What Size Wire You Need for a 40-Amp Breaker

A 40-amp breaker is a reasonably heavy-duty breaker intended for use on a circuit connected by large appliances, such as electric stoves and cooktops. The circuit breaker is only one component in the electrical circuit and is necessary to protect the wiring and the appliance. The wiring of the circuit must also be of an appropriate size to cater to the current draw. So what wire size is suitable for a 40-amp breaker?

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

Using the correct size wire for a circuit and the circuit breaker is essential for safety in the electrical circuit. The unsafe circumstance is to use a wire gauge that is too small for the breaker, but you can use a wire gauge that has a rating of more than 40 amps. The current that will pass through a 40-amp breaker is quite substantial, so it is vital to use the correct wire. Let’s explore which gauge of wire would be the best choice for this breaker.

What Size Wire Do You Need for a 40-Amp Breaker?

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 up to 40 amps only.

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

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

The resulting short circuit has the potential to cause lethal shock to people or start a 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.

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 get 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 ok if the amperage spikes periodically over 80% capacity as appliances startup, but the circuit load must not consistently exceed this maximum.

To calculate the maximum amps that you can put through a 40-amp breaker, use the following formula.

x
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40 amps X 0.8 = 32 amps

The maximum number of amps that 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 size of the circuit breaker providing the wiring gauge is suitable.  

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 will be. For example, a 12-AWG conductor has a diameter of 0.08-inches, while a 6-AWG wire has a diameter of 0.16-inches, making it the thicker wire of the two.

The sizing of a wire does not take the insulation into account 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 are best for taking this measurement.

Here is a table for the most common wiring sizes used in domestic dwellings 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 used as the conductor and the length of the wire.

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

Conclusion

The size wire 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 code.

Using smaller gauge wire for a 40-amp breaker not only violates the code but 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 with a qualified electrician to inspect the system before making any changes to the circuits.

Sources

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Hubert Miles

I've been conducting home inspections for 17 years. I'm a licensed Home Inspector, Certified Master Inspector (CMI), and FHA 203k Consultant. I started HomeInspectionInsider.com to help people better understand the home inspection process and answer questions about homeownership and home maintenance.

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