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  1. #1
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    Jul 2009
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    3 Phase Voltage question.

    I was on a 3 phase condenser today with a compressor that would not start. My voltage coming from my panel was as follows. I wrote it up as I had lost a leg of power. After contacting a electrician he says this is normal and it sends power in on ground. I have never seen or heard of this before. I am going back Monday to check it out again. The compressor would try to start, and sounded like a leg was missing. Didn't sound locked up or anything. Please explain what I was seeing because I have never seen it before.



    L1-L2 Ov
    L2-L3 250v
    L1-L3 250v

    L1 to Ground 273
    L2 to Ground 272
    L3 to Ground 64V

  2. #2
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    Jul 2009
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    I think I have figured out what's going on, and I am not totally crazy with my diagnosis. Thiis is why HVAC techs are great! We come home and study this **** so next time we are smarter then the last time. This is a residential house with a 3ph unit. What I think is going on after some researching, and studying is. They have put in a sub panel and grabbed N to L3. So you have L1, L2, and N to L3. This will give you the 0 volts I had between L1 and L2, and also give the voltage L3 to ground. I have a electrician buddy that told me that could work, but compressor won't last long. Basically Motor spins like this. L1-L2-L333333333333333, L1, L2, L3333333333333. The 64V is just enough to get it pushed to the next winding. Another thing I learned is there is no 3ph to a residential house. When you look up at a pole the Neutral is at the top. The ones below are 2 hots. They put neutral on top so lightning hits it first.

  3. #3
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    Apr 2003
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    If L3 was really N, you should have had 0V from L3 to ground (and then 120v from L1 to L3). Whatever the electrician was trying to tell you makes no sense. The compressor would not be trying to grab power from ground unless the windings were shorted out. The only way a 3 phase motor can get it's power is from between the phases. If you have 0v between L1 and L2, then you have 0v. No matter what kind of magic voodoo you do with that neutral, it can't be made to work.

    There are instances of residential homes that have 3 phase power available, but they are a very rare breed. I don't think I've ever seen one, but I know that they exist.

    If someone absolutely needs to power a 3 phase piece of equipment off of single phase power there are ways to do it. One option is a phase converter which is basically a belt driven 3 phase electric generator that's powered by a large single phase motor. Another option is a frequency drive that can create the extra phase of power through it's electronics.

    I listed those above options just to let you know that ways to do it do exist. They aren't cheap, and most likely aren't realistic options for your situation.

    Your best option would probably be to walk away or sell the guy some new single phase equipment.
    If at First You Don't Succeed, Skydiving Is Not for You.

  4. #4
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    I agree, and the readings I was getting is just crazy as hell! For 1 there is no such thing as 3 Phase in a residential atmosphere, but this unit has been running for a over a year. They are feeding a regular panel into a Sub panel with a 3 pole breaker. I recommended a electrician, and he said it was fine. Trust me, I by no means 100% claim to know whats going on! That is why I recommended a electrician. I can only speculate how they did it. I do know my readings, and that is all I can speculate off of. I told my boss I want to be there when the electrician pulls the panel to see what was done, and how they did it. All I can tell you is there was a 3ph 208/230 compressor running in a residential house off a residential panel for over a year. What I'm trying to figure out is how they did it. I didn't pull the cover off the panel to look. That's electrician Territory. Have I? Yes, Do I like it, Hell NO!

  5. #5
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    Jul 2012
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    WA
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    I work on a 3 phase unit running on single phase service. There is a capacitor bank called roto phase and it reads the same as three phase on a voltmeter.

  6. #6
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    Jul 2014
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    Apache Junction, Arizona
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    I installed a 5 ton Rheem split system Furnace coil box and complete duct system in a home here in phoenix that had a grandfathered 3 phase power... I never seen one in a residential before.. but sure enough there it was...

    from ground to each hot leg on all three you gotta have at least 108 volts on each or it won't work.. I would not try to run it unless it met these requirements.. you may have to reverse couple legs for phasing the compressor unless you have a new phase meter off ebay like me who is dying to try it.

    You can when in doubt simply like ammoniadog suggests recommend a single phase option instead of some bootleg electrical hotwire madness.
    "I never lie because I don't fear anyone. You only lie when you're afraid." - John Gotti

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  7. #7
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    Here where I'm at. There are several old residential neighborhoods with 3 phase power. With a 190v "wild leg" we call it. Most of our light commercial is 208.
    "If you can’t describe what you are doing as a process, you don’t know what you are doing." ~ W. Edwards Deming

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  9. #8
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    Jul 2016
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    I have seen phase convertors or generators as some call them used on 3 phase motors in residential applications. Not just hvac related 3 phase compressors in the garage or a home millwork shop. But clearly if your wire runs directly back to the panel its not the case.

    Sent from my SM-G930W8 using Tapatalk

  10. #9
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    Jan 2008
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    Phoenix Arizona
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    Not all three phase is the same. Those readings look like corner grounded delta...

    Could be wrong though...have to ask the utility.
    "If history repeats itself I am so getting a dinosaur"

  11. #10
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    Jul 2009
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    Ok, Utility company said we are sorry. We had a bad transformer we found. All is good now. I went back today, and went inside. Pulled the shelving away from the walls, and sure enough there is a 3 phase meter in the basement. I stand corrected. This house does have 3 phase. I check voltage at disconnect and still have no voltage between L1 and L3. 0V. Phase is still missing. Go inside and pull the cover off the 3 phase panel. I don't even have it before the breaker. This is the third time Utility company told us and the owner it was fixed. Compressor ohms out good, and is not grounded. I need the dang on power for it to run. I assume there checking power to ground, and not between legs. I'm not sure what’s going on. I'm just trying to get this customer AC. Now I will have to return 1 more time to check rotation after there fix.

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