What Size Wire Do I Need for a 40 Amp Breaker?

When it comes to electrical house wiring, wire sizing is crucial. The normal wire size for a 40 amp breaker is 8 awg. However, this will depend on the type of wire used and the wire length.

If the length of the circuit is over 100 feet, there will be a need to boost the amps.

Circuit breakers and wires are usually rated in amperage, and American wire gauge (AWG) respectively.

If you use a wrong size wire on the circuit breaker or vice vasa, it may cause electrical problems such as overload, or electrical fire.

Benefits of using the right wire size for a 40 amp circuit breaker

Using the right wire gauge can help protect the circuit and appliances. If you use a wrong wire size, it can cause overload and fire hazard. 

The size of wire determines how much current it can safely carry. If the current is higher than the wire rating, it will lead to heat buildup.

But if you choose the right wire size, it will ensure the safety of your appliances, reducing the risk of damage or danger.

Wire size chart table

AWG NumberCross Sectional Area (mm”)Ampacityohms/km
180.821420.95Low voltage lighting
161.311813.18Extension cords
142.08258.28Lighting fixtures
123.31305.21Kitchen appliances
105.26403.28Electric dryers
88.37552.06Electric ovens
613.30751.30Large electric heaters
421.15850.81Large furnace
326.671150.65Large commercial wiring
233.631300.51Car battery cable
142.411450.41Power distribution
1/053.471700.32Power distribution
2/067.431950.26Power distribution
3/085.032250.20Power distribution
4/0107.222600.16Power distribution
250126.682900.13Power distribution
350177.353500.10Power distribution
400202.683800.08Power distribution

Common Appliances That Require 40 Amp Breaker

The most common appliances that require a 40 amp circuit breakers are the high-powered appliances such as

  • Electric stoves
  • Water heaters
  • Ovens
  • Air conditioners
  • Electric dryers, etc.

Factors that determine the 40 amp wire size

When choosing a wire for your breaker, there are some factors that will determine correct wire size. These includes.

1. NEC 80% Rule

This is a safety rule that requires using a wire size that is rated for at least 80% of the breakers amperage, to install your circuit breaker. This is to protect the circuit from overload and electrical fire.

So to get the 40 amp wire size, you will divide 40 amp by 80%. 

40 A divided by 0.8 will give 50 amp. If you go to the wire size chart ampacity table, you will notice there is no 50 amp there.

In that case, you will use the 55 amp wire size, which is 8 gauge wire. So, the correct size wire for 40 amp breaker is 8 awg.

Wire length

NEC 310-16 rule requires we add 10 or 20% amps for every 50 and 100 feet away from the sub panel.

For example, let’s assume you are sending current to a sub panel 50, 100, 150, or 200 feet away.

All you will do is to multiply the ampacity with ohms.

Here is an example:

The 40 amp wire size at 50 feet sub panel is 50 A x 1.1 = 55 amps.

Wire size at 100 ft = 50 A x 1.2 =60 amps

Wire size at 150 ft = 50 A x 1.3 = 65 amps.

40 amp size of wire at 200 feet = 50 x 1.4 = 70 amps.

Then you go to the awg wire size chart table and look for their wire sizes.

Type of wire or conductor used

The type of wire used in running the circuit will determine the correct wire size to use.

Copper wires are best for home wiring as they can withstand any stress without overheating, while aluminium expands and contract with change in temperature

So, the best wire for this installation is copper wires. Using aluminium will not only require larger wire sizes, but can also cause electrical hazards.


From this article, you can see that wire sizing is important in electrical installation. It determines the resistance and the maximum sustainable current.

If you choose a smaller wire size than recommended, it can cause circuit overload and other safety hazards.

The recommended gauge of wire for a 40 amp breaker is 8 AWG copper wire. However, this will depend on some factors, such as the 80% nec rule, length of wire, and the type of conductor used.

You can call an electrician for help if you cannot size the wire on your own.

40 amp wire size FAQ

Can a 10 gauge wire handle 40 amps?

No, a 10 gauge wire cannot handle a 40 amp circuit. 10 gauge wires can only handle 30 amp breakers.

For a normal size wire for a 40 amp service, you will need an 8 gauge copper electrical wire, or 6 gauge aluminium wire.

Can I use 6 gauge wire for a 40 amp breaker?

Yes, you can. The normal size wire for a 40 amp circuit is 8 gauge copper wire. However, you can still use larger wires like 6 awg to install the circuit.

But on no account should you install a 40 amp circuit with smaller wires because it violates electrical code, and can also cause electrical hazards.

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About mariaelectricals

Hi, I am Emmanuel Nwankwo, a commercial electrician and the founder of mariaelectricals.com. I established this blog to share my decades of work experience in electrical installations and repairs.