Circuit Breaker Load Calculator

List everything that runs at the same time on the circuit. Continuous loads (running 3 hours or more) must not exceed 80% of the breaker rating.

Important: This calculation provides a preliminary estimate. Final conductor and overcurrent protection sizing must account for the NEC as adopted locally, conductor material, insulation temperature rating, ambient temperature, installation method, bundling, terminal ratings, equipment specifications and local amendments. Electrical work should be done or checked by a qualified electrician.

Tapeline — Circuit Breaker Load Calculator

Printed September 29, 2026

Loads1500; 800; 100 W
Circuit voltage120 V
Existing breaker rating20 A
Loads run continuously (3 hours or more)No

Results update as you type.

Loads
Load 1
Load 2
Load 3
V

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

Load this example into the calculator

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.