100 Amp Wire Size: What Size Wire Do I Need For 100 Amp Service/Sub-Panel

100 Amps is a very serious current requiring thick wires in order to keep the energy losses to an acceptable level and to keep the wire surface temperature below the maximum allowed temperature.

The thickness of the wire depends on the wire material, conduit size, and maximum allowed energy losses/wire surface temperature, but it can also depend on other details, as well. Personally, if You are going to service or add 100 Amp service, consult or hire a certified electrician, regardless of whether You need a permit for such works or not.

Published: July 12, 2022.

AWG Wire Size Chart

The following AWG wire size chart lists some of the most common wire thicknesses with wire current carrying abilities (Wire Ampacity) and wire dimensions, given at a certain temperature:

As one can see, the Ampacity of wires is given at three different temperatures (60°C/140°F, 75°C/167°F, and 90°C/194°F), but for most residential installations, 60°C/140°F is the maximum allowed wire surface temperature.

Also, if the value is not given for a required current, one has to look for a wire with a larger Ampacity.

For example, if we are going to look for 100 Amp wire size using default Ampacities for solid copper wire, then:

- 60°C/140°F: 1 gauge wire (AWG 1) features an Ampacity of 115A,

- 75°C/167°F: 3 gauge wire (AWG 3) features an Ampacity of 100A,

- 90°C/194°F: 3 gauge wire (AWG 3) features an Ampacity of 115A.

But, these wire thicknesses are not recommended for actual use since wires should never carry current that equals their default values for safety reasons - hence the 80% Rule.

The 80% Safety Rule

80% Rule states that actual wire current should be at most 80% of its default Ampacity. Hence, we should not look for wires being able to carry 100 Amps, but 125 Amps:

I_{(80% Rule)} = 100 Amp / 0.80 = 125 Amp

Thus, for 100 Amp service, the required wire thickness is:

- 60°C/140°F: 0 gauge wire (AWG 0) features an Ampacity of 125A,

- 75°C/167°F: 1 gauge wire (AWG 1) features an Ampacity of 130A,

- 90°C/194°F: 2 gauge wire (AWG 2) features an Ampacity of 130A.

And these values are for relatively short wires, usually less than 50 feet (less than ~15 meters), which raises the question about the wire thickness of longer wires...

Wire Length

There are several rules and standards that may be used when calculating the required wire thickness, but the general rule of thumb is to increase the required Ampacity by 10% for every 50 feet (~15 m) of the wire length - some standards increase Ampacity by 20% for every 100 feet (~30 m) which provides the same or very similar results.

For example, when calculating the required Ampacity for the 50 feet, 100 feet, 150 feet, and 200 feet wires, we can use (default value is 125 Amp, after applying the "80% Rule"):

50 feet wire: Ampacity = 125 Amps * 1.1 = 137.5 Amps

100 feet wire: Ampacity = 125 Amps * 1.2 = 150 Amps

150 feet wire: Ampacity = 125 Amps * 1.3 = 162.5 Amps

200 feet wire: Ampacity = 125 Amps * 1.4 = 175 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:

When calculating the wire thickness of the long wires, always use values from the 60°C/140°F column, since the "wire length" rule, especially after calculating the "80% Rule" is all about energy losses and not wire surface temperature - as long as the wires are properly installed.

So, if You need to know the solid copper wire thickness for 100 Amps service/sub-panel, then:

• <50 feet: AWG 0 (0 or 0/0 wire gauge),
• 50 feet: AWG 2/0 (00 or 2/0 wire gauge),
• 100 feet: AWG 3/0 (000 or 3/0 wire gauge),
• 150 feet: AWG 3/0 (000 or 3/0 wire gauge),
• 200 feet: AWG 4/0 (0000 or 4/0 wire gauge).

However, when adding or servicing a 100 Amp wire(s), it is highly recommended to consult and even hire a certified electrician who is aware of local standards and laws regarding electric wiring. And if a permit is required, get one ...