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What size wire do I need for a 100 foot run?

Instead, we use bigger #12 AWG wire (close to 20 amp sub panel) and #10 AWG wire (100 feet or more from 20 amp sub panel).

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What is 20 amp breaker wire size? If you’re planning to set up a 20 amp circuit, you will need the correct wire gauge for 20 amps. Note: 20 amp 240 volt circuit can generate up to 4,800W wattage.

Here the answer seems obvious:

You need a 14 gauge wire on a 20 amp circuit, right?

Not quite. This is the most common mistake homeowners make when it comes to what size wire you need for 20 amp service. It’s quite understandable why the mistake is made. #14 AWG wire has a 20A ampacity. It seems the perfect choice for a 20 amp circuit.

Why this is a wrong choice?

Because 20 amp current will probably fry a #14 AWG wire. To prevent this from happening, the National Electric Code requirement is set to help you choose an adequate wire size for a 20 amp breaker.

The NEC code, Chapter 3, Segment 220-2 states that:

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 for electrical wire sizing. That means that if you want to have a 20 amp circuit, you need a wire with at least 25A ampacity. These extra 5 amps (or 20% of the total current) are a safety measure that prevents you from overloading the wire. That also means that the #14 AWG wire with 20A ampacity is not sufficient for a 20 amp circuit. The perfect wire gauge of a 20 amp breaker is the #12 AWG wire. This is a bigger wire with 25A ampacity and can handle 20 amp current while also satisfying the NEC 80% rule for safety. You can consult the overall wire gauge chart here. This is the screenshot for the 20 amps circuit we are going to delve in this article: We will look into how you can calculate the ampacity you need from a wire in a 20 amp circuit. On top of that, we will also look at an extra NEC recommendation for 20 amp services with sub panel 100 feet or 150 feet away (you need to account for voltage drop; boost amps by 20% for every 100 feet away from the sub panel).

What Wire Gauge For 20 Amp Circuit? (Apply NEC 80% Rule)

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When you are trying to figure out the wire gauge for 20 amps, you usually look at these 3 wire sizes:

#14 AWG with 20A ampacity.

#12 AWG with 25A ampacity.

#10 AWG with 35A ampacity.

Despite the common sense conclusion, #14 AWG is never used for a 20 amp circuit. Instead, we use bigger #12 AWG wire (close to 20 amp sub panel) and #10 AWG wire (100 feet or more from 20 amp sub panel).

Why is that?

Well, we need to account for the NEC 80% rule. Here is how you account for it by calculating the minimal require ampacity of a wire for 20 amp service:

20 Amp Wire = 20A × 100% / 80% = 25A Ampacity

This means you need a wire with at least 25A ampacity for 20 amps, and the #14 AWG with 20A ampacity just doesn’t cut it.

Which is the best wire gauge for a 20 amp breaker?

#12 AWG wire. This is usually a copper wire with 25A ampacity at a median temperature (75°C or 167°F). If, however, you are sending electricity away from the sub panel at a considerable distance, you would need a bigger wire still. This has to do with the voltage drop:

What Size Wire For 20 Amps At 100 Feet Away?

When you send electricity 100 feet or more away, the voltage will inevitably drop. We talk about the voltage drop. If you still want to get 20 amps on the whole circuit, you have to boost the amps to counter the voltage drop. Boosting the amps is usually connected with a bigger wire. You’re sending a higher-than-20-amps current 100 feet away (or more). According to the NEC 310-16 rule for copper and aluminum 20 amp wires, you need to increase the amps by 20% for every 100 feet. We already know that you need at least a 25A ampacity wire for a 20 amp circuit. If you want to use those 20 amps 100 feet away, you need to increase the minimum ampacity the wire can handle by 20%. Here is how you calculate the ampacity now: 20 Amp Wire (100 ft away) = 25A × 1.2 = 30A Ampacity

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That means that the #12 AWG wire with 25A ampacity won’t be enough any more. Which wire should you choose for 20 amp 220 volt circuit 100 feet away from sub panel?

#10 AWG wire is perfect. It has a 35A ampacity, more than the required 30A ampacity. You can calculate similar minimum amp requirements for a 20 amp circuit at any distance: 50 feet from sub panel. Add 10% and you get 27.5A Ampacity. That’s #10 AWG wire. 100 feet from sub panel. Add 20% and you get 30A Ampacity. That’s #10 AWG wire. 150 feet from sub panel. Add 30% and you get 32.5A Ampacity. That’s #10 AWG wire. 200 feet from sub panel. Add 40% and you get 35A Ampacity. That’s #10 AWG wire. As you can see, close to the 20 amp panel, you use #12 AWG wire. For a 20 amp circuit 50 feet or more from the sub panel, you use #10 AWG wire. Hopefully, this clears things a bit. If you have any questions regarding 20 amp wire sizing, you can pose them in the comments below and we will try to help you out. You are also very recommended to read similar calculations and wire size determinations for:

learnmetrics.com - Wire Gauge For 20 Amp Circuit: 20A Wire Size + 100 Feet Away