40 Amp Wire Size: Wire & Breaker Size Guide

40 amp wire size is an important consideration for electrical wiring. With the correct gauge and type of wire, electricians can ensure that the power transmitted over an electrical circuit is safe and efficient. So, what size wire for a 40 amp breaker do you need?

The minimum wire size you can use with a 40-amp circuit breaker is 8-AWG. An eight-gauge wire is rated to handle a 40 amp breaker’s electric current. You can use a larger gauge 6-AWG wire for future-proofing, but a lower gauge wire will render the electrical system non-compliant with the NEC.

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A 40-amp breaker is a heavy-duty breaker intended for use on a circuit connected by large appliances, such as an air conditioner, or appliances, such as electric cooktops. The common breaker size for electric dryers is 30 amps, and for electric stoves is 50 amps.

The wiring of the circuit must also be of an appropriate size to handle the current draw. Always check the amp rating or wattage on the appliance nameplate for the correct wire size and breaker ratings.

The 40 amp circuit breaker is only one component in the electrical circuit and is necessary to protect the wiring and the appliance.

Using the correct size wire for a circuit and the circuit breaker is essential for safety in the electrical circuit. The unsafe circumstance is using a wire gauge that is too small for the breaker, but you can use a wire gauge with a rating of more than 40 amps.

The current that will pass through a 40-amp breaker is quite substantial, so using the correct 40 amp wire size and type of wire is vital. Let’s explore which gauge of wire would be the best choice for this breaker.

wire size chart - wire gauge sizing conductors

What Wire Size For a 40A 240V Circuit?

The minimum wire gauge for a 40-amp circuit breaker is 8-AWG solid core copper wire. Many installers will use a larger gauge wire, such as a 6-AWG wire, which offers flexibility for future expansion, and still put a 40-amp breaker on this circuit, which is an acceptable practice.

Even though a 6-AWG wire is rated for 50 amps, putting a 40-amp circuit breaker on this wire gauge will cause the breaker to trip when its maximum current draw is exceeded, irrespective of the wire gauge. Thus, the circuit breaker will still protect the appliances connected to a 240v 40 amp wire size.

Electrical current flowing through a copper wire encounters resistance from the wire, generating heat in the copper conductor. The thickness of the wire conductor will determine how much current flow the conductor can handle before the heat build-up becomes dangerous.

The thicker the wire gauge, the more space for current to flow through the wire, thus less resistance and less heat build-up in the conductor.

You can think of it in terms of a hosepipe. A thin pipe will only allow a specific volume of water through the pipe. If you push more volume through the pipe, the pressure will increase, and the pipe can burst.

Similarly, the larger the wire gauge, the larger the “pipe” that the electricity can pass through without the heat build-up of pushing too much current through a thin “pipe” or conductor.

The danger of pushing too much current through a thin wire is that the conductor will heat up to the point where it melts the protective insulator around it. Once this happens, the wire can touch other cables or metal surfaces, causing a short circuit.

The resulting short circuit can potentially cause lethal shock to people or start an electrical fire in your home. The wiring can also melt completely through, breaking the circuit and requiring the wires to be re-installed in your home.

Regulatory authorities have set standards regarding the minimum wire gauge suitable for specific amounts of current draw. The baseline standards used by the building codes are from the American Wire Gauge or AWG, which is a standard for measuring the thickness of a wire.

The wire gauging system works in reverse, so the smaller the wire gauge, the larger the diameter of the wire.

Related reading: 80 Amp Wire Size: Breaker & Wiring Gauge Guide

40 amp wire size

How Many Amps Can You Put On a 40-Amp Breaker?

A circuit breaker will trip or disconnect the circuit once its maximum ampacity has been reached. In the case of a 40-amp circuit breaker, if more than 40 amps are drawn through the circuit, the breaker will trip.

The NEC or National Electrical Code requires that not more than 80% of the circuit breaker capacity be put through the circuit over a long time. The NEC defines a “long time” as more than 3 hours continuously.

This principle means that the continuous current drawn through the circuit must not exceed 80% of the circuit breaker’s capacity for more than 3-hours. It is okay if the amperage spikes periodically over 80% capacity as appliances start, but the circuit load must not consistently exceed this maximum.

