kW to Amps Calculator

The kW to Amps Formula — Single-Phase and Three-Phase

To run these conversions, you must first convert kilowatts back to watts by multiplying by 1,000. Once you have the power in watts, the calculation depends on the phase configuration of the supply circuit. Since industrial and heavy machinery draws power over multiple lines, the calculations incorporate phase shift factors.

If your appliance has a simple resistive heating element (like a hot water system or furnace), the relationship is direct because the voltage and current waveforms are in phase. For these simple situations, you can also use our standard watts-to-amps calculator.

For complex equipment, use the following formulas:

The value 1.732 represents the square root of 3 ($\sqrt3$), which mathematically accounts for the 120-degree phase displacement between the active lines of a three-phase system. In Australia, calculations use the nominal standard voltages of 230V for single-phase and 400V for three-phase.

Worked Example 1 — Current Draw of a 7.2 kW Residential EV Charger

Let's look at a common residential install: setting up a Level 2 single-phase EV wall box charger. The compliance plate lists the charger capacity at 7.2 kW. The electrician must calculate the continuous current draw to select the correct cables and protection device.

Worked Example 2 — Sizing a Three-Phase Supply for a 22 kW Elevator Motor

Now let's calculate the full load current for a commercial elevator motor rated at 22 kW, operating on a three-phase 400V supply with a nameplate power factor of 0.84.

Reading a Motor Nameplate — What Numbers Matter

When sizing supply lines, you need to extract specific numbers from the motor's metal nameplate:

• kW (Output Shaft Power): This is the mechanical power output of the shaft, not the electrical input. Because no motor is 100% efficient, the input power drawn is higher. Sizing calculations must account for this difference.
• Full Load Amps (FLA): The continuous current rating of the motor at rated load and nominal voltage. Sizing cables and thermal overloads is based directly on this value.
• Power Factor (Cos φ / PF): The decimal rating showing the efficiency of phase alignment. Inductive loads cause current to lag behind voltage, dropping PF to 0.80–0.85, which increases total current flow.
• Connection Type (Star vs Delta): Stamped as Δ/Y. Delta (Δ) is typically used for lower voltages, while Star (Y) is used for higher voltages (like 400V). Ensure you check how the terminal block bridges are configured.

To determine the complete load profiles across multiple sub-circuits for switchboard submission, combine individual motor currents into our maximum demand calculator. If you are feeding cables through conduit paths, check our conduit fill calculator to verify spatial spacing compliance.

Common Mistakes When Sizing kW Loads

• Assuming Power Factor is Unity (1.0): Treating inductive motor loads as simple heaters. Sizing a 15 kW motor circuit assuming PF = 1.0 yields a current estimate of 21.6A, whereas the actual current is 25.5A at PF = 0.85. This causes undersized cables that run hot.
• Forgetting Starting Inrush: Expecting standard breakers to survive motor starting currents. Failing to select Type C or Type D circuit breakers causes nuisance trips when motors start up under load.
• Ignoring Cable Grouping Deratings: Selecting a cable directly from standard tables without adjusting for ambient heat or layout. Running multiple power lines through a single conduit decreases their thermal capacity, requiring derating under AS/NZS 3008.
• Confusing Shaft Output with Electrical Input: Sizing cables based on the mechanical shaft power rating instead of the electrical input. Use manufacturer motor efficiency parameters to calculate the true input load.

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