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How this is calculated#

The tool sizes the circuit for one motor: the cable, the circuit-breaker, the contactor, the overload relay and the isolating switch. It reads AS/NZS 3000:2018 Cl 4.13 and sizes the cable with the cable size tool.

1. Full load current. It divides the shaft power by the supply factor, the power factor and the efficiency. This is the same figure as the motor tool.
2. Locked-rotor current. It takes the maker's multiple of full load current. With none, it takes 8 times full load current (Cl 4.13.1.2(i)).
3. Starting current. Direct on line, the start equals the locked-rotor current. Star-delta gives a third of it. A soft starter gives the limit you enter. A drive gives about 1.5 times full load current (generic).
4. Circuit-breaker. It picks the smallest standard rating at or above full load current. The low end of its instantaneous band must clear the start. The bands are generic: type B 3, type C 5 and type D 10 times the rating.
5. Cable. The cable size tool sizes it with that breaker. The cable current capacity must be at least the breaker rating. It also checks the voltage drop and the earth loop with the Ze you type.
6. Contactor and overload relay. It picks an AC-3 frame and a relay range from generic lists. Direct on line and soft start use full load current. Star-delta uses full load current divided by √3 for the main contactor, the delta contactor and the relay. The star contactor uses full load current divided by 3.
7. Isolating switch. It must carry full load current and break the locked-rotor current. An AC-23 switch at or above full load current does both (Cl 4.13.1.2).
8. Motor protection. Overload protection applies above 370 W (Cl 4.13.2). Overtemperature protection applies above 2250 W, or above 240 VA for a motor that runs unattended (Cl 4.13.3.1). A motor on a fire-protection service has neither (Cl 4.13.3.2).

Worked example#

• 15 kW, 400 V, three‑phase pump, direct on line, 30 m, type D. The tool gives a full load current of 28.3 A and a locked-rotor current of 226.4 A. The result is a 16 mm² cable on a 32 A type D circuit-breaker. The contactor frame is 32 A. The relay setting is 28.3 A in the 23 to 32 A range. The isolating switch must be 28.3 A or more at AC-23.

Limits#

• The contactor frames, relay ranges, breaker bands and the soft starter and drive starting currents are generic. They are not from a standard. Check the maker's catalogue and coordination tables.
• The locked-rotor figure of 8 times full load current is a default. Use the maker's figure when you have it.
• The cable is sized to the breaker rating. Where the overload relay protects the cable, a smaller cable can be allowed.
• Star-delta needs six cables to the motor. Each carries full load current divided by √3.
• A drive can draw a different input current. Size the supply from the drive data sheet.
• The tool does not check the prospective fault current or the breaker breaking capacity.

See also: Motor circuit for a 15 kW pump: DOL or star-delta, Motor starters, contactors and overload relays, Motor circuits and starting, the motor calculator and the cable size calculator.