Circuit Load Calculator: Will Your Appliances Trip the Breaker?

How the circuit load calculator works

A household outlet circuit in the US is usually 120 volts and protected by a 15 or 20 amp breaker. The calculator adds the watts of everything on the circuit and divides by 120 to get amps.

Amps = watts / 120 volts

The 80 percent rule for long-running loads

A load that runs for 3 hours or more is called a continuous load. The National Electrical Code (Article 210.20(A)) requires the breaker rating to be at least the other loads plus 125 percent of the continuous load. That is the same as keeping a continuous load to 80 percent of the breaker rating. Space heaters, window air conditioners, furnace blowers and lights often run that long. Microwaves, hair dryers and coffee makers usually do not. Check the code edition your area follows, because local rules differ.

Worked examples

• A 1,500 watt space heater alone on a 15 amp circuit: 12.5 amps. For a long-running load the breaker would need to be 15.6 amps, so it is slightly over the safe limit, even though it may not trip.
• That heater and a 200 watt lamp, both running for hours, on a 15 amp circuit: 1,700 watts, or 14.2 amps. Over the safe limit. On a 20 amp circuit the same load is within the limit.
• That heater and a 1,800 watt hair dryer: 3,300 watts, or 27.5 amps. Far above 15 amps, so the breaker will trip.

What to do if it is over the limit

Unplug the largest load or move it to an outlet on a different circuit. Use one circuit for a heater by itself if you can. If a breaker trips with only a small load, or a plug, outlet or cord feels warm, stop and call a licensed electrician, because the problem may be the wiring or the breaker. For causes of repeated trips, see why a circuit breaker keeps tripping. For a heater in particular, read why a space heater trips the breaker.

What this tool cannot tell you

It uses typical watts, so use the number on your appliance label. It does not add the surge many motors draw for a moment when they start, such as a refrigerator, sump pump or air conditioner. It cannot see other outlets and lights on the same circuit, old or damaged wiring, or loose connections. It covers plug-in loads on 120 volt circuits, not 240 volt circuits such as dryers, ranges and electric water heaters. It is a guide and not an electrical inspection.

More electrical and home tools

Planning for a power cut? Use the generator size calculator. To see what a heater costs to run, try the space heater cost calculator and the space heater size calculator. If a GFCI will not reset, read what to do when a GFCI outlet will not reset. If you must use a cord, check it with the extension cord size calculator.