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HomeTips & GuidesWire for 40 Amp Breaker – How to Choose and Connect It

Wire for 40 Amp Breaker – How to Choose and Connect It

About Wire for 40 Amp Breaker: In all houses, you will always need a circuit breaker to disconnect your electric power when it short circuits. 

Now that your house has many appliances, the breakers you install must have a high current rating of about 40 amps. 

You need a wire to support the same current load for this high-amperage circuit breaker to work securely. 

Hence, I have compiled a comprehensive guide on choosing a wire for a 40-amp breaker and how to wire it.

What Is the Appropriate Wire Size for 40 Amp Breakers?

Before we go into details, note that a low gauge wire can easily burn when serving several appliances. 

In this regard, wires of 8 AWG should be your ideal choice when using 40 amp breaking.

 If you want to feel even safer, use a bigger wire, such as 6 AWG. While here, note that the smaller the number the bigger the wire gauge and the safer it can handle big currents. 

Circuit breaker wiring illustration

Circuit breaker wiring illustration

What Should You Consider When Choosing Ideal Wire Size for 40 Amp Breakers?

Like any other item, rules exist for choosing a wire for a 40 amp breaker, which ensures functionality and safety. If you’re in the market for a wire, I recommend considering the following things:

Appropriate Wire Guage

As we established above, the appropriate wire gauge for 40 amp breakers is 8, and you should always use this. 

While using a bigger gauge than 8 AWG can still work, it may bring issues during wiring and routing. 

You will find that such a wire is stiff and heavy and can add to your budget unnecessarily. On the other hand, using a smaller gauge can result in overheating caused by electrical resistance

This overheating may melt the insulator or the wire and burn your house or the connected electronics.

NEC 80% Rule

First, you should know that the NEC is the National Electrical Code, which determines electrical safety guidelines. 

The 80% rule recommends that your breaker run less than 80% of its total wattage in three hours. 

For example, if you have a 40 breaker in three hours, your devices should draw less than 32 amperes. 

If they draw more in that time frame, you need a bigger breaker, hence bigger gauge wires.

Wire Length

Although we said 8 AWG wires work well with 40 amp breakers, expect this rule to change when involving length. 

For instance, wires less than 100 feet will work perfectly with the breaker if they are 8 AWG. 

However, when using a wire longer than 100 feet, consider the next gauge on the same breaker. In this case, size 6 AWG for a 40-amp breaker.

Wire construction

As you may know, there are different wire materials, each with its pros and cons. When buying a wire for the 40 amp breaker, you mostly get aluminum or copper. 

With aluminum, you get a lighter wire with only 30% copper weight, and you do not spend much.

 However, copper is ideal for better conductivity, heat resistivity, and longevity, especially where the wire length is a factor.

How to Size Wire for a Circuit Breaker

When sizing a wire, you look at the diameter or the thickness. Thicker wires will have a smaller number rating, meaning they have a better gauge. 

For example, 4 AWG wire is better than 6 AWG wire, which is better than 8 AWG wire. Now the bigger the amperage rating of the breaker, the thicker the wire it requires. 

Also note that the longer the wire, the bigger the resistance, explaining why you need thick cables for long-distance wiring.

Electric cables photo

Electric cables photo

How to Wire 40 Amp Breaker

Hoping that you have the right breaker and correct wire gauge, let me show you how to wire it:

Identify installation location

First, locate the main panel where you want to install the breaker. Check for the empty circuit breaker space on the panel where you will install yours. Now, switch off the main panel switch to avoid electrocution or shortcircuiting.

Unscrew the panel cover.

The next step is to remove the electrical panel cover to access the empty breaker spaces inside. 

You can easily do this by using an appropriate screwdriver to unscrew the screws and then remove the cover gently. From there, use an electric tester to ensure your power is off completely.

Breaker Installation

Now that you have the correct size circuit breaker according to the available space, it’s time to install it. 

Start by turning the breaker handle to OFF. Then, align it well in the available space. Now press it gently to ensure it snaps and sits in its position tightly.

Wiring and Testing

After ensuring it sits well, you will need to connect it to the wires. To help connect the wires appropriately, check the breaker’s labels, N for neutral and L for live. 

While here, you should also know the color code of the Live wire is Black and the neutral is White. 

Once you identify them, unscrew the contact bolts on the breaker, insert the wires correctly, and tighten snuggly. From there, you must turn every switch on, replace the cover, and test your connection.

A man installing a circuit breaker

A man installing a circuit breaker


Now you know how to choose the right wire size for your 40 amp breaker and install it. If you aren’t sure about your electrical skills, I recommend hiring a qualified electrician.

Hi I am Christa, sales manager of Cloom.

I have extensive expertise and experience in wiring harnesses and I believe I can help you.

And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

If you also have wiring harness needs, please send me the drawing so that we can give you our quote and start our business.

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