What the animation shows
The animation and the step-by-step video above use your numbers. Here is the same walkthrough written out for the default example.
What you enter
- Loads: 3 loads.
- Circuit voltage: 120 V.
- Existing breaker rating: 20 A.
- Loads run continuously (3 hours or more): No.
What happens, step by step
- Each load’s current is its watts ÷ 120 V, all through one breaker.
- Load 2 passes 80% of the breaker rating: the meter turns orange.
- 20 A is 100% of the rating: fine briefly, not for 3+ hours.
The answer: total current is 20 A.
How it’s calculated
- Current = total watts ÷ volts
- Continuous loads: breaker ≥ 125% of load current
- Standard breaker sizes per NEC 240.6(A)
Worked example
A 1,500 W heater, 800 W microwave and 100 W of lights on a 120 V, 20 A circuit.
Inputs
- Loads
- 1500; 800; 100 W
- Circuit voltage
- 120 V
- Existing breaker rating
- 20 A
- Loads run continuously (3 hours or more)
- No
Results
- Total current
- 20 A
- Share of breaker rating
- 100%
- Minimum standard breaker for this load
- 20 A (the wire must be rated for it)
- Total load
- 2,400 W
Assumptions and limits
- Resistive loads (power factor 1). Motor starting current is not included.
- Never upsize a breaker without confirming the wire is rated for it.
Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.
Questions
How much can I put on a 20 amp circuit?
20 A × 120 V = 2,400 W total, or 1,920 W for continuous loads under the 80% rule.
Can I just install a bigger breaker?
No. The breaker protects the wire. A larger breaker on the same wire is a fire hazard.