What is the minimum copper neutral size permitted in conduit to be run to a service disconnect that serves three phase motors only given that the neutral load is 0 Ohms when the size of the ungrounded phase conductors are two sets of 3/0 Awg copper pulled into separate conduit raceway?

What is the minimum copper neutral size permitted in conduit to be run to a service disconnect that serves three phase motors only given that the neutral load is 0 Ohms when the size of the ungrounded phase conductors are two sets of 3/0 Awg copper pulled into separate conduit raceway?