If you’re setting up a 40 amp circuit, you need to figure out what size wire you need for 40 amps. In particular, you have to choose the right AWG gauge wire to handle that current.
With a 40 amp 240 volt circuit, you will have a circuit that can handle 9,600W wattage. We will also look into what size breaker you need for 40 amp service 100 feet and 150 feet away.
Here’s the deal:
For a 40 amp service, you can’t just pick an AWG gauge wire with 40A amperage. If you do that, you will most likely fry the circuit.
To prevent this, the National Electric Code (NEC) has set a requirement that states the following:
Maximum loading for any branch circuit is 80% of the rating of the circuit for ampacity of wire for any load. (NEC 220-2)
This is known as the 80% NEC rule in wire sizing. Needless to say, it applies to the 40 amp wire sizing as well. NEC also has a recommendation about what size wire you need for 40 amp service 100 feet aways, for example, and we’ll cover it later on.
If this is your first time sizing a wire, it might be useful to know how the AWG gauge is calculated. Here are the basic steps we use:
- Apply the 80% NEC requirement for your amperage. Namely, we will calculate the minimum ampacity a wire has to have (how many amps they can handle) to be adequately sized for a 40 amp circuit.
- Check the wire AWG chart here. Look at the wire sizes with an ampacity around and above 40 amps. This is a screenshot from the chart:
- Pick a wire size that has larger than the minimum required ampacity for a 40 amp circuit.
It’s useful to know that you need the same wire for any voltage: 40 amp 240 volt circuit requires the same size wire as 40 amp 220 volt circuit or even 40 amp 110/120 volt circuit.
With this in mind, let’s determine the wire size for 40 amp breaker:
What AWG Wire Do I Need For 40 Amp Circuit?
As you can see, we can choose between #10 AWG, #8 AWG, #6 AWG, and #4 AWG wires.
Now, let’s start with a minimum ampacity requirement according to the NEC 220-2 requirement. We know that 40 amps have to be 80% of the wire ampacity. What is the ampacity we need?
40 Amp Wire = 40A × 100% / 80% = 50A Amperage
We need 50A amperage. That means the #8 AWG wire (with exactly 50A amperage) is the perfect wire size for 40 amp circuit.
Example: Let’s say you want to create a 40 amp 240 volt circuit. What size wire do you need? You need a wire that can handle at least 50 amps, and that’s the #8 AWG wire.
Now, this is true when the sub panel is close. What about is the sub panel for the 40 amps circuit is 100 feet or 150 feet away? We need to account for voltage drop and get a bigger wire:
40 Amp Wire Size For Sub Panel 100 Feet Or 150 Feet Away
With distance, the voltage in the electric circuit drops. In order to get the same 40 amp at the end of the wire, we need to increase the amps at the sub panel to balance out this drop in voltage.
That, of course, also means that we need a bigger size wire for 40 amps (since at the sub panel we’re putting a larger amp current into the wire).
NEC 310-16 rule states that we need additional 20% amps for every 100 feet away from the sub panel. This also includes what size breaker you need for a 40 amp circuit.
If you’re sending a current at a distance, you have to make this adjustment to the minimum amperage requirements:
- Add 10% to for sub panel 50 feet away.
- Add 20% to for sub panel 100 feet away.
- Add 30% to for sub panel 150 feet away.
- Add 40% to for sub panel 200 feet away.
We know that we need a minimum of 50A amperage wire for 40 amp service close to the sub panel. Now, if the sub panel is 100 feet away, we need to increase this minimum amperage requirement by 20% like this:
40 Amp Wire (100 ft distance) = 50A × 1.2 = 60A Ampacity
That means that we need an AWG wire with at least 60A amperage to send 40 amps 100 feet away from the sub panel. The #8 AWG wire with 50A amperage will not be sufficient here.
We need to get the next size wire: #6 AWG wire. This wire has a 65A ampacity and can handle 60 amps quite easily. That’s why we use #6 AWG wire for 40 amp service 100 feet away. We also use #6 AWG wire for 40 amp service 150 feet away.
When the sub panel is 200 feet away, however, we have to add 40% on top of the 50A minimum ampacity. That’s 70 amps. Even the #6 AWG wire is not sufficient here. For 40 amp service 200 feet away you need #4 AWG wire. This wire has an 85A ampacity and can handle the 70A requirement pretty well.
We hope all of this helps understand how to size a wire. If you have any questions, you can pose them in the comment section below, and we’ll try to help you out.
Of course, you are also encouraged to read similar requirements and calculations for the following amperages:
- Wire size for 30 amps, with 240V example and 100 feet from sub panel.
- Wire size for 50 amps and associated NEC codes.
- Wire size for 60 amps, with 240V example and 100 feet from sub panel.
- Wire size for 100 amps, 100 feet and 150 from sub panel.
- 200 amp wire size and breaker size.
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I need to service an automobile charger system. The garage is about 200 feet away from the main house panel. The charger requires 40 amps and 220volts. The main panel is about 175 feet away from the garage where the car is parked. What size wire and breakers do I need to provide the 40 amp to the car charger running a line 200 feet from the main service breaker,?
Hello Floyd, if we presume that you lose about 20% of amps (due to voltage drop) per 100 feet, you will lose about 40% amps running a 200 feet line. Now, you need 40 amps at the end of the line (this is 60% of the initial amperage). So, here is how you calculate that 40A / 0.6 = 67A. You will have to account for NEC 80% rule as well, so you need 67A × 1.25 = 84A; so you would need a wire with at least 84A ampacity at a median 75°C temperature.
Now, to pick the correctly sized wire, you can check the ampacity chart for copper wires here and ampacity chart for aluminum wires here. In short, you would need a minimum of 4 AWG copper wire (85A ampacity) or a 0 AWG aluminum wire (96A ampacity).
Breaker-wise, you need a breaker with at least 84A rated amps. You can use a single big 90 amp breaker or three 30 amps breakers. Hope this helps.
So, if I need a single big 90 amp breaker, can I use three 30 amps breakers in series… or in parallel… on each of the 2 single phase 220volt lines (black wire line and red wire line)?
Hi Ron, yes, the 90A breaker is basically three 30A breakers; bringing the total to 90 amps.
An 240V appliance requiring a dedicated breaker requires 40A and is short of 100 ft away. 6 AWG carrying 60A would be required. This would require two 60A breakers (one for the black, one for the red)?
Hi John, for a 60 amp current you would need a 60A * 1.25 = 75A breaker. One 60A breaker won’t be enough. You can go with two 60A breakers or a 90A breaker. Hope this helps.