Electrical
Electrical Breaker Size Calculator
Explore a preliminary breaker lookup from an entered current or a simple watts-to-current conversion.
Result
A planning estimate, not a code calculation. Conductor sizing, overcurrent protection and load calculations are governed by the NEC as adopted and amended where you are, and by the listing of the equipment involved. Have the design confirmed by a licensed electrician and inspected. The breaker protects the conductor, so the wire must be sized for the breaker and not merely for the load. Motors, air conditioners and welders have their own sizing rules that override this.
Get a Free Business AuditWhat moves the number
- The input-mode selection controls whether amps or watts are used.
- Supply voltage matters in watts mode, while directly entered current bypasses that conversion.
- The duration setting applies the model's design uplift.
Where this one goes wrong
Watts mode uses a simple power divided by voltage relationship. It has no power-factor or phase input, so it cannot represent every AC load. The presence of a voltage option does not establish that all equipment using that voltage fits the model.
Questions
About this calculator
Which mode should I use with rated input amps?
Use the amperage mode when assessing a stated load current. The watts field is not an additional simultaneous load and should not be added mentally to the result.
Does the standard-size lookup check a panel's compatibility?
No. Device type, permitted panel combinations and interrupting capability are outside this arithmetic.
Can this authorize a breaker change?
It cannot. Treat protection sizing as a code-governed planning check requiring the conductor details, local rules and the manufacturer's instructions.
The formula
design amps = load x 1.25 if continuous; breaker = next standard size at or above that
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How to use this calculator
- Work it out from. Choose the unknown quantity. Only the inputs used by that calculation are shown.
- Load (amps). Enter your own figure; the page starts at 24.
- Load type. Enter your own figure; the page starts at Continuous, 3 hours or more.
The result updates as you type, so you can try a figure, see what it does to breaker size, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Work it out from | Amps |
| Load (amps) | 24 |
| Load type | Continuous, 3 hours or more |
What comes out
| Result | Value |
|---|---|
| Breaker size main result | 30 |
| Actual load | 24.0 |
| After the continuous uplift | 30.0 |
| Spare capacity | 20.0% |
A 24.0 amp load, continuous, becomes 30.0 amps for sizing and needs a 30 amp breaker, leaving 20.0% spare.
How sensitive the answer is
Holding everything else steady and changing only load (amps), this is what happens to breaker size. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Load (amps) | Breaker size |
|---|---|
| 12 | 15 |
| 18 | 25 |
| 24 example above | 30 |
| 36 | 45 |
| 48 | 60 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Work it out from
- Choose the unknown quantity. Only the inputs used by that calculation are shown.
- Load (amps)
- A number, between 0 and 600, starting at 24.
- Load type
- Choose from Normal, Continuous, 3 hours or more.
What the results tell you
| Result | What it is |
|---|---|
| Breaker size | The headline result. A number. |
| Actual load | A decimal figure. |
| After the continuous uplift | A decimal figure. |
| Spare capacity | A percentage. |
Before you rely on this number
Conductor sizing, voltage drop and load figures are governed by code, and the code takes precedence over any calculator. Ambient temperature, conduit fill, continuous load factors and the specific installation all change the answer. Treat this as a first pass, not a design.
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