Determine Proper Electrical Wire Gauge, Ampacity, and Wattage Load

Unfinished electrical wiring

GK and Vikki Hart / UpperCut Images / Getty Images

If you've ever gone to the store with the intent on buying some electrical wire for a project at home, you may have noticed that there are many types and sizes of wire to choose from. These electrical wires are used for powering all types of devices, appliances, and lighting in your home, but knowing what the right size wire is for each is the secret to a safe and effective wire choice.

Here are some great tips for determining the proper wire gauge, ampacity, and maximum wattage allowed. Determining the proper size wire to use can be easy if you know what amperage and wattage a wire can carry per wire gauge. The trick is to have the right sized wired fitted to the power demand it will have on the circuit. Although some wires look the same and even look to be the same size, it doesn't mean that they can handle the amperage. For instance, copper wire can handle more than aluminum wire. Aluminum wiring was used years ago, being cheaper, but due to the softer wiring becoming heated and then becoming loose within the wiring connection points, the practice of using aluminum wire has faded. Copper wire is a superior choice and the standard in wiring techniques.

Wire gauge is the physical size of the wire, rated in gauge size. For instance, common sizes include 14-, 12-, 10-, 8-, 6-, and 2-gauge wire. The gauge of the wire indicates the amount of current that can safely pass through the electrical wire. Electrical current is measured as ampacity. As a guide, #14 wire is good for 15 amps, #12 wire is good for 20 amps, #10 wire is good for 30 amps. As the number gets smaller, the size of the wire gets larger and the amount of amps it can handle also gets larger. This little tip can help you choose the correct sized wire.

Ampacity is defined as the measurement of how much electrical current can flow through an electrical wire safely. This ampacity should match the circuit size, meaning the circuit breaker or fuse that protects it. Speaking of that, remember to calculate the circuit load at no more than 80 percent of the circuit protection. That would mean that a 20-amp circuit should not be loaded more than 16 amps safely. Let's think of a motor on this same circuit that has a startup amperage larger than that of the run ampacity. This practice leaves plenty of startup amperage left over, that being 20 percent or 4 amps, to get the motor spinning. This is also true with many appliances in your home that use compressors and motors to cool like refrigerators and freezers.

Appliances are marked with a tag that informs you of the maximum wattage (load) that it takes to run. The maximum wattage of the appliance should never exceed the maximum wattage rating of the circuit it is connected to. If it does, a dedicated circuit should be run that is capable of handling the load. You want to avoid overloaded circuits, appliances plugged into extension cords that are too light to handle the load, and appliances plugged into power strips that are unable to handle the load. These are great instances of potential electrical fires due to the wrong sized extension cords. In the event that you don't know, there are special extension cords called appliance cords that are designed to handle the load of appliances.

One more thing to keep in mind is to select wire that best fits your needs. Some wire is stranded, while other wire is solid. The solid wire doesn't always pull as easy in conduit with a large number of bends but is much easier to place underwire terminals like those on switches and outlets.

Article Sources
The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Thermal Conductivity. Georgia State University