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40 Amp Wire (Few Things You Must Know)

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Last updated on August 14th, 2022 at 01:25 pm

When setting up your 40 amp circuit, you need to consider the wire size you need for the 40 amp circuit.

Specifically, you need to select the correct American Wire Gauge (AWG) gauge wire that will handle the current.

The 40 amp circuit breaker is best for large appliances like electric stoves and refrigerators. The 40 amp wire size must be suitable for carrying the current. 

According to the National Electric Code, the wire size for a 40 amp circuit breaker is 8 AWG. An 8 AWG wire gauge can handle a 40 amp circuit breaker. However, if you use a low wire gauge, it may damage your appliance and also render your electric system non-compliant.

What Size Wire Do I Need for 40 Amps?

40 Amp Wire

You will require an 8 AWG gauge wire for 40 amps. You can use the larger 6 AWG wire if you work with future-proofing. The National Electrical Code (NEC) requires calculating the AWG gauge.

According to the NEC, the maximum loading for a circuit branch is 80% of the circuit rating for wire ampacity for any load. 

You might also know it as NEC wire sizing, which applies to 40 amp sizing. If you have not sized a wire before, you must understand how to calculate the AWG wire gauge.

Below are the steps to calculate the American Wire Gauge wire sizing:

  • Apply the NEC minimum ampacity requirement for your 40 amp circuit breaker.
  • Multiply 40 to 100 percent and divide the result by 80 percent. This will give you 50A amperage.
  • This means that you need a 50A amperage. An 8 AWG gauge wire has 50A amperage and is the best wire size for your 40 amp circuit. 

Several electric wiring installers use larger wire gauges like the 6 AWG wire because it offers great flexibility in case of a possible future expansion.

Though the 6 AWG gauge wire is for 50 amps, using this wire gauge for a 40 amp circuit breaker will not stop it from tripping when it has reached its max current draw.

The electric current that passes through a copper wire gauge receives resistance from its wire, which causes heat in the conductor.

The width of the copper wire conductor determines the current flow rate that it can handle and prevents the heat from accumulating to a dangerous level.

The wider the AWG wire gauge, the broader the space for current flow in the wire. This subsequently reduces the heat build-up and resistance in the conductor.

You can compare this to a water pipe. A thin pipeline allows a specific amount of water to pass through it. Increasing the volume of water passing through the pipe will increase the pressure and damage the hose.

In the same way, the bigger the wire gauge, the larger the conductor that allows the electric current to pass through.

When you overload the current passing through the wire, the conductor will accumulate to a high point and melt the insulator covering the wire. 

This can cause the open wire to come in contact with metal surfaces, wet surfaces, or other cables, which results in a short circuit.

The short circuit can cause damage to your electrical wiring, dangerous shock, or a fire outbreak in your home. 

Therefore, you should adhere to the standards set by regulatory officials regarding the min and max wire gauge you need for your specific volume of current transfer.

The American Wire Gauge sets the measures for determining the building codes for wire width measurement. The bigger the wire diameter, the smaller the wire gauge you need.         

What to Consider to Select the Best Wire for Your 40 Amp Circuit?

Square D HOM240 Plug-In Mount Standard Miniature Circuit Breaker 2-Pole 40 Amp 120/240 Volt AC Homeline

For your large appliances, a 40 amp circuit breaker is essential for your home appliances to run well.

However, you must understand that other factors affect the wire size you would choose for your circuit breaker. 

Some of these factors include the length of the wire, the circuit amp rating, and the type of wire you need per National Electrical Code requirements.

Considering these factors will help secure your home and ensure you pass any home inspection. Below are some of these factors:

#1. NEC 80% Rule

As per National Electrical Code standards, 80% of the capacity of your circuit breaker is suitable for usage. If you overload your current with more by using over 80%, the circuit breaker will trip off.

This means you can use only a maximum of 32 amps if you use a 40 amp circuit breaker with an 8 AWG wire gauge.

#2. Length of the Wire

The length of the wire is also a significant factor to consider. If your circuit line is lower than 100 feet, an eight gauge wire size is suitable for your 40 amp breaker.

However, if the circuit line is longer than 100 feet, you might need to use a six gauge wire for your 40 amp breaker. This will lower the resistance from curving the wire and the distance obstacle. 

#3. Wire Type

Before selecting the wire size, you need for your 40 amp circuit breaker, consider the wire material. Wiring manufacturing companies usually use copper and aluminum materials for electric cables.

Aluminum wires are more cost-effective than the copper alternative since they are 30 percent lighter

However, copper is better than aluminum when it comes to electricity conduction. It has a better heat resistance and strength than aluminum material.  

40 Amp Wire Size Chart         

Wire lengthAWG
10 inches8
15 inches6
20 inches6
25 inches4
30 inches4

What Size Wire Do I Need for a 40 Amp 240 Volt?   

Southwire Building Wire 8 Ga, 3 Conductor 40 Amp 600 V 90 Deg C 125 ' Black

For your 40 amp, 240 volts circuit breaker, you will need an 8 AWG wire gauge. You can measure the voltage capacity your wire gauge should carry in American Wire Gauge (AWG).

Therefore, wire gauges smaller than 8 amp will be useless since they cannot hold the current flowing through them. 

The thickness of wires varies, and its measurement factor is the AWG wire gauge. Use your wire gauge according to the capacity of the voltage in question.

Using a gauge higher than the appropriate wire gauge might make the current redundant, as eight AWG is a more suitable gauge for the 240 volts.

You should also avoid using a lower gauge for a 40 amps, 240 Volts circuit breaker. In this way, you can ensure the safety of your wire gauge and your home.

What Size Wire Do I Need for 40 Amp 220 Volt? 

The size of wire you require for your 40 amp, 220 volts circuit breaker is an 8 AWG wire gauge. This is the most suitable wire size for your circuit breaker, as it meets the high requirements of the circuit. 

Understanding your wire ampacity ensures you do not exceed the correct size of wire you need to use, as it may cause overheating in your wiring.

To guarantee maximum safety, ensure you use the right size of wire that can carry the current load.

Wires are metallic and thus are high conductors of electricity. As gauge wires vary in width, so do amperage values vary.

This means that for a wire to transmit a 40 amps electric current, it should be wide enough.

What Is a 40 Amp Breaker Used For? 

You can connect your 40 amp circuit breaker to your electric ranges. You can also use it for circuits that carry large appliances.

The 40 amp breaker is a component in your circuit that protects your electric wiring and components. You must match the correct size of your wire to its corresponding circuit breaker.  


The standard wire size for your 40 amps circuit breaker is the 8 AWG wire gauge. This is also in line with the National Electrical Code 80%  standard.

Using the correct wire gauge size for your circuit breaker will keep your home from electrical accidents like shock and fire outbreaks. 

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