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 you need for your breaker.

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).

Gauge is a measurement of a wire, specifically its diameter. AWG is the standardized system of this measurement. Wire gauge is measured from high to low with higher numbers meaning a smaller wire size.

Precise wire sizing is essential for circuit breakers as it helps determine how much electrical current can flow through it and how much resistance it has. The chart below will tell you approximately what wire size is needed for the amount of power that will be running through the line.

Please note that the information within this article is intended only as general guidelines. For any questions, always consult a certified electrician, as well as your local electrical codes.

Wire Size Ampacity Chart

In the chart below, we outline what gauge wire you need for the maximum amperage, or strength, of the electrical current. Note that the gauge can sometimes vary between aluminum and copper wires. Below is the gauge size based on the more commonly used copper wires.

Maximum Amps 7 10 15 20 30 40 55 70 95
Gauge (Wire Size) 18 16 14 12 10 8 6 4 2

Here are some more details about some of the more popular amperage sizes. Remember that when you use an appliance, always check the amp requirements first. You can calculate this by dividing Watts by Volts.

10 AMP Wire Size

For a maximum of 10 amps, you’ll need a wire gauge of 16. Common appliances that require around 10 amps include: a toaster, a hair dryer, a vacuum cleaner, a radiator, a washing machine, a dishwasher, and a refrigerator.

30 AMP Wire Size

For a maximum of 30 amps, you’ll need a wire gauge of 10. The most common household item that requires a 30 amp circuit is a central air conditioner.Often, people living in RVs use 30, and sometimes 50, amp systems to run all the appliances in the vehicle.

40 AMP Wire Size

For a maximum of 40 amps, you’ll need a wire gauge of 8. Many electric cooking appliances require 40 amps such as electric cooktops.

50 AMP Wire Size

For a maximum of 50 amps, you’ll need a wire gauge of 6. Fifty amp breakers are most often used to power many different appliances. However, a kitchen oven can alone require 50 amps. Many electric dryers also require a 50 amp breaker. 

Reading Cable Labels

You’ll often buy wires in cable, but it’s important to know how to accurately read the product labels in order to purchase the correct product. Here’s a quick look at what to remember:

  • First, you’ll often see the AWG of the wire(s) in the cable. So for a 14 gauge wire, it will read “14.”

  • Next, you may see an additional number. This could read in either of these 2 formats: “14-2” or “14/2.” This number signifies the amount of service wires or conductors in the cable. 

  • Then you may see a “G” or “w/G.” Both mean that the cable comes with a ground wire, which does not count towards the total number of wires noted on the label already.

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. For example, let’s say you were using an appliance that requires 30 amps. If you replace this with a new appliance that only requires 20 amps, you will need to change the circuit breaker and outlet receptacle — but you do not need to change the wire.This is because a wire can safely carry less power than it was rated for without any potential issues. However, a wire cannot carry more power than it was rated for, otherwise it could get hot and create a fire hazard.

Other Aspects to Consider

Wire length:

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.

Wire material:

If you are using a wire that is not made of copper, brass or silver, you will want to check to see if you need a different size. Aluminum wires are much less common than copper ones, and they also vary from them in requirements: They offer 61% of the conductivity of copper wires but have only 30% of the weight of copper.

What Will Happen With the Wrong Size Wire?

It is incredibly dangerous to use a wire that has a smaller gauge than needed. The wire could overheat and melt. This could lead to breaker or appliance damage and be a fire hazard.

Using a wire that has a larger gauge than needed is not dangerous. It can be an inconvenience, since a larger wire is usually heavier and stiffer, but it won’t lead to any potential hazards.

Finding the right wire size is an essential step to any electrical project or circuit breaker set-up. It’s especially important to remember that it’s about more than just the number — remember to consider all the possible factors when it comes to wire gauge. The information in this article is only a guide — we highly recommend that you consult with your electrician, and check your local and national electrical codes, before purchasing and installing wires.