Never join them, that is a dead short.
If the motor is operated from a three phase three wire distribution system the motor will not need a neutral wire.
The neutral wire should be the same size as your conducters
In a balanced three-phase system, the current in the neutral wire should ideally be minimal as the three phase conductors carry equal and opposite currents that cancel out in the neutral wire. However, if the loads are unbalanced, the neutral wire may have higher current due to the uneven distribution of power among the phases. This can happen when loads on each phase are different or when single-phase loads are connected between a phase and neutral, leading to increased neutral current.
because its much safer to switch the hot wire then the neutral wire because if you are working on a light fixture for example and the switch is off if you ground yourself out to the neutral you become the load or return. a neutral shock can me more dangerous at times
no
If the phase and neutral wires are shorted together, the voltage in the neutral wire will be the same as the phase voltage. This is because the short circuit effectively bypasses any impedance or resistance in the circuit, causing the potential difference between the phase and neutral wires to be equal.
A smaller neutral wire in a three phase system can be used because it does not carry the full line current. It carries the unbalanced current of all three leg loads. This is one reason that three phase loads on a distribution panel should be equalized as much as possible to reduce the current on the neutral.
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The Neutral wire should be the same gauge and size of the phase wires. Only the Earth or Equipment wire can be smaller than the phase(hot) wires. <<>> On a three phase four wire distribution system the neutral can be reduced in size. The neutral only carries the unbalanced load current and should be sized to that current. The highest current load to neutral of the three legs is used to calculate the neutral sizing. There is no reduced neutral allowed on discharge lighting and non linear loads. A demand factor of 70% shall be permitted to be applied to that portion of the unbalanced load in excess of 200 A.
On three phase services over 200 amps the electrical code allows the electrical contractor to reduce the size of the neutral wire . This saves the customer money. There are conditions as to how much the neutral can be reduced but it is not as low as half the supply conductors. The reason that the neutral can be reduced is that it only carries the unbalanced current on the three phase four wire system.
On a typical house plug, there are three wires - the positive, neutral, and ground. Ideally, the positive and neutral wires carry the current (the neutral wire provides the return path for the current from the positive wire), and the ground wire carries no current. In a three phase system, you have three phase voltages of the same magnitude (ideally), but the three phase voltages are out of phase with each other by 120 degrees - meaning one is at 0 degrees, one is at 120 degrees, and one is at -120 degrees if you looked at them on an oscilloscope, and referenced to one phase. If you take (1 at an angle of 0 degrees) + (1 at an angle of 120 degrees) + (1 at an angle of -120 degrees), you will get zero. Thus the return path in three phase power is shared between the three phases, and the neutral wire in a 4 wire, three phase system is equivalent to the ground wire connected to your wall plug. The ground wire will only carry current when the "vector sum of the phase voltages does not equal zero" (meaning the simple equation at the beginning of this paragraph does not sum to zero - whatever it sums to is what is flowing in the neutral).
If the motor wire numbers are L1, L2 and L3, it is not a single phase motor. It is a three phase motor. Also for future reference, a 220 volt single phase motor does not use a neutral.