A motor circuit has two design currents, not one. The motor draws its full load current for hours and its starting current for seconds. The cable answers to the first. The device and the supply answer to the second.
Full load current
Full load current, or FLC, is the current the motor draws at rated output. It follows from the shaft power, the supply voltage, the power factor and the efficiency. Both the power factor and the efficiency belong to the motor, so both come off the nameplate.
FLC is the design current, IB, for the cable. Cl 2.5.3.1 of AS/NZS 3000:2018 sets the coordination between the device rating and the cable capacity around it.
Starting current
A motor started direct on line draws a large current until it approaches speed. The tool assumes six times FLC for a DOL start, and it states that assumption in every result.
The starting current does not size the cable. It lasts for seconds, and the conductor has thermal mass. It does size two other things. The device has to carry the inrush without tripping. The supply has to hold its voltage while the motor accelerates.
A worked motor
Take a 22 kW four‑pole motor on a 400 V three‑phase supply. Power factor 0.86, efficiency 0.92, 50 Hz, slip 2 %.
The tool reports:
- Full load current: 40.13 A
- Starting current, DOL: 240.81 A
- Synchronous speed: 1,500 rpm
- Shaft speed: 1,470 rpm
- Full load torque: 142.93 Nm
The shaft speed sits below the synchronous speed by the slip. That difference is what produces torque in an induction motor.
Open the motor calculator with these inputs.
The cable for that motor
Now take the FLC into the cable size tool. The inputs are:
- Design current: 40.13 A
- Route length: 35 m
- Cable: four‑core copper, enclosed in air
- Ambient temperature: 35 °C
- Device: 50 A curve C
- External loop impedance: 0.35 Ω
The tool returns:
- Ambient derating factor: 0.94
- Total derating factor: 0.94
- Required tabulated current: 42.69 A
- Selected conductor: 16 mm²
- Derated capacity: 62.04 A
- Voltage drop: 0.79 %
- Earth fault loop impedance: 0.53 Ω
- Earth conductor: 6 mm²
Notice which check drove the size. The load needs 42.69 A of tabulated capacity after derating. The 50 A device needs the cable to carry 50 A after derating, which is the larger requirement. The device, not the motor, set the conductor at 16 mm².
That is the ordering rule at work. IB is 40.13 A, IN is 50 A and IZ is 62.04 A, so 40.13 ≤ 50 ≤ 62.04 holds.
Open the cable size calculator with these inputs.
Starting methods and what they change
Direct on line. Full voltage to the stator. Highest starting current, highest starting torque, simplest starter.
Star-delta. The stator starts in star and transfers to delta. Starting current and starting torque both fall to about a third. It suits a load that starts unloaded, such as a fan on a closed damper.
Soft starter. Thyristors ramp the voltage. Current falls in proportion to the voltage and torque falls with the square of it.
Variable speed drive. The drive controls frequency and voltage together, so the motor accelerates near full torque at a current close to FLC. A drive also changes the harmonic content of the supply current and the earthing arrangement of the circuit.
The tool models a DOL start. For any other method, take the starting current from the starter data rather than from the six-times figure.
Voltage drop while starting
Cl 3.6.2 of AS/NZS 3000:2018 sets the steady-state drop limit. A motor circuit has a second case that the limit does not describe.
Put the starting current of 240.81 A into the voltage drop tool. Keep the same 16 mm² cable and the same 35 m, and enter the motor's power factor of 0.86. The drop is 18.22 V, or 4.55 %, against the same 5 % limit. The running drop was 0.79 %.
That sag reaches every other load on the board while the motor accelerates. A long run to a large motor is checked at starting current as well as at FLC.
Demand and the rest of the board
Table C1 group L and Table C2 group D cover motors in the maximum demand tables. The demand figure is not the sum of the nameplates, because the tables apply diversity to the group.
This page is a design aid. Verify every value against the current edition of the standard.