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  1. #1

    PTAC units and 3 phase

    No sure where to post this question on this site so I thought I would try here. I have a small 30 unit apartment complex that is uses PTAC units to heat and cool. Before we purchased the building, two new dedicated 240volt breaker panels were installed to service just the PTAC units. We've been slowly replacing the units with new Amana 410 systems and having all kinds of voltage problems and only new units randomly shutting off with "high voltage" errors. What I discovered is the third leg coming into building is a "high leg" 240 volt. (I really am baffelled how when you put the two 120 legs together you get 240 but when you put the 240 and 120 together you only get 240.) I think the issue is we're getting 254v from the pole and the Amana plate says operating range is 199v-253v. Every unit that that has the high leg occassionaly will shut off with "high voltage". Power company wants $7000 to change transformer pole. Electrician said we could put expensive transformer on panel. Odly enough, on the Amana PTAC unit the transformer has a alternate 265v leg we can hook up to for the low voltage. I assume 265v is for 480 power but was just curious if that might be an option or am I just going to blow up the board. Just so unfamiliar with 3phase, high legs and newer electronics. Any feedback or suggestions would be greatly appreciated.

  2. #2
    I would check with Amana on that, and I wouldn't blow up the board. That's really dangerous.

  3. #3
    Join Date
    Nov 2007
    Location
    Mississippi
    Posts
    1,307
    I think I would get power company to correct the voltage.

  4. #4
    Join Date
    Jul 2007
    Location
    The Hot South
    Posts
    1,299
    What you have is a 3 phase system wired in 'Delta'. 2 of the phases will read 120 volts to ground and the 3rd phase will read a higher voltage to ground. All 3 phases should read 240 volts when measured phase to phase. It really doesn't matter what you voltage you read to ground as long as you read 240 volts (or voltage range of 199 - 253 volts) phase to phase. If you are out of that range, then you need to get the power company to fix it.

  5. #5
    Join Date
    Oct 2012
    Location
    Sandy, Utah
    Posts
    93
    R123,
    The Delta 3 phase...
    Is the high voltage leg because the Delta system has a "floating ground"....
    I'm probably wrong but I'm thinking I learned that some time ago.
    Is that why voltages to ground can be confusing and as you say, that doesn't matter. It's the phase to phase your concerned with?

  6. #6
    Join Date
    Jul 2007
    Location
    The Hot South
    Posts
    1,299
    Name:  3 phase delta high leg.png
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    There is no 'floating ground'. One of the windings has a 'center tap' in the middle of the winding which is the neutral and is also grounded. The voltage measured from Phase A or Phase C to the center tap (ie. neutral or ground) will be half the voltage measured from phase to phase. The voltage measured from phase B to the center tap will be the square root of 3 (1.732) times the voltage measured from Phase A or Phase C to the center tap because the power is going across one full winding (across B to C and B to A) and across half a winding again (C to center tap and A to center tap). In the case of a 240/120 volt 3 phase delta system, phase B will read 208 volts to ground.

  7. #7
    Join Date
    Oct 2012
    Location
    Sandy, Utah
    Posts
    93
    Thanks for the clarification.
    I stand corrected with my confusion.
    Great explanation
    Thanks

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