Circuit Load Calculator
For a single-phase circuit the current is the load divided by the voltage times the power factor: I = P ÷ (V × PF).
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the total connected load in watts (add up the wattage of everything on the circuit).
- Enter the supply voltage and the breaker (circuit) rating in amps.
- Optionally set a power factor (use 1 for purely resistive loads like heaters and incandescent lamps).
- Read the current draw, percentage of the breaker used, and whether you are within the 80% continuous limit.
How it works
For a single-phase circuit the current is the load divided by the voltage times the power factor: I = P ÷ (V × PF). The percentage of the breaker used is then the current divided by the breaker rating. A power factor of 1 applies to resistive loads; motors, LED drivers and switch-mode supplies draw more current for the same watts, so a lower power factor raises the current.
The 80% continuous-load rule (common in AS/NZS, NEC and IEC-based codes) says a circuit protected by a standard breaker should carry no more than 80% of its rating for a continuous load — one running for three hours or more. This calculator flags whether your draw is within that continuous limit, over it, or above the breaker rating itself. Always confirm requirements against your local wiring rules and have circuits designed or checked by a licensed electrician.
Worked example
1,800 W on a 230 V, 20 A circuit. At power factor 1.0, current = 1,800 ÷ (230 × 1) = 7.83 A. That is 39.1% of the 20 A breaker, with 12.17 A spare. The 80% continuous limit is 16 A (3,680 W), so this load is comfortably within limits.
Common mistakes
- Adding up amps from mixed-voltage appliances instead of adding watts first — always total the watts, then divide by the circuit voltage once.
- Assuming a 20 A breaker can safely carry a continuous 20 A load; the 80% rule limits a continuous load to about 16 A.
- Leaving power factor at 1 for motors, LED drivers or electronics — their lower power factor means the real current draw is higher than watts ÷ volts suggests.
Frequently asked questions
How do I calculate the current on a circuit?
For a single-phase circuit, current = watts ÷ (volts × power factor): I = P ÷ (V × PF). Total the load in watts, then divide by the supply voltage (and power factor for non-resistive loads). For example 1,800 W at 230 V and PF 1 draws 1,800 ÷ 230 = 7.83 A.
What is the 80% continuous load rule?
Codes based on AS/NZS, NEC and IEC limit a continuous load (running 3 hours or more) to 80% of the breaker rating, so a 20 A breaker should carry at most 16 A continuously. This tool flags whether your draw is within that limit, over it, or above the breaker rating.
Do I need to set a power factor?
Use a power factor of 1 for purely resistive loads like heaters and incandescent lamps. Motors, LED drivers and switch-mode supplies have a lower power factor (often 0.6–0.95), which raises the real current for the same wattage — set it to match your load.
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