We use 2/0 for TPG's (Temporary Protective Grounds) in the power utility industry all the time.

 

TIP #2: Price the wire at a welding supply shop, it'll be cheaper. Same with the quick-connects. Warn wants $65 for their push-type quick connects, I paid $9.95 a piece for twist-lock ones at my local welding shop. Plus that type can take anything from 4 AWG to 2/0 AWG, you simply cut the wire insertion end at the line marked for the wire gauge you put in. Plus they are screw lock, so all you do is trim the end of the wire, stick it in and tighten two screws...

 

I guess that I work for a company that has 2/0, and 4/0 just laying around doing nothing in up to 50' lengths is a good thing, and yes, if you are pulling 400 amps, you need 4/0 for sure.