How to Prevent Circuit Overloads
Updated: Jul. 26, 2024
When you plug in a blender, turn it on, and the power goes off, you've got a circuit overload. Here's how to prevent that from happening again.
A perfect example of how a circuit overload can be dangerous occurred as my caregiver friend tried to relieve the excessive heat affecting her 100-year-old client. The solution seemed simple: plug in a portable air conditioner and turn it on. Unfortunately, the breaker tripped when she did that because the current demand for the air conditioner exceeded the breaker’s capacity. My friend tried several outlets in the house and got the same results until she found one that didn’t trip the breaker. It was so far from the bedroom that she needed a 75-foot extension cord to reach it.
Breakers are supposed to trip when the current demand exceeds their capacity because that prevents overheating wires and possible fires. It’s usually just an inconvenience, but in this case, the payoff for fire safety was extreme discomfort and maybe worse for an elderly person in the midst of a heat wave.
When you have a circuit overload, you can do what my friend did and use a different plug, and chances are you’ll have better luck finding one than she did. You can also unplug something from the overloaded circuit. However, the best strategy for avoiding overloads is to install more receptacles in the house, which may involve adding one or more new circuits to the main panel.
This post will help you decide whether or not that’s an option.
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Don’t Overload the Panel
When you add more circuits to a panel, you increase the total load on the panel, and that load can’t exceed the panel’s capacity.
The electrical load is simply the sum total of the current drawn by all the devices demanding power at a given time, but electricians use a detailed and somewhat complicated method to calculate it. If you don’t feel confident doing the numbers yourself, hire a licensed electrician to evaluate your electrical load and help you determine if it’s safe to add to it.
The panel’s capacity is easier to determine: It’s the amperage rating of the main circuit breaker, which shuts off the entire panel in the event of an overload. The National Electrical Code (NEC) requires panels to have a minimum capacity of 100 amps, but the standard capacity for new construction these days is 200 amps, which is typically enough to allow for the addition of more circuits. On the other hand, some older homes that have never had an electrical upgrade may have panels rated for 60A or less.
According to NEC rules, the total load on the panel shouldn’t exceed 80 percent of its capacity. If adding a new circuit would increase the load above this threshold, you should upgrade the panel to be safe and code-compliant (which are the same thing).
The panel must have space for a new breaker
Each circuit in the house connects to an individual circuit breaker in the panel, and these are stacked vertically in two columns. Some of the breakers control individual appliances, and some control multi-use circuits for lights and receptacles. Multi-use breakers are rated for either 15 or 20 amps, and if you want to add a new receptacle circuit, you need to install a new one of these.
Panels have limited space for breakers. The NEC allows up to 42 in a 100-amp panel, but most panels are smaller and can only handle 20 or fewer. If your panel already has the maximum number of breakers it can handle, you need to install a bigger panel if you want to add another circuit.
Can you swap out a breaker with a bigger one?
Suppose you have a 15A breaker that keeps overloading and tripping. Can you replace it with a 20A breaker? Maybe.
Circuits rated for 15 amps may be wired with 14-gauge wire, but 20-amp circuits require 12-gauge wire, which is thicker. You can’t upgrade a breaker from 15 to 20 amps if the circuit is wired with a 14-gauge wire. Some electricians use 12-gauge wire exclusively when wiring a home just to avoid such situations. If all the 120-volt circuits in your house have been wired with 12-gauge wire, you might be able to upgrade the breaker, but for safety, you should have a licensed electrician evaluate the circuit before you do anything.
Plugging Devices to Other Receptacles
The most common way to handle a circuit overload is to move one or more loads from the overloaded circuit to another one. This usually involves little more than unplugging a device and plugging it into an outlet on another circuit. There’s a danger of overloading that circuit, though, so you need to know what’s on it, and this may involve a little detective work.
Trace the circuit
The labels on the main panel are the starting point for tracing circuitry in the house. If the panel is properly indexed, the label next to each breaker (or on the panel door) will tell you — in general — what the circuit runs. Unfortunately, the index doesn’t provide many details, so you may have to map out the circuit yourself to find out what’s on it.
To trace a circuit, turn off its breaker at the main panel (or unscrew the fuse), then go through your home testing outlets — flipping on light switches and plugged-in devices and plugging a test light into open receptacles. Test both the upper and lower receptacle of standard duplex receptacles because they’re sometimes wired to different circuits. And make sure switched receptacles are “on” before testing them. Check outdoor lights and receptacles, too. Outlets and lights that don’t work are connected to the circuit that’s off. Write your results down, or put them on a simple floor-plan diagram so you won’t forget or skip locations.
Calculate the circuit load
Once you know which devices, including lights and receptacles, are on a circuit, you can calculate the load for that circuit in watts. The wattage rating for each light bulb is marked on the bulb, and each device permanently plugged into the circuit (such as a TV), has a label on the back indicating its wattage rating. Simply add up the wattage ratings. Devices that plug in temporarily, such as vacuum cleaners, don’t count in this calculation.
The maximum allowed wattage on a 15A circuit is 1,800 watts, and for a 20A circuit, it’s 2,400 watts. For safety, you should conform to the 80-percent rule, which would set these maximums at 1,440 and 1,920 watts, respectively.
- Pro tip: Any device plugged into a multipurpose circuit that draws more than half the allowed maximum wattage should have its own dedicated circuit. This includes refrigerators, microwaves and similar devices.
After calculating the loads on your general-purpose circuits, you can redistribute the loads (plug-in devices) so no single circuit has more than its maximum allowed wattage. This isn’t always convenient, however. You might have to add a new circuit or install a new outlet to get the power where you want it.
Adding a New Receptacle
The easiest way to add a new outlet is to find a circuit with sufficient capacity with a convenient junction box to tap into. Sometimes, you can tap into an existing outlet, but if not, you can sometimes find easy access to lights or switch boxes in an unfinished basement. Otherwise, look to the attic. The junction boxes in most attics (assuming your attic is accessible) are usually buried under insulation, so you’ll probably have to rake the insulation aside. (Wear a dust mask, goggles and long sleeves if you do this!) Look for junction boxes near the access hole first or over ceiling light fixtures in the rooms below. The power must come directly to the light box, not from a switch.
- Caution: Electrical boxes might contain wires from several circuits. Test the wires with a voltage tester to ensure the power is off before touching them.
- Pro tip: Make sure the existing box is large enough to accommodate the additional new cable. Wires packed into too small a box can overheat. If the wiring in a box looks complicated, find a different box or call in an electrician to make the connections.
If you can’t find an existing circuit with sufficient capacity and a convenient tap-in location, you may be better off running an entirely new circuit from the main panel, assuming the panel has the capacity for one (see above). You can install the new outlet yourself and run the new cable back to the main panel, but let an electrician make the hook-up inside the panel. Working inside the panel is dangerous. Let a pro do it.
Safety First
Always apply for an electrical permit from your local building inspections department when undertaking an electrical project. The permit not only ensures that your work will be inspected for proper technique and safety but also that you’ve properly analyzed your home’s circuitry and are following a sound plan.
Here’s what you need to know about overcurrent protection.
About the Expert
- John Williamson is a master electrician and the retired chief electrical inspector for the Minnesota Department of Labor and Industry.
Originally Published: October 18, 2019
