Matching Wire Size to Circuit Amperage

Wire Gauge and Ampacity

tangled electrical wires
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Whenver a circuit is extended or rewired, or when any new circuit is installed, it is critical that the new wiring is made with wire conductors that are properly sized for the amperage rating of the circuit, as determined by the size of the circuit breaker controlling it. Higher amperage circuits require wires of larger diameter to avoid excessive heat and reduce the danger of fire. In smaller wires, too much current flowing through them creates excessive resistance and more heat.

But how do you know what size wire to use?

Wire is sized by the American Wire Gauge (AWG) system. What wire size and circuit size is proper for your installation is determined by several factors, including the planned load on the circuit, the number of outlets or light fixtures, and the length of the circuit. What is critical, though, is that the wire gauge matches the circuit breaker size.

If you've shopped for electrical wire, you have likely noticed that there are many types and sizes of wire to choose from. Different types of wire are intended for different uses, but with any of these wire types, knowing the right wire size is key to a safe and effective wire choice. For a summary of what the different wire designations are and their usage, see this article on interpreting the letter designations printed on electrical wiring. 

  • If that isn't complicated enough, be aware that while copper wire is more or less the standard, there is also aluminum wiring in some homes, which has its own ampacity-carrying capabilities. Aluminum wiring was one widely used, but because it was found that aluminum had a greater expansion profile under load, it often loosened wire connections and sometimes caused electrical fires. That is not to say you are necessarily at risk, because those connections may work forever if not overloaded, but an evaluation and replacement with copper wire may be a good idea. 

    Wire gauge refers the physical size of the wire, rated with a numerical designation that runs opposite to the diameter of the conductors--in other words, the smaller the wire gauge number, the larger the wire diameter. Common sizes include 14-, 12-, 10-, 8-, 6-, and 2-gauge wire. The size of the wire dictates how much current 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

    Electrical current is measured in ampacity. As a guide, #14 wire is designated for 15-amp circuits, #12 wire is designated for 20-amp circuits, #10 wire for 30 amps.  his ampacity should match the circuit size, meaning the circuit breaker or fuse that protects it. This little tip can help you choose the correct-sized wire if you are replacing circuit wires or installing or extending circuits.

    One more thing to keep in mind is to select wire that best fits your needs. Some wire is stranded, while other wire consists of a solid copper conductor. The solid wire doesn't always pull as easy in conduit with a large number of bends but is sometimes easier to secure under wire terminals such as those on switches and outlets. In standard usage, though, the wire conductors in conduit or NM cable for household wiring will be 14-, 12- or 10-gauge wire that is a solid copper conductor.

     

    The following chart gives some example of common household devices and appliances, and maps them against their amperage capacity and recommended wire gauge for circuits supplying those devices.   examples of devices in your home, the ampacity that they are rated for, and the wire gauge recommended for that amperage. 

    • NOTE: the potential for danger occurs when a device or appliance draws more power than the wire gauge is rated to handle. For example, plugging a heater rated for 20 amps into a 15-amp circuit wired with 14-gauge wire poses a distinct danger. On the other hand, plugging in a laptop computer (a mild electrical load) to a circuit rated for 20 amps poses no danger whatsoever. The potential for danger is most pronounced with the use of light household extension cords. Many a household fire has occurred when a light extension cord with 16-gauge wire is used to power a heater or heating appliance of some sort. Most manufacturers will discourage the use of any extension cords with portable heaters, but if one must be used, it has to be a heavy-duty cord with a high amperage rating. 

    Wire Gauges and Uses

    Wire UseRated AmpacityWire Gauge
    Low-voltage Lighting and Lamp Cords10 Amps18 Gauge
    Extension Cords13 Amps16 Gauge
    Light Fixtures, Lamps, Lighting Runs15 Amps14 Gauge
    Receptacles, 110-volt Air Conditioners, Sump Pumps, Kitchen Appliances20 Amps12 Gauge
    Electric Clothes Dryers, 220-volt Window Air Conditioners, Built-in Ovens, Electric Water Heaters30 Amps10 Gauge
    Cook Tops45 Amps8 Gauge
    Electric Furnaces, Large Electric Heaters60 Amps6 Gauge
    Electric Furnaces, Large Electric Water Heaters, Sub Panels80 Amps4 Gauge
    Service Panels, Sub Panels100 Amps2 Gauge
    Service Entrance150 Amps1/0 Gauge
    Service Entrance200 Amps2/0 Gauge