Wire Size Guide: What Size Wire Do I Need?

Whether you have a 30 amp or a 40 amp breaker, wire size is important. Here’s how to determine what size wire for your breaker you need.

Wire is sold based on several factors. The most important part that will be covered here is the gauge, sometimes referred to as AWG (American Wire Gauge).

Wire gauge is measured from high to low with higher numbers meaning a smaller wire size. The chart below will tell you approximately what wire size is needed for the amount of power that will be running over the line.

Maximum Amps 7 10 15 20 30 40 55
Gauge 18 16 14 12 10 8 6

Under certain circumstances a large wire size may be needed even if your amp requirement does not seem to justify it. Go to the next largest wire size if your run is more than 100 feet, inside a conduit, or ganged with other wires where the heat dissipation may be inhibited. As with all electrical work, consult a professional with any questions about special circumstances.

After the gauge number, there will be a number signifying the amount of service wires in the cable. The picture above shows two service wires plus a ground. If this wire was 14 gauge, it would be labeled "14-2" or "14/2". (The ground wire does not count.)

In the example above, if this wire was 12 gauge it would be labeled "12-3" or "12/3." This wire has three service wires plus a ground.

The example above is 14 gauge 4 conductor stranded wire which is used for most mini split air conditioners. This is known as the control wire. This is the minimum gauge that should be used between the indoor and outdoor units. This is not the correct wire to connect your outdoor unit to your circuit box. For the main unit, refer to the chart above.

Circuit Breakers

When updating an appliance or air conditioner, it is important to determine the correct current draw.

If your circuit breaker is 20 amps, you cannot install a product that requires 30 amps. It is not as simple as just changing the circuit breaker. The wiring to the appliance also needs to be changed.

Although, when the opposite occurs, the solution is easier. If you are going from an appliance that required 30 amps and your new appliance requires only 20 amps, the wire does not need to be changed. The circuit breaker and the outlet receptacle will need to be changed. This is because a thicker wire can safely carry less power, but a thinner wire cannot carry more than it is rated for. If you try to run a 30 amp appliance on a wire rated for 20 amps, the wire will get hot and potentially create a fire hazard.

Below are examples of typical circuit breakers. The one on the left is an example of a 110 volt 15 amp breaker. The 15 on the switch indicates that it is 15 amps. The one on the right is an example of a 220 volt 20 amp breaker. Once again, the 20 on the switch indicates the amperage. These are only examples and your breakers may be different. If you are not sure, consult with your electrician before ordering.

This information is only a guide. If you have questions please consult with your electrician and check your local codes.