Electrical
Breaker Size Calculator
Compare a watts-based load estimate with a standard breaker lookup while separating operating current from the tool's design allowance.
Result
Electrical work is life-safety work governed by the NEC or your local code, and must be designed and installed by a licensed electrician. Conductor and overcurrent sizing depends on ambient temperature, conductor bundling, termination ratings, continuous-load rules and more. This is an educational estimate and must never be used as the basis for an installation. The breaker must also be matched to conductor ampacity. A breaker larger than the wire it protects is a fire risk. Motors, welders, air conditioners and transformers have their own sizing rules that override this.
Get a Free Business AuditWhat moves the number
- Phase and power factor affect current derived from watts.
- The continuous-load choice changes the design allowance used here.
- Rounding to a standard device creates numerical headroom, not unrestricted spare capacity.
Where this one goes wrong
A larger breaker is not a remedy for repeated tripping. The cause could involve an overloaded circuit, a fault or equipment behavior this model does not represent. The displayed selection does not establish conductor protection or coordination.
Questions
About this calculator
What belongs in the load-watts field?
Use electrical input power for the load being assessed. Mechanical horsepower converted directly to watts leaves out losses and may understate input current.
Can the spare-capacity figure justify another appliance?
Not by itself. The complete circuit load, duration, wiring and equipment restrictions need assessment before adding anything.
Does this cover motor or transformer protection?
It does not apply their detailed rules. Breaker sizing is a code-governed planning check here; the locally adopted requirements and equipment instructions determine the final choice.
The formula
design current = load current × 1.25 for continuous loads; round up to the next standard device
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How to use this calculator
- Load (watts). Enter your own figure; the page starts at 3,600.
- Voltage (V). Enter your own figure; the page starts at 240.
- Load type. Enter your own figure; the page starts at Continuous (3+ hours).
- Phase. Enter your own figure; the page starts at Single phase.
- Power factor. Enter your own figure; the page starts at 1.
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 |
|---|---|
| Load (watts) | 3,600 |
| Voltage (V) | 240 |
| Load type | Continuous (3+ hours) |
| Phase | Single phase |
| Power factor | 1 |
What comes out
| Result | Value |
|---|---|
| Breaker size main result | 20 A |
| Actual load current | 15.0 |
| Design current | 18.8 |
| Spare capacity on that breaker | 25.0% |
A 3,600 W load draws 15.0 A. Sized at 18.8 A, the next standard device is a 20 A breaker, leaving 25.0% headroom.
How sensitive the answer is
Holding everything else steady and changing only load (watts), 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 (watts) | Breaker size |
|---|---|
| 1,800 | 15 A |
| 2,700 | 15 A |
| 3,600 example above | 20 A |
| 5,400 | 30 A |
| 7,200 | 40 A |
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
- Load (watts)
- A number, between 0 and 10000000, starting at 3,600.
- Voltage (V)
- A number, between 0 and 100000, starting at 240.
- Load type
- Choose from Non-continuous, Continuous (3+ hours).
- Phase
- Choose from Single phase, Three phase.
- Power factor
- A number, between 0 and 1, starting at 1.
What the results tell you
| Result | What it is |
|---|---|
| Breaker size | The headline result. A category or planning selection, rather than a measured quantity. |
| Actual load current | A decimal figure. |
| Design current | A decimal figure. |
| Spare capacity on that breaker | 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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