A workplace adds a 22 kW three-phase charger to a fleet car park. The nameplate gives kilowatts. The cable schedule and the supply application both need amps.
The question
The circuit has to carry whatever a 22 kW three-phase charger draws from a 400 V board. That one number sets the design current for the circuit. It also sets the entry in the demand calculation.
The inputs
- Phases: 3
- Line voltage: 400 V
- Rating kind: kW
- Value: 22
- Power factor: 1
- Efficiency: 1
An AC charger controls its own input stage. It draws close to unity power factor at full output. Efficiency 1 tells the tool to treat 22 kW as power drawn at the board. The losses in the charger and the losses in the vehicle sit outside this calculation.
The clause and the table
Cl 7.9.3 of AS/NZS 3000:2018 covers the electric vehicle charging circuit. Read it for the circuit arrangement and the residual current device type. Table C2 group C(ii) of AS/NZS 3000:2018 is the demand group that carries a charger in a non-domestic installation.
Neither reference gives a current. They tell you where the current belongs once you have it.
The steps
The tool works in three moves.
- Output power: 22 kW.
- Apparent power drawn: 22 kVA. A power factor of 1 makes the two figures equal.
- Full load current: 31.75 A. The tool divides 22 000 VA by 400 V and by the square root of three.
The square root of three is the only three-phase term in the line. It appears because the tool works from the line voltage, not the phase voltage. Drop that term and the answer comes out much too high.
The result
The charger draws 31.75 A per phase. The tool reports no warnings for these inputs.
Three things follow from that figure.
The device. 31.75 A sits just under the 32 A frame size. A 32 A device leaves very little margin. Check the charger instructions before you settle on a rating.
The cable. Carry 31.75 A into the cable size tool as the load current. Then add the route: the length, the install method, the ambient temperature and the grouping. The cable size tool applies the derating and the voltage drop check together.
The demand. Carry the same figure into the maximum demand tool under Table C2 group C(ii). A single charger on a large board changes the total less than people expect. Several chargers on one submain change it a lot.
Power factor moves the answer. A charger that runs below unity draws more current for the same kilowatts. Set the power factor from the nameplate rather than from habit. Efficiency works the other way. Set it below 1 only when the kilowatt figure on the plate is output power, not input power.
The current also scales with the voltage. The same charger on a 415 V board draws less current, and on a 380 V board it draws more. Run the number again rather than adjusting it in your head.
This page is a design aid. Verify every value against the current edition of the standard.
See also: EV charger cable size for a 7 kW 32 A circuit, AS/NZS 3008.1.2 Tables 22 to 29: derating and Current-carrying capacity and derating.