Common Mode Voltage

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We have single phase (120v)branch outlet from a 3 ph. Y sub-panel. There is only one outlet on the circuit. The panel itself serves many electronic devices. We measure between 1 to 2 volts between the neutral and the IG on the branch ckt. Is there a "plain english" explanation of how this voltage is caused and is an isolation xfmr at the point of use the best fix?
The neutral to ground voltage is creating problems with an electronic scanner.

Ed MacLaren

Senior Member
Re: Common Mode Voltage

The 1 to 2 volts you are measuring could be the voltage drop in the branch circuit neutral conductor. The amount will be determined by the neutral conductor size, length, and load current.

If this is voltage drop, it will disappear if the branch circuit load is removed.


[ May 17, 2004, 10:01 PM: Message edited by: Ed MacLaren ]


Staff member
Plano, TX
Re: Common Mode Voltage

I agree, it is just the voltage drop (I*R) developed along the length of the grounded conductor. The only way to reduce it is an isolation transformer to establish a new ground reference point.

I must ask how your scanner scanner uses ground? 1-to-2 volts between N-G is within specs of most sensitive electronic equipment. It equates to less than a 3% voltage drop.
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