The minimal wire gauges for a 40 Amp circuit breaker are 8 AWG copper or 6 AWG aluminum. This is based on the allowable ampacity of these particular cable sizes: the ampacity of the 8 AWG copper wire is 50 amps at 75°C, and the ampacity of the 6 AWG aluminum wire is 50 amps at 75°C. 

6 AWG copper wire with an ampacity of 65 amps at 75°C and 4 AWG aluminum wire will also be excellent choices if you are dealing with higher loads or want to be safe from an overload.

If your wire for a 40 AMP breaker runs for a long distance, you should accommodate a voltage drop when choosing a 40 Amp breaker wire size. This means using 6-gauge copper wire or 4-gauge aluminum wire for cables running over 100 feet and using 4-gauge copper wire or 2-gauge aluminum wire for cables running over 200 feet.

What can a 40-Amp Circuit Handle?

A 40-Amp circuit, in most cases, is a dedicated circuit set aside for a specific device. A 40-amp breaker is capable of handling up to 4800 watts. Some of the common applications of 40-Amp dedicated circuits include:

• Common models of electrical ranges and ovens
• Large electric water heaters (75 gallons and above)
• Central air conditioning units
• Some Level 2 EV Charging Stations
• Hot tubs and spas

You should always refer to the manufacturer information to ensure that your device requires a 40 Amp dedicated circuit. 

 Is 10 Gauge Wire Good For 40 amps?

10 AWG wires and 12 AWG wires are not suitable for 40-amp circuit breakers. 10 AWG and 12 AWG copper wires have 35 Amp and 25 Amp wire ampacity. Using them for a 40 Amp circuit is inherently wrong and might result in an electric accident. 

The only reason people might consider using 10 AWG or 12 AWG for 40- Amp circuits is that it is a popular wire size that they have in storage. This is not the right motivation to use an electrical cable that does not fit the size required for a 40-amp circuit.

Which Cables are Used For 40 Amp Wiring?

Common cables for dedicated 40-Amp circuits are NM-B, UF-B, MC, and THWN.

• The most commonly employed cable for 30 Amp circuit breakers in indoor applications is NM-B Romex® Cable, provided all components are indoors, and the cables do not need extra mechanical protection.
• UF-B cables are an appropriate choice for appliances entirely or partially placed outdoors or those that come into contact with water and dampness, such as water heaters. Note that UF-B cables cannot be used for hot tubs as per NEC because of specific grounding requirements for this application.
• If the cable is going to be run exposed or subject to mechanical damage, MC metal-clad cables are a suitable option for 30-Amp circuit breakers. However, these cables are not suitable for instances where there is direct exposure to moisture.
• Copper and aluminum THWN can be used as an alternative in conduit for exposed applications. The installation process is more complex, however, as it necessitates the separate placement of hot, neutral, and ground wires. THWN is an excellent option for hot tub wiring. However, hot tubs do not favor aluminum wiring, so opting for copper is best.