the locked-rotor current.
Step 4. Check the motor protection
At 15 kW the motor needs overload protection (Cl 4.13.2). It needs overtemperature protection too, because it is above 2250 W (Cl 4.13.3.1). A pump on a fire-protection service has neither (Cl 4.13.3.2).
Step 5. Compare the starters
Each row changes only the starter. The cable and the breaker stay at 16 mm² on 32 A type D.
| Starter | Starting current | Contactors | Relay setting and range |
|---|---|---|---|
| Direct on line | 226.4 A | Main 32 A | 28.3 A in 23 to 32 A |
| Star-delta | 75.47 A | Main 18 A, delta 18 A, star 12 A | 16.34 A in 12 to 18 A |
| Soft starter, limit 3.5 times | 99.05 A | Bypass 32 A | 28.3 A in 23 to 32 A |
| Drive | 42.45 A | None | Set in the drive, 28.3 A |
Open the star-delta case. A star-delta starter needs six cables to the motor. Each one carries 28.3 ÷ √3 = 16.34 A.
Step 6. Try type C
A type C breaker trips from about 5 times its rating. Open the same pump on type C. The 32 A rating no longer clears 226.4 A, so the tool moves up to 50 A. The result is 10 mm² on 50 A type C. The breaker is larger, but the cable is smaller. Type D suits this start better.
The result
| Item | Direct on line |
|---|---|
| Full load current | 28.3 A |
| Locked-rotor current | 226.4 A |
| Cable and breaker | 16 mm² on 32 A type D |
| Contactor, AC-3 | 32 A |
| Overload relay | 28.3 A in 23 to 32 A |
| Isolating switch, AC-23 | 28.3 A or more |
The contactor frames, relay ranges and breaker bands are generic. They are not from a standard. Check the maker's catalogue and coordination tables before you order. For the earth loop and the disconnection time, see Earth fault loop impedance and disconnection time. For the running current alone, use the motor calculator.