Use the following formula to calculate the maximum amps you can put through a 40-amp breaker.

40 amps X 0.8 = 32 amps

The maximum number of amps you should put through a 40-amp circuit breaker is 32 amps. 

If the circuit regularly draws more than this current, you should consider increasing the circuit breaker size if the wiring gauge is suitable.  

Related reading: 70 Amp Wire Size: Breaker & Wiring Gauge Guide

How Do You Size a Wire?

The sizing of electrical wiring is according to the American Wiring Gauge standard, which gives the conductive wire a rating according to the diameter of the wire and the amount of current it can carry safely.

The AWG number will usually be printed on the insulation of the wiring, and the smaller the number, the larger the wire diameter. For example, a 12-AWG conductor has a diameter of 0.08 inches, while a 6-gauge wire has a diameter of 0.16 inches, making it the thicker wire of the two.

The sizing of a wire does not consider the insulation and only corresponds to the thickness of the conductor itself.

If you cannot see the gauge rating on the insulation, you can use a vernier caliper to measure the thickness of the conductor. A set of digital calipers that will give accurate readings is best for this measurement.

The following chart shows domestic dwellings’ most common wiring sizes as a basic guideline.

Wire GaugeDiameter in InchesRated Amps
12-AWG0.080820 amps
10-AWG0.101930 amps
8-AWG0.128540 amps
6-AWG0.162055 amps

These are, however, basic guidelines for circuits where the conductor is copper. Other aspects should be considered to properly size electrical wire, such as the material of the wire used as the conductor and the wire length.

Not all wires use copper or brass as conductive metal; although not common, sometimes aluminum is used as an alternative conductor. Aluminum wire has different conductive properties, and the requirements vary compared to copper wire conductors.

40 Amp Wire Size FAQs

You may have additional questions about 40 amp wire sizes and breakers. Here are some frequently asked questions about 40 amp wire sizes. Remember that you need an 8-gauge copper wire or a thicker 6-gauge aluminum wire.

Can 10 gauge wire handle 40 amps?

No, a 10-gauge wire cannot handle 40 amps. It is recommended that 10 gauge wire handle up to 30 amps. For a 40 amp wire size, you’ll need an 8 gauge copper wire or a 6 gauge aluminum wire.

What size wire do I need to run 40 amps 100 feet?

You would need to use an 8 AWG (American Wire Gauge) wire for a 40 amp load that is 100 feet long. Beyond 100 feet distance, you should opt for one larger gauge wire to offset voltage drop due to the increased distance.

How far will 8 gauge wire carry 40 amps?

If the 40 amp wire size you have chosen runs farther than 100 feet, you should opt for one larger gauge to account for voltage drop. This will ensure that all necessary safety protocols are followed and proper power flow is maintained.

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

The correct wire size you need to operate with a 40 amp breaker safely is an 8-AWG wire. It is possible to install a 40-amp circuit breaker on a larger gauge wire, such as a 6-AWG wire, but installing it on a smaller gauge wire will violate the electric code.


The correct 40 amp wire size needed to operate with a 40-amp breaker safely is an 8-gauge wire. It is possible to install a 40-amp circuit breaker on a larger gauge wire, such as a 6-AWG wire, but installing it on a smaller gauge wire will violate the code.

Using a smaller wire gauge for a 40-amp breaker violates the code and presents a safety hazard for your home.

If the wiring in your home is not a copper or brass conductor, it may be aluminum, which has different applicable standards. If you are unsure or concerned about any of the wiring standards in your home, it would be safer to consult a qualified electrician to inspect the system before doing any electrical work and making any changes to the circuits yourself.


Hubert Miles | Licensed Home Inspector, CMI, CPI

Hubert Miles is a licensed home inspector (RBI# 2556) with more than two decades of experience in inspection and construction. Since 2008, he has been serving South Carolina through his company, Patriot Home Inspections LLC. As a Certified Master Inspector, Hubert is dedicated to providing his expertise in home inspections, repairs, maintenance, and DIY projects.