Wire Size Chart: What Size Wire for Amp Service?

Every electrician knows that a 50-amp wire has to have a larger diameter than a 20-amp or 30-amp one, and it's a good thing because undersizing an electrical wire can start a fire. Amp is short for ampere, and it's a measure of electrical current, which is typically likened to a flow of negatively charged particles called electrons. You can compare the flow of electricity to the flow of water in a river: The faster the river flows, the more water particles pass a given point per unit time and the more powerful the flow. Ampacity is the measure of the number of electrons that pass a given point per unit time, which amounts to the strength of the current.

In this comparison, wire size is analogous to the size of the riverbed. If the water flow is greater than the bed can contain, water spills over the sides and erodes the banks, and something similar happens when you try to pass too much current through a wire. Electricity generates electrical resistance and heat in conducting wires, and the amount of resistance is inversely proportional to the cross-section of the wire, which means the smaller the wire, the higher the resistance and the more heat produced. Passing too strong of an electrical current through a wire can cause overheating, which can melt the wire insulation and start a fire.

To prevent this, the National Electrical Code (NEC) has established a wire size chart. Note that nm (nonmetallic) wire, such as Romex, always has a lower amperage limit than purely metallic wire of the same gauge, so it's important to always refer to the NEC chart.

Electricians measure wire size by its gauge number, which is based on how many wires can pass through a hole of a given size. The fatter the wire, the fewer strands will pass through the hole, so smaller gauge numbers refer to fatter wires, and larger gauge numbers refer to thinner ones. In the American Wire Gauge (AWG) system, each gauge number corresponds to a particular cross-sectional diameter or area. For example, 14-gauge wire has a cross-sectional diameter of 1.63 mm and a cross-sectional area of 2.08 mm², while 10-gauge wire has a diameter of 2.59 mm and an area of 5.26 mm².

The NEC establishes a limit on how many amps can safely pass through a wire of a particular gauge. The ratings chart distinguishes between copper wire and aluminum wire, although aluminum wire isn't commonly used in residential wiring because it requires special connectors to prevent overheating.

In residential wiring, 12 AWG is the most common wire gauge, typically used for wiring a light fixture as well as a standard or GFCI receptacle, and many electricians use it even when 14 AWG wire would be acceptable. Standard 12-gauge two-conductor cable (which also includes a ground wire, which isn't counted as a conductor) is identified on the sheathing as 12/2 cable, and if the cable includes a third conductor, which is needed for special applications, such as three-way light switches, it's 12/3 cable.

A 240-volt appliance, such as a water heater, air conditioner, or stove, draws more current than a 120-volt light or receptacle and calls for a lower wire gauge. A typical electric dryer draws 30 amps and must be wired with 10-gauge cable, while an electric range or hot tub, which draw 50 amps each, must be wired with 8-gauge copper wire. Some energy-efficient home air systems draw only 40 amps and can also be wired with 8-gauge cable, while less-efficient ones may draw 60 amps and call for 6-gauge cable.

Appliances that operate at 240 volts require two-conductor or three-conductor cable. They need two hot wires that carry 120 volts each for a total of 240 volts. Circuits that require a neutral conductor are wired with three-conductor cable. Each of the hot wires forms a leg of the circuit and must be controlled by a separate circuit breaker device, and the two breakers are coupled to ensure that if one leg trips, the other does also. This is usually accomplished with a single unit called a double-pole circuit breaker. Each half of the breaker has a heavy-duty lug to hold the large-diameter wire. The NEC requires most 240-volt appliances to be controlled by dedicated circuits, and the breaker rating must correspond to the current draw of the appliance and the wire size.

Aluminum wiring was popular in the 1960s and '70s because aluminum is cheaper than copper, conducts electricity almost as well, and has a lower temperature rating, which means that smaller-gauge aluminum wires can be used at higher temperatures. It fell into disuse mainly because it becomes less conductive as it rusts, which doesn't happen with copper. The lower conductivity creates more resistance and can cause overheating, which is why it requires special connectors.

Wire length affects both amperage and voltage, and the voltage drop across from one end of a long wire to the other can affect the performance of electrical equipment. When voltage drop is significant, the solution is to increase the wire gauge. For example, the 50-amp wire size over a large distance should be increased to 6 AWG to avoid the effects of voltage drop.

When you wire an electrical circuit with an undersized wire, you might not notice any effect until the day you run one too many appliances, drawing current that is below the rating of your 50-amp breaker but above the capacity of the wire. The wire can overheat, causing the insulation to melt, which in turn exposes bare conductors that can pass electricity between them. This is known as a short circuit, and it creates arcing and high temperatures that can start a fire. By contrast, there is no danger in oversizing wires, so when in doubt, it's always safer to opt for a lower wire gauge. The answer to the question "What size wire for a 50-amp circuit?" is actually anything larger than 8 gauge.

To avoid electrical accidents, you should have electrical work completed by a licensed pro, and when you choose to do your own work, you should have it checked by a pro before turning on anything.