For my examples, all wires are copper with 90C insulation and 75C terminals.

I understand it for single conductor per phase feeds. As a simple example, consider a 64A load on an 80A breaker that would normally have #4 wire, #8 ground, for local cases. Upsize the wire to #2 for voltage drop, and the proportional increase (160%) yields #6 Cu for ground.

**Usually I consider 500 kcmil or 600 kcmil to be the maximum size before thinking about paralleling.**

What happens when your situation is on the cusp of either paralleling vs not paralling, and voltage drop causes the decision to parallel?

What happens when your situation is on the cusp of either paralleling vs not paralling, and voltage drop causes the decision to parallel?

Example: a 400A breaker with a 320A load. Locally, it could have either 600 kcmil, or parallel 3/0, and in either case, a #3 ground per raceway. If a single raceway is shared, it would be parallel 4/0 because of the bundling derate.

Suppose I were upsize the live feeders to parallel 350 kcmil, for voltage drop. At this size of wire, if it is 3-phase AC, I wouldn't even about sharing a raceway. Dedicated raceway each set.

I understand that whatever the ground may be, each raceway must contain a full size. No dividing up the kcmil, as I originally would guess.

If 600 kcmil were considered the default, then I've upsized by a ratio of 7/6. Meaning the upsized ground would be #2.

If parallel 3/0 were considered the default, then I've upsized by a ratio of 2.08. Therefore the ground would be #2/0.

So which strategy is correct?