EV Charging Load Calculator
Size the circuit for an EV charger by converting its power rating into a supply current and applying the 125% continuous-load rule. It handles single-phase and three-phase supplies and returns the charging current and the minimum circuit rating.
Enter Values
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How to use this calculator
- Enter the charger power in kW and the supply voltage in volts.
- Set the number of phases to 1 or 3 (default 1) to match your supply.
- Read the charging current and the minimum circuit rating at 125%.
How it works
For a single-phase supply the current is the charger power in watts divided by the voltage. For three-phase it is the power divided by the square root of 3 times the line voltage. Because EV charging draws current continuously for hours, the circuit and protection are sized at 125% of the charging current, so the minimum rating is the current multiplied by 1.25.
Worked example
Worked example. A 7.2 kW single-phase charger on 230 V draws 7200 / 230 = 31.3 A. At 125% the minimum circuit rating is 31.3 * 1.25 = 39.13 A, so a 40 A circuit is the practical minimum.
Common mistakes
- Sizing the circuit at the raw charging current instead of 125% for a continuous load.
- Using the wrong phase setting, which changes the current by a factor of the square root of 3.
- Adding the charger to a supply that is already near capacity without checking spare headroom.
Frequently asked questions
Why multiply by 1.25?
EV charging runs at close to full current for three hours or more, which wiring rules classify as a continuous load. The circuit and overcurrent protection must be rated for at least 125% of that current to avoid overheating.
Which voltage should I enter for three-phase?
Use the line-to-line voltage, typically 400 V, together with 3 phases. The calculator applies the square-root-of-3 factor to convert three-phase power to line current.
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