Where does you neutral come from single phase transformer?

In a single phase system the neutral wire is the return path of the circuit and may be near ground potential, in multiphase systems it carries the unbalanced current.

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In a single phase system where only one "hot" wire supplies current to the load, the "neutral" wire completes the circuit and carries the current flowing from the load back to the power station.

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As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.
Before you do any work yourself,
on electrical circuits, equipment or appliances,
always use a test meter to ensure the circuit is, in fact, de-energized.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB
SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS.