Breaker Size Calculator
This breaker size calculator works out the circuit breaker rating a load needs, starting from the power the load draws and the voltage it runs on. Enter the load in watts, the supply voltage, and whether the load is continuous, and the calculator finds the current in amps, adds the standard continuous-load allowance, and rounds up to the next standard breaker size. For a load that runs for three hours or more, a breaker should carry no more than 80% of its rating, so the load current is multiplied by 1.25 before the breaker is chosen. The result is picked from the common miniature circuit breaker ratings used in New Zealand homes and light commercial work, 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100 and 125 amps, so you get a size you can actually buy. This is a planning and sanity-check tool for a single circuit: it tells you the smallest standard breaker that will carry the load without tripping, and shows how hard that breaker will be working. The cable feeding the circuit must be rated to carry the breaker current, and final circuit design, especially anything touching the switchboard, must be signed off by a registered electrician under AS/NZS 3000.
A 3,000 W continuous load at 230 V draws 13.0 A. Adding the 125% continuous-load allowance gives 16.3 A, so the next standard MCB up is 20 A, running at about 82% of its rating.
Assumes a resistive load at unity power factor on a single-phase supply. The cable must be rated for the breaker current. Final design must be certified by a registered electrician under AS/NZS 3000.
How it works
The load current is the power divided by the voltage, so a 3,000 W load on 230 V draws about 13.0 A. A breaker is then sized so a continuous load sits at no more than 80% of its rating, which is the same as multiplying the load current by 1.25 to get the required rating. That figure is compared against the standard breaker ratings and rounded up to the next size, because you cannot buy an in-between breaker. Utilisation shows the required rating as a percentage of the chosen breaker, so you can see how much headroom is left before it would need the next size up.
Worked example
Take a 3,000 W continuous load on a 230 V circuit. The current is 3,000 / 230 = 13.0 A. Because it is continuous, multiply by 1.25 to get a required rating of 16.3 A. The standard ratings are 6, 10, 16, 20, 25 and so on, and 16 A is below 16.3 A, so the next size up is a 20 A breaker. At 16.3 A the 20 A breaker runs at about 82% of its rating, leaving a sensible margin. The cable to this circuit must be rated to carry 20 A.
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