“What size wire for 30 amps?”

You want to choose the correct wire size for 30 amp service. This can be a 30 amp breaker size, 30 amp circuit, and so on.

We are going to look into what wire size you need for:

- 30 amp 240 volt circuit. This circuit can handle up to 7,200W of electrical power.
- 30 amp at 0 feet distance.
- 30 amp 100 feet and 150 feet away from sub panel.

Example: “What is wire size for 30 amp 240-volt circuit?”. In most cases, you will need an #8 AWG wire for a 30 amp 240V circuit.

The key for 30 amp wire sizing is to account for 2 NEC codes, namely:

- NEC 220-2 Code. This requires that a conduit wire’s maximum load (30 amps) represents 80% of ampacity of that wire.
- NEC 310-16 Code. This determines what size wire for 30 amp service at a distance. Usually, you have to add 20% to the wire’s ampacity for every 100 feet away from 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 check the wire sizes with 30 or more ampacity. Here is the screenshot from that part of the chart:

The most common mistake homeowners make is this:

They choose a #10 AWG wire for a 30 amp circuit. Some questions even ask how far can you run 10 gauge wire for 30 amps? The mistake is understandable: #10 AWG wire has a 35A ampacity. Surely it can handle 30 amps, right?

Wrong. You need to account for the NEC 80% rule (we’ll explain this further on).

In short, the perfect wire size for most 30 amp services is the #8 AWG wire with 50A ampacity. This is the common size wire for 30 amps 240 volt service, 30 amp 220 volt service, and so on.

## What Size Wire For 30 Amps? (80% NEC Code)

For adequate 30 amp wire sizing, we need to account for the National Electric Code (NEC) requirements. You can’t just use 30A ampacity wire for 30 amps service. As we’ll see, you shouldn’t even use #10 AWG wire with 35A ampacity.

Here is why:

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 ampacity requirement. It tells us that those 30 amps have to represent at most 80% of the ampacity of the wire (copper or aluminum wire).

Here is how we calculate what ampacity wire we need for 30 amp circuit:

**30 Amp Wire** = 30A × 100% / 80% = **37.5A Ampacity**

When we account for 80% NEC requirement, we see that 35A ampacity #10 AWG is not big 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 is the size after #10 AWG wire (35A) is the #8 AWG wire (50A).

With 50A ampacity, the #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 a 30 amp service. What size wire you should use?

The choice of the wire size is irrelevant of the voltage. That means that you can get 110/120 volt 30 amp, 220 volt 30 amp, and 240 volt 30 amp service with the same wire. The only thing that changes is the wattage such a circuit can deliver, namely:

- 110 volt 30 amp wire can deliver 3,300W wattage.
- 120 volt 30 amp wire can deliver 3,600W wattage.
- 220 volt 30 amp wire can deliver 6,600W wattage.
- 240 volt 30 amp wire can deliver 7,200W wattage.

You just have to make sure that your wire has an ampacity of at least 37.5 amps (as calculated from the NEC code). That’s not a #10 AWG wire since it only has 35A ampacity. The correct choice is the #8 AWG wire that can handle up to 50 amps.

Now, one of the most common questions regarding 30 amp wire is what happens if the sub panel is some distance away. Example: What size wire do I need for 30 amp sub panel 100 feet or 150 feet away? Let’s use the 2nd NEC rule to figure out 30 amp wire sizes some distance away from sub panel.

## What Size Wire Do I Need For 30 Amp Sub Panel 100 Feet Or 150 Feet Away?

Sending electricity over some distance along a 30 amp copper wire or 30 amp aluminum wire cause a voltage drop. At below 10 feet distance, the voltage drop is contained at below 3% and you don’t really have to account for it.

At 50 feet, 100 feet, or 150 feet, for example, you do have to account for the voltage drop. And you account for it by increasing the amperage (boosting the amps, so to speak). By how much?

NEC 310-16 tells us (roughly speaking) that you have to increase the amps by 20% for every 100 feet away from the 30 amp sub panel.

That simply means that you have to:

- Increase amps by 10% for 30 amp wire size 50 feet away from sub panel.
- Increase amps by 20% for 30 amp wire size 100 feet away from sub panel.
- Increase amps by 30% for 30 amp wire size 150 feet away from sub panel.

Here’s how you calculate the 30 amp amperage at a distance:

Let’s say you want 30 amp service 100 feet away from sub panel. We already know that you need at least 37.5A amperage at 0 feet of distance. Now you have to increase the amps by 20% on top of that to account for 100 feet distance away from sub panel:

**30 Amp Wire (100 ft away)** = 37.5A × 1.2 = **45A Amperage**

For 30 amp service 100 feet away you would need a wire that can handle at least 45 amps. Luckily, the #8 AWG wire has a 50A ampacity. You can use the #8 AWG wire for sub panel 50 feet, 100 feet, and even 150 feet away.

If, however, the sub panel is 200 feet away, you have to boost the amps by 40% to counter the voltage drop. That would result in you needing a wire that can handle 52.5 amps and #8 AWG can’t do that. In this case, you should opt for the #6 AWG wire with 65A ampacity.

Hopefully, now you understand what size breaker you need for a 30 amp circuit. If you have any questions, you can use the comments below and we’ll try to help you out. On top of that, you can see similar calculations for:

- 20 amp wire size and breaker size.
- 40 amp wire size and breaker size.
- 50 amp wire size, breaker size, sub panel at 100-150 feet distance.
- 60 amp wire size, breaker size, sub panel at 100-150 feet distance.
- 100 amp wire size, breaker size, sub panel at 100-150 feet distance.
- 200 amp wire size and breaker size.

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Very useful information!

Great information answered a lot of the problems we have with my rv. One other question that brings up, I’m approx 50 feet from the sub panel so I’ll get some 8 gauge for that so should I increase my sub panel breaker to 40 or 50 Amp then at the box for my 30 Amp rv plug run a 30 Amp breaker before the plug?

Hi Hugh, you have the right idea. Increasing the sub panel breaker to 40A is a sound idea.

Hi Hugh I Have a question, I have purchased a small generator Firman model P03602, it is 4560 starting watts, 3560 running watts,120volts,30amps, I want to make an extension cord using L5-30P which is a 3pin plug, to L14-30R which is 4 slot receptacle, My question is, Can i use S00W 12/3 Black wire for this, if not what would you recommend, thanks for any advise.

Hi Malcolm, 12/3 wire has three 12 AWG wires, each of them having a 25A median ampacity (copper, 167 Kelvins). That’s 75A total ampacity, which translates to 60A after you apply the 80% rule. For 4560 starting watts at 120V, you need a wire that can handle at least 38 amps. So, the 12/3 wire with the allowed 60 amps current will do just fine.