You want to choose the right wire size for 30 amp service. It can be 30 amp breaker size, 30 amp circuit, and so on.
We’re going to look at what wire size you need:
- 30 amp 240 volt circuit. This circuit can handle up to 7,200W of electrical power.
- 30 amp at a distance of 0 ft.
- 30 amp 100 ft and 150 ft away from the sub panel.
The key to 30 amp wire sizing is for 2 NEC codes, namely:
- NEC 220-2 code. This requires that the maximum load (30 amps) of a drain wire represents 80% of that wire’s capacity.
- NEC 310-16 code. This determines what size wire for 30 amp service at a distance. Typically, you’ll need to add 20% to the wire’s ampacity every 100 feet from the sub panel.
The first step in checking what size wire you need for a 30 amp circuit is to look at the AWG wire size chart here. You calibrate the wire size with 30 or more ampacity. Here’s a screenshot of that part of the chart:
The most common mistake homeowners make is:
They choose #10 AWG wire for a 30 amp circuit. The mistake is understandable: #10 AWG wire has 35A ampacity. Surely it can handle 30 amps, right?
Wrong. You have to heed the NEC 80% rule (we’ll explain this later).
In short, the correct wire size for most 30 amp services is #8 AWG wire with 50A ampacity. This is common size wire for 30 amps 240 volt service, 30 amp 220 volt service, etc.
What size wire for 30 amps? (80% NEC Code)
For a sufficient 30 amp wire size, we need to account for National Electric Code (NEC) requirements. You can’t just use 30A ampacity wire for 30 amps service. As we’ll see, you also shouldn’t be using #10 AWG wire with 35A ampacity.
The maximum loading for any branch circuit is 80% of the circuit’s rating for the wire’s capacitance for any given load. (NEC 220-2)
This is known as the 80% NEC Ampacity Requirement. This tells us that those 30 amps are supposed to represent at most 80% of the capacity of the wire (copper or aluminum wire).
Here’s how we calculate what ampacity wire we need for a 30 amp circuit:
30 Amp Wire = 30A × 100% / 80% = 37.5A Ampacity
When we account for the 80% NEC requirement, we see that 35A Ampacity #10 AWG is not large enough. With 35A, it’s almost big enough but not quite.
To use a 30 amp breaker, we need a wire that can handle at least 37.5 amps. The next size after #10 AWG wire (35A) is #8 AWG wire (50A).
With 50A ampacity, #8 AWG wire is the perfect wire size for 30 amp service.
Example: Let’s say you have a 220 volt or 240 volt circuit and you want to get 30 amp service. What size wire should you use?
Wire size selection is irrelevant to voltage. This means you can get 110/120 volt 30 amp, 220 volt 30 amp, and 240 volt 30 amp service from the same wire. The only thing that changes is the wattage such a circuit can deliver, namely:
- 110 volt 30 amp wire can provide 3,300W wattage.
- 120 volt 30 amp wire can provide 3,600W of wattage.
- A 220 volt 30 amp wire can provide 6,600W of wattage.
- A 240 volt 30 amp wire can provide 7,200W of wattage.
All you have to do is make sure your wire has at least 37.5 amps of capacitance (as calculated from the NEC code). This is not #10 AWG wire because it only has 35A of ampacity. The correct choice is #8 AWG wire which can handle up to 50 amps.
Now, one of the most common questions regarding 30 amp wire is what if the sub panel is at a distance. Example: What size wire do I need for a 30 amp sub panel 100 feet or 150 feet away? Let’s use the second NEC rule to find the size of the 30 amp wire at some distance from the sub panel.
What size wire do I need for a 30 amp sub panel 100 feet or 150 feet away?
Sending power over a distance with 30 amp copper wire or 30 amp aluminum wire results in a drop in voltage. At a distance of less than 10 feet, the voltage drop is less than 3% and you don’t really have to account for it.
For example, at 50 ft, 100 ft, or 150 ft, you will need to account for voltage drop. And you account for this by increasing the amperage (by increasing the amps, so to speak). by how much?
The NEC 310-16 tells us (roughly speaking) that you need to increase the amps by 20% for every 100 feet away from a 30 amp sub panel.
It simply means that you have to:
- Increase the amps by 10% for a 30 amp wire size 50 feet away from the sub panel.
- Increase the amps by 20% for the size of the 30 amp wire at 100 feet from the sub panel.
- Increase the amps by 30% for a 30 amp wire size 150 feet away from the sub panel.
Here’s how you calculate 30 amp amperage over a distance:
Let’s say you want 30 amp service 100 feet from the sub panel. We already know that you need at least 37.5A amperage at a distance of 0 feet. Now you need to increase the amps by 20% to account for 100 feet from the sub panel:
30 amp wire (100 feet away) = 37.5A × 1.2 = 45A amperage
For 30 amp service 100 feet away you will need a wire that can handle at least 45 amps. Luckily, the #8 AWG wire has 50A ampacity. You can use #8 AWG wire for sub panels 50 feet, 100 feet and even 150 feet away.
If, however, the sub panel is 200 feet away, you will need to increase the amps by 40% to counteract the voltage drop. As a result you will need a wire that can handle 52.5 amps and #8 AWG can’t. In this case, you should opt for #6 AWG wire with 65A ampacity.
Hopefully, you now understand what size breaker you need for a 30 amp circuit. If you have any questions, you can use the comments below and we will try to help you. On top of that, you can see similar calculations for: