20 Amp Wire Size: What Size Wire For 20 Amp Circuit

20 Amp wires are often used for household outlets, extension cords, and similar applications, where the circuit must be strong enough to power one larger or a few smaller tools and/or appliances, with the maximum current through the wires of 20 Amps.

However, when calculating the right wire thickness, one has to consider safety measures, wire length, type and material, and a few more details.

Published: August 12, 2022.

Maximum Theoretical Wire Current

If we check the AWG wire size chart, we can easily find a wire size (copper solid wire) that can transfer the required 20 Amps with the surface temperature not reaching above the requested temperature:

As one can see, 12 gauge wire size features physical dimensions of:

• diameter: 2.0525 mm, 0.0808 inches,
• cross-section area: 3.3088 mm², 0.0051 inches²,

and can transfer 20 Amps @60°C/140°F, 25 Amps @75°C/167°F, and 30 Amps @90°C/194°F.

However, for residential use, 60°C/140°F is the highest allowable conductor wire surface temperature.

So, is 12 gauge wire our required 20 Amps wire? No, it is not.

80% Rule: The Safety Rule

While 20 Amps circuits must be protected by 20 Amps circuit breakers, 20 Amps circuits must use wires dimensioned according to the 80% safety rule:

I (A) = 20 A /0.8 = 25 Amps

So, for 20 Amps circuits, we will not use wires with the limit of 20 Amps, but wires that can transfer 25 Amps without their surface temperature reaching above the requested temperature.

If we check the AWG wire size chart, we can find:

• @60°C/140°F: AWG 10, 30 Amps,
• @75°C/167°F: AWG 12, 25 Amps,
• @90°C/194°F: AWG 12, 30 Amps.

So, for 20 Amps circuits, we need to use AWG 10 wires since the maximum allowed wire surface temperature in residential use is 60°C/140°F.

And that is only for shorter wires, since the longer the wire, the larger energy losses are in the wire.

Wire Length

When the wire length is longer than a few dozen feet, in order to keep the energy losses to an acceptable level, wire Ampacity (wire ability to carry the current) must be increased by 10% for every 50 feet of the wire.

50 feet wire: Ampacity = 25 Amps * 1.1 = 27.5 Amps

100 feet wire: Ampacity = 25 Amps * 1.2 = 30 Amps

150 feet wire: Ampacity = 25 Amps * 1.3 = 32.5 Amps

200 feet wire: Ampacity = 25 Amps * 1.4 = 35 Amps

Now, we have to check the required AWG value for given wire lengths, depending on the wire surface temperature - values are given in the following chart:

Note: the actual surface temperatures due to the current flowing through the wires will be lower, but to keep calculations simpler, maximum allowed currents are calculated using these formulas - remember that the actual goal is to keep energy losses low in longer cables AND to keep their maximum surface temperatures at the certain level.

So, for residential use, AWG 12 wires are simply too thin to be used as 20 Amp wires - wires for 20 Amps currents must be able to transfer those 20 Amps "with ease" - that is why the "80% Rule" is used.

And for longer wires, wires must be thicker to keep the energy losses at an acceptable level - that is why the Ampacity is increased by 10% for every 50 feet.

If unsure what to do, contact the locally certified electrician regarding wire thicknesses, local laws, safety margins, and similar - whatever You do, stay safe ...