1. When Neutral Wire should be the same size as Phase Wire?
-- The Neutral Wire should be the same size as Phase Wire in two wire single phase circuit. the neutral wire is also a part of the live circuit because of the same current will flow through the phase wire and neutral wire. As here, the current in both wires is equal so the size of both wires should be equal.
The neutral wire should be treated as the live or phase conductor.
The neutral conductor should have the same cross-sectional area as the phase conductor in two wire single phase circuit.
2. When Neutral Wire should be thinner than Phase Wire?
The size of the neutral wire may be taken as less than phase wire in case of three-phase balanced load where the neutral wire mainly connected to the ground.
The LT side of the transformer or alternator neutral terminal is connected to the ground. Here the main purpose of the neutral wire is to ground the fault current in case of unbalance or harmonics. Here the neutral wire does not carry the load current. In this case, the size of the neutral wire may be less than phase wire.
When each phase conductor has a cross-sectional area greater than 16 mm-square(for copper) or 25 mm-square(for aluminum) in the polyphase system, then the neutral conductor may have smaller cross-sectional area than the individual phase conductor.
If the harmonic is greater than 10% in a polyphase system, then the neutral conductor should have the same size as the phase conductors.
3. When Neutral Wire should be thicker than Phase Wire?
The neutral wire should be thicker than phase wire when no. of loads uses a common neutral.
Generally, in house wiring purpose a common neutral wire is used for all loads, in this case, the neutral should be thicker because the neutral wire carrying more current than phase currents.
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