What size copper wire is needed for a 400 amp three phase feeder?

600 kcmil copper per phase at 75 degrees C, or two parallel sets of 3/0 copper, per NEC Table 310.16.

Does a 200 foot 400 amp feeder have a voltage drop problem?

Two parallel sets of 3/0 copper come in around 2.5 percent at 208 volts, inside the 3 percent recommendation in the NEC informational notes.

What size ground goes with a 400 amp feeder?

3 AWG copper or 1 AWG aluminum from Table 250.122, and NEC 250.122(F) requires one in each parallel raceway.

What size wire do I need for a 400 amp 120/208 volt three phase subpanel on a 200 foot run?

Start with ampacity, then check voltage drop, then size the neutral and the equipment grounding conductor. For a 400 ampere feeder at 75 degrees C from Table 310.16, copper gets you there with a single 600 kcmil conductor per phase rated 420 amperes, or with two parallel sets of 3/0 copper rated 200 amperes each for a combined 400 amperes. Aluminum needs 750 kcmil at 435 amperes for a single set, or two parallel sets of 250 kcmil at 205 amperes each.

Section 310.10(G) permits paralleling only in sizes 1/0 AWG and larger, and requires that the paralleled conductors be the same length, same material, same size, same insulation type, and terminated the same way. Section 215.2(A)(1) sets the feeder minimum at the calculated load, and 215.3 covers the overcurrent device.

Voltage drop over 200 feet

Section 210.19(A) and 215.2(A) informational notes recommend a maximum of 3 percent on the feeder and 5 percent total. On a 120/208 volt three phase system at 400 amperes, 3 percent of 208 volts is 6.24 volts. Using the approximate formula for three phase, Vd equals 1.732 times K times I times L divided by circular mils, with K near 12.9 for copper at 75 degrees C.

For two parallel sets of 3/0 copper the total area is 2 times 167,800, or 335,600 circular mils. Vd equals 1.732 times 12.9 times 400 times 200 divided by 335,600, which is about 5.3 volts, or 2.5 percent. That passes. A single 600 kcmil copper conductor per phase gives 1.732 times 12.9 times 400 times 200 divided by 600,000, about 2.98 volts, or 1.4 percent, which is comfortable.

Aluminum with K near 21.2 needs more copper equivalent area. Two parallel 250 kcmil aluminum sets give 500,000 circular mils total and produce roughly 5.9 volts, or 2.8 percent, which just clears 3 percent with nothing to spare. Two parallel 350 kcmil aluminum sets drop it to about 2 percent and are the safer choice on a 200 foot pull.

Neutral and grounding conductor

Section 220.61 lets you calculate the feeder neutral for the maximum unbalanced load, and 220.61(C) prohibits reducing the neutral below the required grounding electrode size for the portion of the load that is electric discharge lighting or nonlinear. On a 120/208 volt three phase four wire feeder serving significant nonlinear load, run a full size neutral.

The equipment grounding conductor comes from Table 250.122. For a 400 ampere overcurrent device the minimum is 3 AWG copper or 1 AWG aluminum. Section 250.122(F) requires a full size EGC in each parallel raceway, so if you pull two sets in two conduits, each conduit gets its own 3 AWG copper EGC. Section 250.122(B) also requires the EGC to be increased proportionally if you upsize the phase conductors for voltage drop.

Field takeaway

Two parallel sets of 3/0 copper in two conduits is the common build for 400 amperes at 200 feet, with a 3 AWG copper EGC in each raceway and a neutral sized per 220.61. If you prefer a single set, 600 kcmil copper or 750 kcmil aluminum works and gives better voltage drop margin. Verify the terminal temperature rating on both the breaker and the subpanel lugs before you commit, because 110.14(C) forces you back to the 60 degree C column if any terminal is rated only 60 degrees C.