Circuit Load Calculator
The current a circuit will carry from the watts of everything on it, and the breaker rating that suits — with the 80% continuous-load rule and the standard breaker sizes for your supply.
Current is watts divided by volts (and by power factor for motors and fluorescent gear).
How the circuit load calculator works
Current is watts divided by volts (and by power factor for motors and fluorescent gear). A breaker should not run continuously above 80% of its rating, so a load that runs for hours is sized against 0.8 × the breaker. Pick the next standard size up: 6, 10, 16, 20, 25, 32, 40 A on 230 V systems; 15, 20, 30, 40, 50 A on 120 V. The cable must be rated for the breaker, not the load — check the wiring rules for your country.
Formula: I = P ÷ (V × PF); breaker ≥ I ÷ 0.8 for continuous loads
Worked examples
| Inputs | Current drawn | Note |
|---|---|---|
| 2,400 W at 230 V, continuous | 10.43 A | 10.4 A → 16 A breaker |
| 1,500 W at 120 V, continuous | 12.5 A | 12.5 A → 20 A breaker |
| A 3 kW motor at 0.8 PF, intermittent | 16.3 A | 16.3 A → 20 A |
FAQFrequently asked questions
Why the 80% rule?
Breakers are tested to hold their rating but can trip or overheat when held at it for hours; codes (the NEC explicitly, IEC practice similarly) size continuous loads so they do not exceed 80% of the device rating.
Does the breaker protect the appliance?
No — it protects the cable from overheating. The cable size must match the breaker rating; a 32 A breaker on a cable rated for 20 A is a fire risk however small the load.
What about motor start-up?
Motors draw several times their running current for a moment at start; breakers have a time delay for that, but a motor near the breaker’s rating may nuisance-trip. Use a motor-rated breaker or size up with the right cable.
Can I do this work myself?
In most countries fixed wiring must be done or certified by a licensed electrician. Use this to plan and to sanity-check what you are told, not to substitute for it.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Cement Concrete & Aggregates Australia — concrete basics and yields — bag yields, density 2,400 kg/m³
- Australian Building Codes Board — the National Construction Code
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.