Electrical Load Calculator
Uses nameplate watts at unity power factor. This is an estimate; NEC Article 220 load calculations, local code and a licensed electrician govern panel and service sizing.
How the electrical load calculation works
Add up the watts
Use the nameplate wattage of each device times how many you have. 300 W of lights, two 250 W computers and a 900 W coffee maker total 1,700 W.
Amps = watts ÷ volts
1,700 W on a 120 V circuit draws 14.17 A. On 240 V the same load draws half that. For three-phase, divide by 1.732 × volts.
Hold continuous loads to 80% of the breaker
A load that runs three hours or more should not exceed 80% of the breaker rating. A 20 A breaker carries 16 A continuous; a 15 A breaker carries 12 A.
Smallest breaker = amps ÷ 0.8, rounded up
14.17 A ÷ 0.8 = 17.7 A, so the next standard size is 20 A. The wire must be rated for that breaker too.
Worked example. Lights at 300 W, two 250 W computers and a 900 W coffee maker total 1,700 W. At 120 V that is 14.17 A. A 20 A breaker allows 16 A for continuous use, so the circuit is at 88.5% of its limit with 1.83 A (220 W) to spare.
Electrical Load FAQ
How do I calculate the electrical load on a circuit?
Add the watts of everything that runs at the same time and divide by the circuit voltage. 1,800 W on 120 V is 15 A.
What is the 80% rule for breakers?
A continuous load, one that runs three hours or more, should use no more than 80% of the breaker's rating. That is 12 A on a 15 A breaker and 16 A on a 20 A breaker.
How many watts can a 20 amp circuit handle?
2,400 W at 120 V in theory, and 1,920 W for a continuous load under the 80% rule. On 240 V the figures double.
How many watts can a 15 amp circuit handle?
1,800 W at 120 V in theory, and 1,440 W for a continuous load under the 80% rule. A heater with a 1,500 W nameplate draws 12.5 A at 120 V, which is already over the 12 A continuous limit.
What size breaker do I need for a load?
Divide the amps by 0.8 for a continuous load and round up to the next standard breaker size. 6,200 W on 240 V is 25.83 A, and 25.83 ÷ 0.8 = 32.3 A, so the calculator shows 35 A. The wire must be rated for that breaker, and a licensed electrician should confirm the size against the NEC and local code.
How do I calculate the load for an electrical panel?
A full panel or service load uses NEC Article 220, which applies demand factors to lighting, appliances, heating and cooling. This tool adds connected loads only, so it overstates a real service load; have an electrician do the Article 220 calculation.
Can I put a bigger breaker on an overloaded circuit?
No. The breaker protects the wire. Upsizing it without upsizing the wire is a fire risk; split the load across circuits instead, and have a licensed electrician check the circuit.
Sources: NFPA 70, National Electrical Code 2023 (free read-only access)