Most hand-crank Meggers have output voltages from 250 to 500 volts DC. All wiring on transformers should be Meggered at 250 or 500 VDC.
Meggering transformer wiring is emphasized because of the numerous small terminal boxes mounted on large power transformers. the conduit connecting them can have moisture accumulation or water leaks. In addition, when wiring is pulled through the metal conduit on a transformer, occasionally the insulation is scraped down to the bare wire.
Also, note that any box mounted on a vertical surface should have a small drain hole drilled at the bottom in case water leaks in from a loose conduit joint. Larger boxes or cabinets usually have resistive heaters and air-vent holes covered by screens to prevent moisture accumulation. Terminal boxes mounted on horizontal surfaces must have good weather seals for their covers. Any gasket with questionable ability to provide a watertight seal should be replaced.
Early completion of wire checking and low-voltage-component moggering is advisable, especially when large transformers are to be tested. Completion of these tasks up front is important because it allows the application of power to alarm and control circuits without worry of causing damage. Having auxiliary power available helps facilitate operational checks, especially when UL tap changers need to be operated to perform various tests. Changing tap positions manually by hand cranking the mechanism is slow and tiresome
process.