Why Neutral Cable Size is Half of Phase Cable?

The neutral wire is half the size of the phase wire because it carries only the imbalance current between phase A and phase B.

The neutral wire is tied to ground and also the center tap on the secondary (output) of your transformer out on the pole (right in the center of the secondary coil, making a reference point to 0v gnd, that splits the voltage between two 120v legs).

That’s why it is allowed to be smaller.

The cross section of the neutral conductor can be less than the cross section of the phase conductor when the cross section of the phase conductor is greater than 16mm2 with a copper cable, or 25mm2 with an aluminium cable, if both the following conditions are met:

1. The neutral conductor is protected by a device that will automatically disconnect all conductors of the circuit including the neutral conductor in case of a fault between any live conductor and earth.

2. The maximum voltage to earth does not exceed 50 V for single-phase circuits and 30 V for three-phase circuits.