For 400 amp service, we can rule out the standard AWG wires. We have to use thick kcmil copper and aluminum wires. On top of that, we have to account for voltage drop for services some 100s feet away.
Alright, here is how you can figure out any wire size, including for 400 amp service:
For a 400 amp circuit, you need a wire that can handle at least 400 amps. That means that that wire should have at least 400A ampacity. We can check this NEC 310.16 table with listed wire gauges and their corresponding ampacities.
For 400 amps, we have to check the kcmil wire sizes. Here is the screenshot from that NEC 310.16 chart with highlighted 400 amp wires:
As we can see from this screenshot, we need either:
- 600 kcmil copper wire for 400 amp service. It has 420A ampacity; more than the minimum required 400A ampacity.
- 900 kcmil aluminum wire for 400 amp service. It has 425A ampacity; more than the minimum required 400A ampacity.
Now, if you want to run a 400 amp service some distance away (100, 200, 300 feet, and so on), you have to account for voltage drop. The maximum allowable voltage drops are:
- 3% for branch circuits.
- 5% for feeders and sub-panels.
- 10% for non-crucial circuits like general appliances and general lighting.
At lower amp services, the voltage drop is significant. You will quickly have to use thicker wires. At the big 400 amperes current, the voltage drop is less significant.
To help you out, we have prepared 2 charts for running a 0-500 feet wire for 400 amp service at 120V and 220V voltages. The 1st one accounts for a 3% allowable voltage drop, while the 2nd one accounts for a less rigorous 10% allowable voltage drop.
Here are the wire gauges for 400 amp service at a distance:
400 Amp Wire Gauge Source 0-500 Feet Away (3% Allowable Voltage Drop)
| Wire Length (3% Vd): | 400 Amp Wire Size At 120V: | 400 Amp Wire Size At 220V: |
| 0 Feet | 600 kcmil Copper Wire | 600 kcmil Copper Wire |
| 50 Feet | 600 kcmil Copper Wire | 600 kcmil Copper Wire |
| 100 Feet | 600 kcmil Copper Wire | 600 kcmil Copper Wire |
| 150 Feet | 600 kcmil Copper Wire | 600 kcmil Copper Wire |
| 200 Feet | 600 kcmil Copper Wire | 600 kcmil Copper Wire |
| 300 Feet | 900 kcmil Copper Wire | 600 kcmil Copper Wire |
| 400 Feet | 1250 kcmil Copper Wire | 700 kcmil Copper Wire |
| 500 Feet | 1500 kcmil Copper Wire | 800 kcmil Copper Wire |
At a 3% allowable voltage drop, we see that we can use the standard 600 kcmil copper wire for service up to 200 feet away (for 120V and 220V circuits).
For 120V circuits at a 3% allowable voltage drop, we see that we have to:
- Upgrade to 900 kcmil copper wire for service 300 feet away.
- Upgrade to 1250 kcmil copper wire for service 400 feet away.
- Upgrade to 1500 kcmil copper wire for service 500 feet away.
For 220V circuits, the voltage drop is less severe, and we have to:
- Upgrade to 700 kcmil copper wire for service 400 feet away.
- Upgrade to 800 kcmil copper wire for service 500 feet away.
Now, let’s have a look at the 400 amp wire sizes for less rigorous 10% allowable voltage drop:
400 Amp Wire Gauge Source 0-500 Feet Away (10% Allowable Voltage Drop)
| Wire Length (10% Vd): | 400 Amp Wire Size At 120V: | 400 Amp Wire Size At 220V: |
| 0 Feet | 600 kcmil Copper Wire | 600 kcmil Copper Wire |
| 50 Feet | 600 kcmil Copper Wire | 600 kcmil Copper Wire |
| 100 Feet | 600 kcmil Copper Wire | 600 kcmil Copper Wire |
| 150 Feet | 600 kcmil Copper Wire | 600 kcmil Copper Wire |
| 200 Feet | 600 kcmil Copper Wire | 600 kcmil Copper Wire |
| 300 Feet | 600 kcmil Copper Wire | 600 kcmil Copper Wire |
| 400 Feet | 600 kcmil Copper Wire | 600 kcmil Copper Wire |
| 500 Feet | 600 kcmil Copper Wire | 600 kcmil Copper Wire |
Here we see that the 600 kcmil copper wire for 400 amp wire doesn’t need to be upgraded even if you run the wire for 500 feet (for 120V and 220V voltage).
In short, for wiring general appliances and general lighting, you don’t have to worry about voltage drop, and can go straight ahead with 600 kcmil copper wire or 900 kcmil aluminum wire.
We hope that this illustrates well how to go about figuring out a 400 amp wire size (or any wire size, for that matter). If you need a bit of help, you can use the comment section below, and we will help you out figuring the correctly sized 400 amp wire size.
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