If you were to pull 2 three phase circuits in the same conduit would you need more than 1 neutral and why?

Answer 1

If the load requires a single circuit breaker, as most three phase loads do, you need a separate neutral for each. This allows you to check the balance between each phase and the other phases as well as neutral, in order to detect a ground fault.

It depends on the purpose of the three phase circuits - if this were CT cables, then yes, unless you intentionally over sized the ground wire. Otherwise this would cause unusually high voltage rise in the secondary of the CT during ground faults, which could lead to saturation (unless CT ratio is intentionally set high). As noted above, having separate neutrals helps to determine balance, although you can measure 3I0 from the phase currents for a similar relationship without actually looking at the neutral at all (some relays doe this for ground protection instead of directly measuring neutral current, see SEL's 351 for an example relay). No matter what, this would be highly irregular, and I wouldn't recommend scrimping on a neutral. In my experience, the more you move away from accepted practices (not to dampen ingenuity...), the more trouble you create for the future.

If the loads were strictly three phase then there would not be any neutral needed. If from a three phase distribution panel the loads were three single phase loads then you can use one neutral for the three phase wires. This is a code rule that has to be adhered to. A same N, B same N, C same N and only in this combination. Conduit fill 4 wires, two three phase four wire circuits, conduit fill 8 wires.