Circuit Breaker Load Calculator
Safe continuous watts for any breaker at 120 or 240 volts.
What it does
Breakers protect wires, not gadgets, and continuous loads get only 80% of the handle rating. A 20 A bedroom circuit therefore carries 16 A all evening, which is 1,920 W at 120 V, while short bursts may use the full 20.
The tool above takes breaker size with voltage. It returns continuous amps and watts with the typical copper wire beside it. The default 20 A at 120 V gives 16 A continuous (1,920 W) on 12 AWG.
Rules follow National Electrical Code practice through federal energy references: volts times amps is watts, continuous means 3 hours or more, and feeders size at 100% of the rest plus 125% of the continuous share. Energy Star panel guidance frames the same 120 versus 240 V upgrade choices.
Inverse sizing runs the math backward for new loads. A 32 A continuous EV charger needs 40 A of breaker, and a 1,500 W heater fits a 20 A 120 V circuit with room to spare.
How to use
- Pick the breaker handle size.
- Pick 120 or 240 volts.
- Read continuous amps first, watts beside it.
- Keep steady loads under the continuous line.
- Pair the listed copper wire before derates.
- Confirm panel capacity with an electrician.
Formula
- 0.8
- NEC continuous load share of rating.
- 1.25
- Inverse sizing factor for new continuous loads.
- wire
- Typical copper pairing before temperature derates.
Continuous means 3 hours or more at max current. Motors and inductive loads need power factor math beyond this page.
Worked example
Example: 20 A at 120 V
Defaults: 20 A breaker, 120 V.
- Amps: 20 x 0.8 = 16 A continuous.
- Watts: 20 x 120 x 0.8 = 1,920 W.
- Wire: 12 AWG copper typical.
- Heater check: 1,500 W fits under 1,920.
- 2,000 W continuous would need 20.83 A, so a 25 A circuit.
- Tool shows 16 A, 1,920 W, 12 AWG.
Reference table
| Circuit | Continuous | Wire typical |
|---|---|---|
| 15 A x 120 V | 12 A, 1,440 W | 14 AWG |
| 20 A x 120 V | 16 A, 1,920 W | 12 AWG |
| 20 A x 240 V | 16 A, 3,840 W | 12 AWG |
| 30 A x 240 V | 24 A, 5,760 W | 10 AWG |
| 40 A x 240 V | 32 A, 7,680 W | 8 AWG |
| 50 A x 240 V | 40 A, 9,600 W | 6 AWG |
Limits
- Single phase resistive math; motors differ.
- Typical wire before derates and long runs.
- Panel total capacity separate from branch math.
- Local NEC edition and inspector rule.
Common mistakes
- Loading 20 A continuous on a 20 A breaker.
- Sizing wire to the load instead of the breaker.
- Forgetting the 3 hour continuous definition.
- Mixing 120 and 240 V in one product.
- Backstabbing instead of proper terminations.
Try it yourself
Quick checks. Answers sit tucked under each question.
15 A at 120 V?
12 A continuous, 1,440 W, 14 AWG.
50 A at 240 V?
40 A continuous, 9,600 W, 6 AWG.
32 A continuous charger?
32 x 1.25 = 40 A breaker minimum.
Questions
Default capacity?
16 A (1,920 W) continuous on 20 A at 120 V.
Why 80%?
NEC continuous rule: heat builds over 3 plus hours.
30 A dryer at 240 V?
24 A (5,760 W) continuous on 10 AWG.
Wire pairing?
Typical copper before derates; confirm locally.
240 V effect?
Doubles watts per amp against 120 V.
Panel full?
Branch math passes but the service total still rules; ask an electrician.