The force between two parallel wires is defined by F/L = mu0 * I1 * I2 / 2Pid.
Two wires that are touching (insulation to insulation) and carrying 1000A between them will push apart with a force of about 1lb/ft. If they're hit with 20KAIC durring a bolted fault, that jumps to about 275lbs/ft! This is why you have to tie down cables in some applications.
If, instead of running parallel conductors through one conduit, you run the 3 phases together (or better yet, triplex them) and split the parallel conductors up in different conduits like that, the magnetic fields will cancel and pull the cables together as opposed to tearing them apart. This will cause less inductive losses in the conduit, too.
[This message has been edited by SteveFehr (edited 11-17-2006).]
