125 Amp Wire Size Calculator + Chart (0-500 Feet Away)

Figuring out a 125 amp wire size is fairly easy. We’ll show you how to figure that out. Where it gets a bit more difficult is at 125 amp service 50 feet, 100 feet, 150 feet, 200 feet, and up to 500 feet away. Here we have to account for voltage drop; we’ll explain how to do that as well.

To help you adequately determine the 125 amp wire size at a distance, we have also prepared these two useful resources:

• 125 Amp Wire Size Calculator. Here you input wire length, voltage, and allowable voltage drop percentage, and the calculator will size a 125 amp copper wire size automatically.
• 125 Amp Wire Size Charts (2 of them). We have calculated wire gauges for 125 amp service at 0-500 distance away from the source and for 12V, 120V, and 220V voltages. You will find two charts further on; 1st one if for a 3% allowable voltage drop, and the 2nd one is for a 10% allowable voltage drop.

Let’s start by illustrating how you determine the 125 amp wire size (including the 125 amp breaker wire size; not the same as the 125 amp wire size):

How To Determine 125 Amp Wire Size? (+ 125A Breaker Wire Size)

For 125 amp service, we need a wire that can handle at least 125 amps. That’s pretty obvious. That means that a wire has to have at least 125A ampacity.

We consult this NEC 310.16 chart with specified copper and aluminum wire sizes and corresponding ampacities. Here is the screenshot for both copper and aluminum wire ampacities with highlighted 125 amp wire sizes:

From this NEC wire ampacity charts, we see that we need either

• 1 AWG copper wire for 125 amp service. This wire has 130A ampacity; more than the minimum required 125A ampacity.
• 2/0 AWG aluminum wire for 125 amp service. This wire has 135A ampacity; more than the minimum required 125A ampacity.

Note on 125 amp breaker wire size: For breakers, we use the 80% NEC rule. That means that a 125A breaker can only be loaded up to 80% of its capacity; that’s 100 amps. So, don’t need a 125 amp wire to wire it; we need a 100 amp wire. From the chart above, we can see that we can use either 3 AWG copper wire (100A ampacity) or 1 AWG aluminum wire (100A ampacity) as a 125 amp breaker wire sizes.

Now, the 1 AWG copper and 2/0 AWG aluminum wires are the standard 125 amp wires we use for long distances. However, if we have a service that is 50+ feet away from the source, we do have to account for voltage drop.

Example: If you want to run a 125 amp service 100 feet away on a 120V voltage and at a 3% allowable voltage drop, the standard 1 AWG copper wire is not thick enough. Instead, we have to upgrade to 1/0 AWG copper wire to run it 100 feet away and still be within the 3% allowable voltage drop.

Let’s look at the math first, and then you can use the calculator that does this calculation automatically, and the corresponding two 125 amp wire size charts:

How To Calculate 125 Amp Wire Size At A Distance? (Voltage Drop)

The wire itself has a little resistance that impedes the flow of current. That’s why the voltage is 125 amp wire can drop, and we have to stay within the allowable voltage drop, as dictated by the NEC code. These voltage drops are:

• 3% allowable voltage drop for branch circuits.
• 5% allowable voltage drop for feeders and sub-panels.
• 10% allowable voltage drop for general appliances and general lighting.

To illustrate how to account for voltage drop in 125 amp circuits, let’s use our previous example. We have a 125 amp service at 120V that we want to deliver to a source 100 feet away. This is a branch circuit, so the allowable voltage drop is 3%, and we are using copper wires. Here is the equation for voltage drop:

Voltage Drop = 2 × L × K × 125 Amp / Circular Mils

At 120V, the 3% voltage drop is 3.6V (120V×0.03 = 3.6V). The L in the equation stands for wire length, and K is the specific resistivity of the wire material (12.9 ohms for copper wires and 21.2 ohms for aluminum wires).

