What Size Wire Do You Need For A 40 Amp Breaker?

Knowing what size wire do you need for a 40 amp breaker doesn’t mean just considering the length of the wire. It also involves assessing its thickness and the electrical load it can handle.

Having the right wire size ensures you get consistent power, minimal outages, and also a safer electrical set-up, and helps you avoid frustration, financial pain, and terrible accidents. 

What Size Wire Do You Need For A 40 Amp Breaker?

But still the question remains, how do you know what size wire do you need for a 40 amp breaker?

Is it better or worse to underestimate or overestimate your wires, and what about the materials of the wire? Is that important too? That’s what we’ll be exploring in our guide on knowing what size wire do you need for a 40 amp breaker today!

Here you’ll find everything you need to know about what size wire do you need for a 40 amp breaker (see also: What Wire Size Do You Need For 60 Amps?) breaker, and why size actually matters in this case so you can have a safe and efficient electrical set-up.

Let’s get into it!

So What Size Wire Do You Need For A 40 Amp Breaker?

So, what size wire do you need for a 40 amp breaker? The recommended wire size for a 40 amp breaker is an 8-gauge wire.

The accurate wire size is crucial in ensuring the breaker is protected from excessive voltage. Also, it should not trip in the event of an overload.

Meanwhile, using a wire that is smaller than what you need can be very frustrating, as it creates popping sounds when using it for a piece of equipment such as a welder.

This can not only harm your appliance, but you as well.

If the wire is larger than 8-gauge and does not have armored cable markings, this will be suitable for most regular, residential usages like space heaters and dryers. 

8-gauge wires tend to be made of copper. Yet, you can also find these wires made from other materials such as aluminum. 

Aside from knowing what size wire do you need for a 40 amp breaker, you need to learn different wire types. There is a variety of wire types out there for a range of applications, such as:

Cables With Rubber Insulation

These are normally used for wiring plugs and transformers as well as in commercial buildings. 

Bare Copper Wire

This is commonly used in residential applications in ceilings or walls (see also: Where To Buy RV Paneling? (Ceiling And Wall Panels))because of its flexibility.

Wire Carrying A Current To An Outlet

You can tell the amount of current in a cable by its thickness. It is recommended to use 14-gauge wire for 15A circuits, 12-gauge wire for 20A circuits and 10-gauge wire for 30A circuits.

Breaker Box Wires

These wires should be bigger than the wires that go out of the box. It is recommended to use 4/0 AWG wire for 100 ample, while 6/3 or 4/4 AWG wire for 200 amps.

How long a cable is will decide its resistance as well as how fast heat will run through it. Therefore, longer cables require thicker gauges if rated right under the maximum load of the circuit. 

Coils of multicolored electrical wires with exposed copper ends, featuring blue, gray, and green colors, isolated on a white background. RV Lifestyle Experts

Why Getting The Right Wire Size Is Important

Using the incorrect wire size will put unnecessary pressure on your appliance, and therefore draw upon too many amps from the electrical system. This highlights the importance of knowing what size wire do you need for a 40 amp breaker.

It may even result in an electrical fire if not dealt with quickly enough. The wires may overheat, causing them to melt or begin a fire, destroying your appliances and maybe even your home.

A certified electrician should handle the wiring, as they will be more knowledgeable on all the safety requirements and know what to do if an emergency occurs.

As a homeowner, you can set up your electrical system by yourself, but if something goes wrong it is best to seek help from someone who is knowledgeable about wiring.

However, the most important thing is to regularly check the wires to avoid fires, overheating, and shock hazards.

Let’s take a look at more reasons why getting the right wiring is so important.

Drops In Performance

If wires are too small for the load being used, there will be drops in performance.

This is because when using an insufficiently sized wire, it will not be able to power your appliances properly and overheat. 

This can result in an overload of certain appliances in the circuit, or an outage that could cause a fire or lead to other serious issues.

Increased Power Outages

Electrical services might be disrupted because of a drop of voltage in overloaded wires.

The wires are overloaded because they can’t support the load. This can lead to issues such as flickering lights or further outages.

How Do You Make Sure You Get The Correct Wire Size?

Now that you know what size wire do you need for a 40 amp breaker, its time to know to get the correct wire size. To make sure your appliances are always running at optimum performance, you should use an appropriate wire gauge for the length and amount of current the wire can run.

Using a smaller wire can be incredibly dangerous, especially when it’s powering appliances like air conditioners, electric dryer units, stoves, and water heaters, as they have a particular amount of rated load that needs to be met. 

What Size Wire Do You Need For A 40 Amp Breaker (1)

AWG Vs SWG: What’s The Difference?

AWG is an acronym for American Wire Gauge, and – as the name suggests – is a standard used in the US. Meanwhile, SWG is an acronym for Standard Wire Gauge, and is the standard used in the UK.

While the systems have a lot in common, there are some small differences in their size charts. 

For instance, in the AWG system, 8 gauge wire is rated for 40 amps, while it’s rated for 30 amps in the SWG system. 

What Are The Different Kinds Of Wire?

