Insulation resistance for of generator

First of all let me clarify this,
Insulation value does not depend on KVA rating of the Generator.
It depends only on the rated voltage of the Generator.

The insulation resistance is measured between the Conductor (Phases) and earth. If the neutral is not earthed  it becomes as good as a conductor. Hence its insulation value should be Rated Voltage in KV + 1 M.ohm.
For eg. if the rated voltage of your generator is 415V, then insulation resiatance should be
1 + 0.415 = 1.415 Meg.Ohm.
This is the thumb rule which works pretty well

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