What Size Wire for a 40 Amp Breaker

For many high-powered appliances such as cooktops, electric dryers, and electric stoves, you’ll need a 40 amp breaker.

Having a 40 amp breaker helps protect your electrical appliances. But if you don’t pick the correct gauge wire, you’ll risk the safety of circuits and components. So, if you are having trouble deciding what size wire for a 40 amp breaker, I’m here to give you some tips.

In general, 8 AWG copper wire is the minimum wire size for a 40 amp breaker. However, 8 AWG wire is only suited for under 100 feet run. Apart from that, you can use a 6 AWG wire as well. 

I’ll go into more detail below.

Wire Size for 40 Amp Circuit Breaker

8 AWG copper wire
Video | PSE Mentalist

The 8 AWG copper wire is the most suited option for a 40 amp breaker. We can label this wire as the minimum wire size for 40 amp. However, you should check the following two factors.

Length of the Wire

If you are using an 8 AWG wire on a 40 amp circuit breaker, the length of the wire should be below 100 feet. In electrical circuits, the resistance gets increased with the length of the wire.

According to Ohm’s Law,

V = I.R

  • V= Voltage
  • I = Current
  • R = Resistance

Hence, the voltage drop changes according to the resistance.

In other words, if you run 240V voltage through an 8 AWG wire for 50 feet, at the 50 feet mark, you won’t get the full 240V. Instead, you’ll get a lesser value. We called this the voltage drop. There is a recommended value for this voltage drop. You should keep the voltage drop under 3% for lights and 5% for other appliances.

When you run an 8 AWG wire 100 feet or above, the voltage drops below the recommended levels. When that happens, the electrical appliance won’t get the required voltage. This could lead to damaged electrical appliances.

Built Material of the Wire

Apart from the length, the built material of wire is a major factor to consider while choosing 8 AWG wire. For instance, when you ask for 8 AWG wire from your local hardware store, the merchant might ask what type of wire you need, aluminum or copper.

There is a good reason for this question. Even though copper and aluminum are excellent electrical conductors, copper is a far better choice than aluminum. Copper is the better conductor.

So, for the 40 amp and 240V circuit, 8 AWG copper wire is an excellent choice. However, if you are going with aluminum, you’ll need a 6 AWG wire for the same circuit.

Is 6 AWG Copper Wire Good for a 40 Amp Breaker?

In truth, the 6 AWG wire is most suitable for 65 amp circuit breakers. But that doesn’t mean you can’t use it with a 40 amp breaker. Using a 6 AWG wire will give you space for expansion. If needed, you can change the 40 amp breaker to 50 or 60 amp.

Tip: Using 6 AWG wire on a 40 amp circuit breaker doesn’t violate NEC rules.

How Many Amps Can a 40 Amp Circuit Breaker Handle?

woman's hand holding a 40 amp breaker
Video | Sparky Direct

Every circuit breaker will start to trip when they reach maximum ampacity. For a 40 amp breaker, the maximum ampacity is 40 amps. So, the breaker will handle 40 amps without any issues. But when the load exceeds the 40 amps, the breaker will trip.

Even though circuit breakers can handle their respective maximum load, according to the NEC, the maximum capacity should be 80%. Here is some information about this rule.

80% NEC Rule

Only 80% of the total capacity of the circuit breaker should be used. The circuit should be able to maintain this limit for 3 hours or longer.

Therefore, if we consider a 40 amp breaker,

40 amp × 80% = 32 amps

So, keep the load to a 32 amp limit for 40 amp breakers. Use a breaker that has a higher ampacity if your circuit draws more than 32 amps regularly. For instance, the 50 amp breaker will be a good choice.

Tip: Some circuit breakers come with a 100% rating.

Why Should I Follow the 80% NEC Rule?

If a circuit draws too much load continuously, it will heat up with time. The same thing will happen to the circuit breakers. So, the circuit breakers will start to trip. Or overheating might start an electrical fire. (1)

How Many Watts Can a 40 Amp Breaker Handle?

Whenever we calculate the watts, we use Joule’s Law.

Wattage = Voltage × Amperage

When we consider a 40 amp, 240V power supply:

In theory, the wattage = 40 × 240 = 9600 watt

But you cannot use the maximum capacity of the circuit breaker. You should only use 80%.


Wattage (when considering the 80% capacity) = 40 × 80% × 240 = 7680 watt

So, a 40 amp breaker (rated 80%) can handle up to 7680 watts. If the circuit breaker is rated 100%, it can handle 9600 watts.

Keep in mind: Whether you use an 80% or 100% rated circuit breaker, the 8 AWG wire is a good choice for the above circuits.

How Do You Size a Wire?

American Wire Gauge, aka AWG, is the standard in North America for wire sizes. It will give us a rating according to the wire’s diameter and the load that particular wire can carry.

orange copper wire with label writing
Video | PSE Mentalist

The corresponding number will be printed on the wire insulation. The marking will show 4 AWG, 6 AWG, 8 AWG, 10 AWG, etc. According to these numbers, you can figure out the diameter of the wire and the current that the wire can carry.

For instance, 12 AWG wire has a diameter of 0.0808 inches, and 6 AWG wire has a diameter of 0.162 inches.

As you can observe, with a higher gauge number, the thickness of the wire gets increases.

Does Wire Gauge Diameter Include Insulation?

diameter of copper wire
Video | PSE Mentalist

AWG wire manufacturers do not include the wire insulation in the wire size. So, the wire diameter is the conductor diameter.

What If I Cannot See the Rating on the Wire Insulation?

Digital Vernier Caliper
Video | Just Vlad

If you cannot see the rating on the wire insulation, use a Digital Vernier Caliper to measure the thickness of the wire. Here’s a simple guide on how to do that.

  1. First, strip the wire that you want to measure.
  2. Then, place the exposed wire conductor on the fixed jaw of the Digital Vernier Caliper.
  3. Next, bring the moveable jaw to the wire.
  4. After that, get the reading on the digital display.
  5. Finally, use an AWG wire diameter reference table to get the corresponding rating for that wire.

Use this link for AWG wire diameters.

Tip: Some Vernier Calipers might show the scale in millimeters. And some will show in inches.

Here are some common copper wires with their diameters and rated amp values.

Wire GaugeDiameter (Inches)Rated Amps
12 AWG0.080820 amps
10 AWG0.101930 amps
8 AWG0.128540 amps
6 AWG0.162065 amps

Wrapping Up

For modern kitchens, having a reliable electrical circuit is crucial. You won’t get good quality electrical circuit without proper wire sizes. So, when you use a 40 amp breaker at home, remember to use 8 AWG or 6 AWG wires. It will surely protect you and your home appliances. (2)

Take a look at some of our related articles below.

(1) fire – https://www.britannica.com/science/fire-combustion
(2) modern kitchens – https://www.houzz.com/photos/modern-kitchen-ideas-phbr1-bp~t_709~s_2105

Video References

Sparky Direct

PSE Mentalist

World Electrical & Electronic Engineering Academy


Just Vlad

About Sam Orlovsky

b1d87d2ee85af3e51479df87928bdc88?s=90&d=mm&r=gI realized early on carpentry was a huge passion for me and I’ve stayed in the industry for over 20 years now. This gives me a unique ability to really be able to tell you what the best tools and recommendations are. I’m not only a carpenter but I also like machinery and anything to do with electrics. One of my career paths starting off was as an apprentice electrician so I also have a lot of experience with electrical products and anything related.