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40 Amp Wire Size: Everything You Need to Know!

Picking the correct size wire for your circuit breaker is extremely important. Because otherwise, it can create a hazardous situation.

 A technician can give you a solid answer but they require a fee. But luckily, you can determine the required size of wire by yourself. 

What is the best choice for a 40 amp wire size

8 AWG copper wire or 6 AWG aluminum wire are the only options. Because if you use thinner wire like 10 AWG wires, it might melt. It can also catch on fire which has a high probability. But choosing a thicker wire will allow more electricity flow. That will trip your circuit breaker. 

Anyways, that was just nothing but just a brief answer. I have discussed further and elaborated on the ampacity and wire sizes. 

Keep reading if you want to find out more about wire gauges and ampacity!

Why Is Wire Size Important?

Let’s start with the basics before we start talking about the wire sizes. In case you’re new, you should understand the fundamentals first. 

Because that would help you size wire if the ampacity changes in the future. 

For example, the wire size for a 70 amp breaker would be totally different. Because higher ampacity will generate more heat.

But what is this heat and should you pay attention to it? Yes, absolutely. 

You see when electricity flows through the wire, it meets obstacles. A lot of obstacles, to be precise! These obstacles are mostly atoms of the wire material. 

These atoms are called resistance. Thicker wires have less resistance because they carry less current. 

But a longer wire will have more resistance due to having more wire material. 

Let me make this easier for you. Imagine the wire as a water pipe of a specific width. Because of the lower width, less electricity would flow. 

Because of that less heat will be generated. 

But now, increase the width to allow more electricity flow. This means more electrons trying to pass through. They’ll clash against the wire atoms; producing a higher heat. 

And that’s why you can’t just use any wire of your choice. When there’s too much resistance, it’ll generate a lot of heat. As a result, the wire would get melted. 

Melted wires can be key reasons behind a short circuit. Wires are flammable so fire hazard is also something to think about. 

That’s why determining the correct wire size is important and should be encouraged. 

On that note, you can check the total amp size by yourself. Don’t forget to do it unless you’re absolutely sure about the total amp. 

Choosing the Wire Size Based on The NEC Chart 

To avoid potential disasters, the United States has implemented an NEC chart. This chart indicates what wires should be used for what ampacity.

Houses in the United States have to comply with the NEC chart. This NEC code makes sure the wires don’t get too hot to avoid fire hazards. 

Explaining without enough context might confuse you. That’s why I have provided the chart first. Afterward, I will proceed to explain the contents of the chart. 

Here’s what you’re looking for:

AmpacityAluminum ConductorCopper Conductor
20 Amps#12 AWG12 AWG
30 Amps#10 AWG10 AWG
40 Amps#6 AWG8 AWG
50 Amps#4 AWG6 AWG
60 Amps#3 AWG4 AWG

As you can see, there are two different values because of two different materials. But what do these numbers mean and what are they based on? 

The AWG determines the thickness of a wire. The higher the number; the smaller the wire. Because it’s thinner and can easily get hot. 

Here, your perfect choice for a 40 Amps breaker is the 8 AWG copper wire. If you use an aluminum wire, then 6 AWG is the only option. 

For your convenience, I have listed some of my favorite picks: 

Product 1
Product 2
Product 3

Choose whichever you like more and get started! 

But if you put a smaller wire which is rated for 30 amps, it’ll melt. That’s why the 10 AWG copper is the best wire size for 30 amps

The 10 AWG won’t be able to handle 40 amps. It may even catch on fire which you definitely want to avoid. 

But what about thicker wires? You may wonder. 

To understand that, you’ll have to understand the circuit breaker first. A 40 amp circuit breaker will trip if more than 40A current passes through. 

A thicker wire such as 4 AWG copper wire will allow 60 amps to pass through. In the event, 60A does pass through, the breaker will trip. 

But your appliances will be safe from electrical damage. 

You should only get thicker wire if you have plans to install more appliances. Otherwise, stay with 8 AWG copper wire for now. 

6 AWG copper wire or 4 AWG copper wire are great for 50 and 60 amps respectively. Choose 4 AWG copper wires because it’s the best wire size for 60 amps


Question: Can you put a 40 amp breaker on a 10 gauge wire?

Answer: It’s always possible to hook up a 40 amp breaker with a 10 gauge wire. But it’s not completely safe. Because the max ampacity a 10 gauge wire can handle is 30 amps. It’ll melt if more than 30A current flows through. 

Question: Can I use a 40 amp breaker on a 6 gauge wire?

Answer: Yes you can. A 6 gauge copper wire has a capacity of 60 amps. This option can also be regarded as futureproof. By installing a 40 amps breaker, you’ll be protecting all the appliances. 

Question: Can I replace a 40 amp breaker with a 50 amp?

Answer: Replacing a 40 amps breaker with 50 amps is extremely dangerous. Because appliances rated below 40 amps will get damaged if more electricity passes through. It can set your house on fire even. 

Final Words

That was all I could gather on 40 amps wire size. I hope this discussion was able to help you understand wire sizes more.

Here’s one last piece of advice since you’ve stayed all the way. Local electrical services are a great option to ask for an inspection. Ask the experts if you’re not sure about the wire size. 

Finally, good luck!