There are two basic types of wire you can use for a 40 amp breaker. These are THHN and XHHW.

THHN is a popular type of wire that is made from one strand of copper (and sometimes aluminum wires).

It is rated for up to 90 degrees Celsius, and is commonly used in interior wiring, and can be found in both stranded and solid forms.

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

Meanwhile, XHHW wire is a stronger kind of wire that is made from more than one strand of copper. It is often used in exterior wiring, and can also be found in both stranded and solid forms. 

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

What Is The Best Wire To Use?

What kind of wire you use will depend on the application. For example, it’s best to use THHN wire for interior wiring. Meanwhile, XHHW wire is more suitable for exterior wiring.

THHN wire is the best choice for indoor wiring as it is fire and corrosion resistant. THHN wire also has a high insulation rating, which makes it great for electric circuits that may experience regular disruption.

Meanwhile, XHHW is the best option for outdoor wiring as it is more resistant to moisture and heat.

It’s also better at withstanding physical damage, making it the ideal choice for electric circuits that are buried or exposed.

How To Read Cable Labels

Reading cable labels can be confusing to say the least, especially when big wires are in the mix. But once you get your head around a couple of concepts, it becomes surprisingly easier. 

The first thing you will see on the wire is the AWG rating. For example, 8-gauge wires will display “8,” and an extra number such as 8-2 or 8/2.

This figure indicates how many service wires are in the cable. Meanwhile, a ‘G’ or ‘w/G’ indicates that there is a ground wire.

Close-up of multi-colored electrical cables with exposed copper wires at the ends, isolated on a white background. RV Lifestyle Experts

How To Size A Wire

Electrical wiring is sized according to the AWG standard, which gives conductive wire a rating according to the wire’s diameter and how many currents it can safely carry.

The AWG figure is commonly printed on the wiring insulation, and the smaller the figure, the larger the wire diameter.

For instance, a 12-AWG conductor has a diameter of 0.08 inches, while a 6-gauge wire has a diameter of 0.16 inches, so it’s the thickest one of the two. 

The size of a wire doesn’t concern insulation and only relates to how thick the conductor is. If the gauge rating on the insulation isn’t visible, you can use a vernier caliper to measure how thick the conductor is.

A set of digital calipers that can give correct readings is the best tool for taking this measurement. 

Factors To Consider When Choosing Wire Size


The material of the wire is a huge factor in selecting a wire gauge. Copper is the most popular material used in household wiring, as it is cost-effective, efficient, and safe.

Other materials have their own unique properties that impact the size of the wire required to carry a suitable current without overheating at high levels of resistance.


To prevent damage to the wires during installation, all bends need to be carried out using either 45 degree or 90 degree bends.

This will boost the longevity of the wires and help to avoid fire hazards.

Thickness Of Insulation

How thick the insulation is can impact how much load can be carried by a specific wire gauge.

Thicker insulation may need larger diameter conductors while thinner ones let smaller conductors carry higher loads of power. To correctly size your wire, you need to know what your insulation will be.

Wire Run

This refers to the length you intend to run your wire. The general rule is to up a size if you intend on running the advised wire size over 100 feet.

So if you intend on running an 8-gauge wire for a 40 amp breaker over 100 feet, you should opt for a 6-gauge wire, as you will lose voltage over that distance, reducing the available amps.

Final Thoughts

Size matters when it comes to wiring. The recommended wire size for a 40 amp(see also: What Size Wire For 20 Amp Wire Size) breaker is an 8-gauge wire. Using a smaller gauge will lead to popping sounds when electricity courses through it.

This can lead to damage to your welder and to any appliances that are also plugged into these outlets. 

In short, using insufficient wires can be frustrating, ineffective, and dangerous too.

We hope our article has given you all the information you need on finding on what size wire do you need for a 40 amp breaker (see also: What Size Wire For 15 Amps? Your Complete Guide), what types of wires are available, what kind of wires you should use for indoor and outdoor usage, and how to correctly size a wire.

But if you have any more questions, check out our frequently asked questions below to see if we have the answers you’re looking for. Good luck planning your electrical set-up!

Frequently Asked Questions

Can A 10-Gauge Wire Cope With 40 Amps?

No, a 10-gauge wire will be unable to handle 40 amps. It is advised that you use a 10-gauge wire to handle only 30 amps.

For a 40 amp wire size, you will need an 8-gauge copper wire or a 6-gauge aluminum wire.

What Wire Size Should You Use To Run 40 Amps 100 Feet?

For a 100 ft, 40 amp load you will need an 8 AWG wire. For anything over 100 feet, you should go for a larger gauge wire to compensate for any drop in voltage because of the increased distance.

Eddie Meyers
author avatar
Eddie Meyers
My name is Eddie, and I am an RV enthusiast! I have enjoyed traveling all my life, and want to give my family the same opportunities to see as much of the world as possible. As I’ve gotten older, I’ve discovered how great RVs can be for vacations with a young family. Here at RV Lifestyle Experts, I want to share my experiences, the tips I’ve picked up along the way. I hope that our guides can encourage you to try the RV lifestyle. I promise you that you won’t regret it!
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