To determine the wire size, we want to calculate how many circular mils should our wire have, and then we can use this AWG to circular mils chart to figure out AWG wire size for 125 amp current at a distance. Let’s express Circular Mils and put all of this into the equation:

Circular Mils = 2 × 100 Feet × 12.9 Ohms × 125 Amp / 3.6V = 89,583 Circular Mils

We see that we need a wire with at least 89,583 circular mils cross-section area. Now, we look at which AWG copper wire is that. Here is the screenshot from the AWG to circular mils chart:

Here we can clearly see that we can no longer use 1 AWG copper wire for this circuit because it has 83,690 circular mils; less than the required minimum of 89,583 circular mils.

The next AWG wire that has more than 89,583 circular mils is the 1/0 AWG copper wire with 105,531 circular mils cross-section area. We can use any thicker wire, but this 1/0 AWG copper wire is the optimum wire size to use for 125 amp service 100 feet away at 120V and 3% allowable voltage drop.

Now, it’s a bit tedious to calculate this manually. That’s why you should use the following 125 Amp Wire Size Calculator; you input wire length, voltage, and allowable voltage drop percentage, and the calculator will determine the 125 amp wire size automatically.

Below the calculator, you will also find two 125 amp wire gauge charts for 3% and 10% allowable voltage drops at 0-500 feet distance at 12V, 120V, and 220V voltages that contains all the relevant 125A wire sizes:

125 Amp Wire Size Calculator (Accounted For Voltage Drop)

Here is how this calculator works:

Let’s say you have a 240V circuit and want to deliver 125 amps to a feeder that is 400 feet away. The allowable voltage drop for feeders is 5%. To calculate the adequately sized 125 amp wire, just slide the 1st wire length slider to ‘400’, the 2nd voltage slider to ‘240’, and the 3rd allowable voltage drop percentage slider to ‘5’. Here is the result:

For this 400 feet away 125 amp 240V circuit, you will need to use 2/0 AWG copper wire, not the regular 125 amp 1 AWG copper wire.

Note about the short wire lengths: This calculator accounts for voltage drop very well. However, we need to keep in mind that we always need a 1 AWG copper wire or thicker for a 125 amp circuit (never 2, 3, 4 AWG copper wires, for example).

Here are the 2 charts that specified the 125 amp wire sizes at 0-500 feet distance and at 12V, 120V, and 220V voltages. The first one is for a 3% allowable voltage drop, and the second one is for a 10% allowable voltage drop:

125 Amp Wire Gauge Source 0-500 Feet Away (3% Allowable Voltage Drop)

We see that the voltage drop is especially important when we use low voltages (case and point: 12V DC current). Here is what gauge wires we need to use for 125 amp service 200 feet away at a 3% allowable voltage drop at different voltages:

• At 12V, we have to use super thick 2000 kcmil copper wire. That’s because this thick wire has a very low resistance and can keep the voltage drop below the 3% allowable limit.
• At 120V, we have to use 4/0 AWG copper wire.
• At 220V, we have to use 1/0 AWG copper wire.

We see that at higher voltage (220V), the voltage drop is not all that significant. In this case, we only have to upgrade from the standard 1 AWG copper wire for 125 amp current to a 1/0 AWG copper wire.

Let’s look at what gauge copper wires we need to use at a less rigorous 10% allowable voltage drop for 125 amp service:

125 Amp Wire Gauge Source 0-500 Feet Away (10% Allowable Voltage Drop)

If we look at 200 feet wire length again, we see that at 10% allowable voltage drop, we have to use:

• 600 kcmil copper wire for 125 amp service at 12V.
• 1 AWG copper wire for 125 amp service at 120V.
• 1 AWG copper wire for 125 Amp service at 220V.

We see that we don’t have to upgrade wire sizes at 120V and 220V voltages (we can use 1 AWG copper wire) since the 10% allowable voltage drop is less restrictive. For 12V DC current 200 feet away, however, we still have to use thicker 600 kcmil copper wire.

Hopefully, the calculator, along with these two charts, helps you out in adequately sizing 125 amp wire. If you need any help, you can use the comment section below, give us some numbers, and we can help you out.