If you’ve ever plugged in one too many holiday lights, switched on a vacuum, or cranked up a space heater only to have the lights or appliance suddenly shut off, you’ve created an electrical circuit overload. The shutdown was triggered by the circuit’s breaker (or fuses) in your home’s service panel. And while circuit breakers are reliable and do a good job preventing house fires due to overloads, the safest strategy is to manage your electricity usage to prevent overloads in the first place.
What Is an Electrical Circuit Overload?
An electrical circuit overload occurs when you draw more electricity than a circuit can safely handle.
How Do Electrical Circuit Overloads Work?
Electrical circuits are designed to handle a limited amount of electricity. Circuits are made up of wiring, a breaker (or a fuse, in old wiring systems), and devices (such as light fixtures, appliances, and anything plugged into an outlet). The electricity usage of each device (when running) adds to the total LOAD on the circuit. Exceeding the rated load for the circuit wiring causes the circuit breaker to trip, shutting off the power to the entire circuit.
If there were no breaker in the circuit, an overload would cause the circuit wiring to overheat, which could melt the wire insulation and lead to a fire. Different circuits have different load ratings so that some circuits can provide more electricity than others. Home electrical systems are designed around typical household usage, but there’s nothing to prevent us from plugging in too many devices on the same circuit. However, the more you know about the layout of your home’s circuits the more easily you can prevent overloads.
Signs of Overloaded Circuits
The most obvious sign of an electrical circuit overload is a breaker tripping and shutting off all the power. Other signs can be less noticeable:
- Dimming lights, especially if lights dim when you turn on appliances or more lights.
- Buzzing outlets or switches.
- Outlet or switch covers that are warm to the touch.
- Burning odors from outlets or switches.
- Scorched plugs or outlets.
- Power tools, appliances, or electronics that seem to lack sufficient power.
Buzzing sounds, burning smells, and unusually warm devices also can indicate other wiring problems, such as loose connections or short circuits. If any of these problems persist after you’ve taken steps to prevent circuit overloads, contact an electrician.
Mapping Your Home’s Circuits
The first step to preventing electrical circuit overload is to learn which circuits power which devices. When you’ve mapped the basic circuit layout, you can calculate the safe load rating of each circuit to get a sense of how many things you can operate on that circuit. For example, if your kitchen lights dim when you turn on your toaster oven (a power-hungry appliance), that tells you that the toaster and lights are on the same circuit (even though they shouldn’t be) and that you’re close to maxing out the circuit capacity. Mapping the circuits also can tell you if there’s a need for new circuits to meet the normal demands of the household.
Mapping circuits are simple (if repetitive): Get a notepad and a pencil. Open the door to your home’s service panel (breaker box) and turn off one of the breakers with the number 15 or 20 stamped on the end of the breaker switch. (Don’t bother with the breakers stamped with 30, 40, 50, or higher numbers; these are high-voltage circuits for appliances like electric ranges, water heaters, and clothes dryers, and you’re not plugging ordinary appliances into these circuits.) Note on the pad where the circuit lies in the panel so you can identify it later.
Next, walk through the house and try all the lights, ceiling fans, and plug-in appliances. Write down everything that doesn’t have power, and note the room it is in. Also, test each outlet with a voltage tester or receptacle tester, or even a plug-in light or lamp, recording all that don’t work. You don’t necessarily have to go through the entire house for each circuit. And if your electrician was thorough, there may be helpful labels next to the breakers, indicating the circuit areas (“Southeast bedroom,” “Garage lights,” etc.). But for accurate mapping, you should test each area broadly because circuits can have oddball members—a microwave on a hallway lighting circuit, for example.
After you’ve tested the circuit area, go back to the panel, turn on the first breaker, then turn off the next one in the row, and repeat the test. Repeat the process for all of the “15” and “20” circuits.
Calculating Circuit Loads
Your circuit map tells you which devices are powered by each circuit. Now you have to calculate how much power those devices are using. To do that, you need a quick lesson in electrical energy. Electricity is measured in watts; a 100-watt light bulb uses 100 watts of electricity. A watt is the product of voltage (volts) and amperage (amps):
1 volt x 1 amp = 1 watt
To calculate the total load on each circuit, add up the wattage of all the devices on that circuit. Light bulbs and many small appliances have labels noting their wattage. If a device gives you only amps, multiply the amp value by 120 (the voltage of standard circuits) to find the wattage. Include all devices that are permanently wired to the circuit as well as plug-in appliances that you don’t move very often (like a toaster oven, or a heater in a particularly cold room).
Compare the total wattage of each circuit to the load rating of that circuit. The circuits with “15” breakers are rated for 15 amps. The maximum load rating of one of these circuits is 1,800 watts:
120 volts x 15 amps = 1,800 watts
If you try using more than 1,800 watts on that circuit, you will overload it, and the breaker will trip.
The circuits with “20” breakers are rated for 20 amps and have a maximum load rating of 2,400 watts:
120 volts x 20 amps = 2,400 watts
Compare the wattage total (how much electricity you’re using) and the load rating for each circuit. For example, a 15-amp circuit serving lights and outlets in a living area might be providing power for 500 watts for lighting, 500 watts for the TV and cable box, and 200 watts for the sound system, for a total of 1,200 watts. If you plug in a 700-watt vacuum while the TV, stereo, and lights are on, you’ll exceed the 1,500-watt rating on the circuit breaker, causing it to trip and shut off the power.
The maximum load on each circuit isn’t the ideal target. For a margin of safety, it’s best if the normal load on a circuit does not exceed 80 percent of the maximum (rated) load. For a 15-amp circuit, the safe load target is 1,440 watts; for a 20-amp circuit, the safe load is 1,920 watts.
If your circuit calculations indicate that you’re drawing more wattage from a circuit than the safe load number—or you’re exceeding the rated load and frequently overloading the circuit—there are a few ways to reduce the load on the circuit to prevent overload:
- Move plug-in appliances to a circuit that is less-used (use your mapping and circuit calculations to identify circuits that have plenty of available wattages).
- Remember not to turn on too many things at once. For example, turn off the TV and sound system while you vacuum (you can’t hear them anyway).
- Reduce lighting loads by replacing incandescent or halogen light bulbs with energy-efficient LED (preferably) or CFL (fluorescent) bulbs.
- Install new circuits for high-demand devices. For example, if you run a lot of power tools in your garage workshop, but your garage is wired with all of the outlets and lights on the same 15-amp circuit, install a new 20-amp circuit supplying a few new outlets for your tools.
Electrical Safety in the Home. National Fire Protection Agency.