Large gauge building wire in the 2-0 to 4-0 range is made of 19 strands of soild wire about 10 gauge each.It's not very flexibe and can put a strain on battery terminals if not bent just so.It will work, but fine strand welding cable is better.
2-0 19 strand building wire is rated at 300 amps,600 volts in free air at 90 degrees Centigrade.This is for continious duty on power distribution ,not for 10 seconds of cranking on a engine starter.For short lengths, the conductor can take probably 100 percent overload.But with only 6 volts when cranking,a 10 percent voltage loss is a big deal.
Pay attention to the terminations on the cable, a large conductor is useless if the cable ends can't take the current.
