A/C supplied with both 120v and 240v


Old 10-04-18, 06:30 PM
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A/C supplied with both 120v and 240v

Our makerspace has 3 identical HVAC units. Each condenser runs off of a 30 Amp 240v breaker. However, when I check the power being supplied at the evaporation, I read confusing voltage. I checked the black to ground (green to a case ground block) and red to ground. On two of the units, I get 240v on the red wire and 110v on the black wire. On one unit, I get 120v on black to ground and 120v on red to ground.

Shouldn't I get 240v between either red to ground or black to ground?

Incidentally, two units function fine. On one A/C condenser the fan runs, but the compressor does not. This is one of the units that has 240v on red to ground and 120v on black to ground.


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Old 10-04-18, 07:01 PM
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Welcome to the forums.

On a 240v circuit...... you should measure 240v across the two hot wires and 120v from each hot wire to ground. How do you have red, black and ground...... where's the white ?

It's virtually impossible to measure from any single wire to ground and see 240v in a home.

On edit:
Is this a commercial application ?
Possible three phase electric in that location ?
You could be getting a red or wild leg to ground. Not usually 240v.... usually slightly higher.

Last edited by PJmax; 10-04-18 at 07:38 PM.
Old 10-05-18, 05:01 AM
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Is this a new installation? Also I think that you need to answer @PJMax's questions. Much more information is necessary as to the age of the units, size of the units, commercial installation, Single or 3 phase electrical service. Also is the power to these units all from the same breaker panel or from different panels. It may be wise to have a registered electrician check the primary feed wiring of these units.

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