My shop addition (21'x60' added on to an existing 40'x60') is coming along.
Had to let go of my electrician due to the fact that he never shows up etc.
He installed a new 3-phase service entrance. From said entrance we are
feeding a sub-panel to the old shop. This subpanel is single phase 100 amp.

I am to install a second 50 amp 3 phase sub-panel to another section of the
new shop. My question is wiring for the run to the sub-panel. I purchase
some 3-#6 wire (52' run). This is 3 #6's and a ground wire. 4 wires total.
Out of this 3-phase sub, I will be running circuits for my wife's kilns
(220v), my 3-phase machines and a few 20 amp (120v) plug circuits. I am
confused about the 120 volt circuits. Being that I only have 4 wires (3
current, one ground), how do I run a neutral wire to these circuits. Even
more confusing is that the neutral and the ground all run to the same bus
bar, so what is the difference?

I can give up my investment in the 52' of 6-3 cable, but I can't seem to
find any 6-4 wire (4-#6's, plus ground), at least not at any one of the
home clubs. I could simply add a fourth wire (white neutral) to the 3 phase
sub-panel, where it will simply be bonded with the ground. Or, I could
simply create a neutral by bonding to the ground in the sub-panel and not
run a fifth wire. All very confusing.

Any and all advice appreciated. BTW, I do have on older copy of the
National Electric Code, but can't seem to find the answer therein.

Thanks, Ivan Vegvary