What Size Wire For 40 amps Breaker?

When it comes to figuring out what size wire you should use for your 40 amps breaker, there are a few things you need to take into account.

The amperage of the breaker is going to dictate how much power can flow through the circuit, and the wire gauge is going to dictate how much current can flow through the wire.

In most cases, you're going to want to use a wire gauge that is rated for at least 40 amps.

What is the Recommended Size Wire for 40 Amp Breaker?

The recommended size wire for a 40 amp breaker is 8 AWG. This is the American Wire Gauge, and it is the standard that is used in the United States for determining wire size.

8 gauge wire is rated for 40 amps, and it can safely handle up to 50 amps.  This 40 amp wire size can safely handle most needs.

Why You Should Use the Recommended Size Wire

It's important to use the recommended size wire for your 40 amp circuit breaker because if you use a wire that is too small, it can overheat and cause a fire.

If you use a wire that is too large, it can be difficult to work with, and it may not fit in the breaker properly.

You also need to make sure that the circuit is properly grounded. This will help protect you from a short circuit, shock and other hazards.

To properly ground your circuit, you'll need to use a grounding rod or plate.

Finally, make sure that you have all of the proper tools and supplies before you start any work. This includes a wire stripper, a screwdriver, and a hammer.

When in doubt, it's always best to consult with a professional electrician like the expert electricians at Bates Electric to ensure that your circuit is properly wired or if you need an upgrade to your electrical panel.

How Many Outlets Can I Wire On The Same Circuit

How Do You Size a Wire?

When you're sizing a wire, you need to take into account the amperage of the circuit and the length of the wire. The longer the wire, the more resistance it's going to have, and that resistance is going to cause heat.

The thicker the wire, the less resistance it's going to have, and that means it can carry more current without getting too hot.

In general, you want to use the thickest wire possible for your circuit. For a 40 amp circuit, that would be 8 gauge wire.

What Is the Difference Between AWG and SWG?

AWG stands for American Wire Gauge, and it is the standard that is used in the United States. SWG stands for Standard Wire Gauge, and it is the standard that is used in the United Kingdom.

Both systems are similar, but there are slight differences in the size chart between them.

For example, in the AWG system, 8 gauge wire is rated for 40 amps, but in the SWG system, it is only rated for 30 amps.

In general, you want to use the AWG system when you're working in the United States and the SWG system when you're working in the UK.

What Are the Different Types of Wire?

There are two main types of wire that you can use for your 40 amp breaker: THHN and XHHW.

THHN wire is a common type of wire that is made from a single strand of copper (there is aluminum wires as well, but copper is more widely used). It is rated for up to 90 degrees Celsius. It is usually used in interior wiring, and it is available in both solid and stranded forms.

For example, 14 AWG THHN wire is rated for 15 amps, and 12 AWG THHN wire is rated for 20 amps.

XHHW wire is a more heavy-duty type of wire that is made from multiple strands of copper. It is typically used in exterior wiring, and it is available in both solid and stranded forms.

For example, 14 AWG XHHW wire is rated for 30 amps, and 12 AWG XHHW wire is rated for 40 amps.

Size Wire for 40 Amp Breaker

Which Wire Should You Use?

The type of wire you use is going to depend on the application. If you're doing interior wiring, THHN wire is a good choice. If you're doing exterior wiring, XHHW wire is a better choice.

THHN wire is a good choice for indoor wiring because it is resistant to both corrosion and fire. It also has a high insulation rating, which makes it a good choice for use in electrical circuits that are subject to frequent interruption.

XHHW wire is a better choice for outdoor wiring because it has a higher resistance to both heat and moisture. It is also more resistant to physical damage, making it a good choice for use in exposed or buried electrical circuits.

What Other Factors Do You Need to Consider?

In addition to the amperage and the length of the wire, you also need to consider the voltage of the circuit. The higher the voltage, the thicker the wire needs to be.

For example, a 240-volt circuit requires 10 gauge wire, but a 120-volt circuit can use 12 gauge wire.

You also need to take into account the environment where the wire will be used. If it's going to be exposed to weather or chemicals, you'll need to use a wire that is rated for that environment.

80% Rule for NEC

The 80% rule for NEC is a safety guideline that recommends using a wire gauge that is rated for at least 80% of the breaker's amperage. This will help to prevent overloading the circuit and potentially causing a fire.

For a 40 amp breaker, this would mean using a wire gauge that is rated for at least 32 amps. This is due to the fact that the 80% rule is a conservative estimate, and using a wire gauge that is rated for more amps than the breaker will not cause any problems.

If you're unsure of which wire gauge to use, it's always better to err on the side of caution and use a wire gauge that is rated for a higher amperage than the breaker. This will help to ensure that your electrical circuit is safe and up to code.

40 Amps Wire Length & Type Of Wire

The NEC also has a rule that requires the use of larger wires for longer runs. This is to prevent voltage drop, which can cause the circuit to overheat and potentially start a fire.

For example, a 14 AWG THHN wire should not be more than 100 feet, and a 12 AWG XHHW wire should not be more than 100 feet.

The type of wire you use is also going to play a role in the length of the wire. In general, THHN wire can be used for longer runs than XHHW wire. This is because THHN wire has a lower resistance, which means it can carry more current without getting too hot.

XHHW wire is a better choice for shorter runs because it has a higher resistance, which means it will get hotter faster than THHN wire. For example, a 14 AWG XHHW wire should not be more than 50 feet, and a 12 AWG XHHW wire should not be more than 25 feet.

The type of wire you use will also play a role in the overall safety of the circuit. For example, if you're using THHN wire in an exposed electrical circuit, it's important to make sure that the wire is protected from physical damage.

To do this, you can use electrical tape or conduit to cover the wire.

In general, you should always use the heaviest gauge wire that your application allows. This will help to prevent overheating and potentially starting a fire. It will also reduce wear and tear on your breaker, as using the wrong wire gauge can result in the breaker tripping very often.

Final Thoughts

Overall, the size and type of wire you use is going to depend on the application. THHN wire is a good choice for indoor wiring, while XHHW wire is a better choice for outdoor wiring.

When choosing a wire gauge, it's important to follow the 80% rule for NEC. This will help to prevent continually tripping your breaker or potentially causing a fire.

If you're unsure of which wire gauge to use, it's always better to err on the side of caution and use a wire gauge that is rated for a higher amperage than the breaker.

Sometimes wiring is best left to the trusted experts.

Feel free to reach out to one of the experienced electricians here at Bates Electric for a free quote.  We will help to ensure that your electrical circuit is safe and up to code.

Contact us here by phone to schedule a free quote or fill out the form below.

Contact Form

  • This field is for validation purposes and should be left unchanged.

Blog Categories

Scroll to Top