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#165219 06/23/07 08:10 AM
Joined: Sep 2005
Posts: 99
Tripp Offline OP
Got a call about a rooftop A/C unit not working. Open the panel and find it is three-phase, with those tell-tale blanks every few spaces. The A/C is on a 3-phase breaker. With my meter I get the following readings, on both the main lugs and the A/C breaker: For all three phases I get 120v to ground. Phase A to Phase B I get 240v. Phase A to Phase C I get 240v. But Phase B to Phase C I get NOTHING. I expected to get 208v. Is this a problem at the transformer? How can B have voltage to ground, but not to another phase?

Joined: Aug 2001
Posts: 7,520
Did you try measuring voltages with the main open?

If you lost the incoming phase B from the xfmr, you could be reading voltages on that phase via some 240V device which is connected between B and C (maybe the A/C unit itself).

Joined: Jun 2007
Posts: 301
Checking voltage to ground is what has you confused. I don't use the practice as I have found that it complicates simple problems.
Ignore the ground readings for a moment. If you do not have voltage between two legs of the 3 phase, you can count that the voltage is not present.
If you are missing a leg you have to look for it. Start somewhere closest to the supply power and work your way back to the equipment. Possible areas of concern are XFMR's, breakers, contactors and possibly the wiring which is doubtful. Just work your way through this thing and keep the meter on the wires, NOT to ground.

Joined: Feb 2003
Posts: 939
As what JValdes saying but let me add one more thing is check the poco supply sometime they will supply the open delta format or open wye so check out the can[s] if you see just two cans both not same size that is open delta but if you see 3 cans if two are same and third one is diffrent that genrally it is delta but some case they used same size all the way thru with cans so watch for secondary taps that is surefire hint the first and third can will have no netural hook up at all but if this is on underground service the bet is off due the transformer set up.

What ya have to do is go back to the main panel and check the voltage on main breaker to make sure you have straght 240 v delta some case you will run into corner grouned delta [ not the case if ya have 4 wire service ]

Merci, Marc

Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

Joined: Jul 2004
Posts: 625
What you describe sounds like 120/240 single phase, connected to a three-phase panel, with one leg of the 240 connected to both the B and C phases of the panel.

Also, why are you expecting 208 between the B and C phases? I know of no system that would have 240 between two pairs of phases, and 208 between the remaining pair.

Joined: Jan 2005
Posts: 5,411
Likes: 1
Cat Servant
No voltage between phases means voltage is not present? EXCUSE ME?????
Maybe ... just maybe ... this is the wrong panel for the service. It's time to start over.


How many wires are coming in from the PoCo? How many transformers do you see on the pole?

B and C leg will show no voltage between them if they are both powered by the same leg. Yet, they'll still read 120 to ground.
I see this when someone puts a 3-phase panel in a 1-phase location. (As Solar Power described)

Joined: Jul 2002
Posts: 8,443
Likes: 3
I'd like to get more info on this type of system, there is a bit of talk about "high legs" and grounded delta's in the chat room from time to time.
I can't say I understand this system at all, it's totally foreign to what I've been trained in.
Can anyone point me to a thread or site that explains it all in detail?.
Mike. cool

Joined: Jun 2007
Posts: 1
New Member
as pauluk said you would be reading voltage via some other 208 volt breaker feeding through a load.working back to the source for that panel until you get voltage between phases narrows down the problem area.question though,could a amprobe be put on the phases and look for the low or no existant amperage on the feeder to narrow down the offending bad phase. this reminds me of a tip to offer.some of you have been to a building with partial lights and sproatic power outages throughout building and know that there is a lost phase.besides checking the neighbors and opening panel, go out to the meter and look for a missing or flashing a,b or c.then you know its on the poco side.

Joined: May 2002
Posts: 1,716
Mike, although I don't believe this has any bearing on the OP's problem, (it sounds like a phase has been lost for whatever reason)see the illustration below of a "Center Grounded Delta"


[Linked Image]

Note that the term"Center Grounded" is only refering to the center of one winding when speaking of a Delta configuration of windings.

Last edited by Roger; 06/23/07 09:22 PM.
Joined: Mar 2005
Posts: 212
One thing you're all missing, from a service man's point of view is that a short time ago this AC worked, just as it's connected. No one jumped up on that roof and did some wierdo wiring just for fun. I can't quite work it out from the info given, but one of the phases is out upstream from the disconnect. Find the point where you DO have correct voltages and work downstream.

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