How to use the circuit load calculator
- Enter the breaker rating in amps and the circuit voltage.
- Check the load limit as a percent of the rating. It starts at 80% as an example you can change.
- List the devices on the circuit with the watts from each nameplate and how many you have.
- Read the total load, the current drawn, the limit and the room left under it.
Formula
Total load (W) = sum of device watts x quantity
Current drawn (A) = total load / voltage
Breaker capacity (W) = breaker rating x voltage
Load limit (W) = breaker capacity x limit percent / 100
The tool treats every device as running at the same time and compares the total watts with the limit.
A 15 amp circuit at 120 volts
A 15 amp breaker at 120 volts has a capacity of 1,800 watts. With an 80% limit, that is 1,440 watts. A 750 watt heater, 150 watts of lights and 100 watts of TV and console total 1,000 watts, which draws 8.33 amps and uses 69.4% of the limit. That is 440 watts under the limit.
Two 1,500 watt heaters on the same circuit total 3,000 watts, which is 1,560 watts over a 1,440 watt limit. On a 20 amp, 240 volt circuit the capacity is 4,800 watts.
Watts, volts and amps
Power in watts is volts times amps, so dividing watts by volts gives the current a device draws. A breaker is rated in amps, and the sum of the devices on the circuit, drawing at once, is what it has to carry.
The limit percent is editable because the right figure depends on how long a load runs and on your local code. The 80% in the box is an example, and the page does not state what any code requires.
Safety and the limits of this tool
This is arithmetic on numbers you type and not an electrical code check. Wire size, breaker type, startup surges from motors and compressors, shared neutrals and other loads on the same circuit all matter. Electrical work should follow your local electrical code and be done by a qualified person.
If a breaker trips often or a plug or outlet feels warm, stop using the circuit and call an electrician. To convert power and current use the kW to amps calculator, to size a generator see the generator wattage calculator, and for running costs the electricity cost calculator. The Ohm's law calculator covers resistance.
Assumptions and limits
- Every device listed runs at the same time.
- Watts come from nameplates and do not include startup surges.
- The load limit percent is an input you can change and is not a statement of any code.
- The tool does not check wire size, breaker type or other loads on the circuit.
Frequently asked questions
How many watts can a 15 amp circuit handle?
Multiply 15 amps by the voltage to get the capacity, 1,800 watts at 120 volts. The limit you choose, such as 80%, gives 1,440 watts.
Why is the load limit a percent of the breaker rating?
Some people keep loads that run for a long time below the breaker rating to leave margin. The percent is editable, and your local code and an electrician decide what applies.
How do I find the watts of a device?
Look at its nameplate or label. If it lists amps and volts, multiply them. Motors and compressors can draw more at startup than the label shows.
Does this tell me my wiring is safe?
No. It only compares the numbers you enter. Wiring must follow local code and be checked by a qualified person.