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Thread: I need help with this HVAC label.......

  1. #11
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    Quote Originally Posted by JoeStillman View Post
    kwired, I agree, that MCA is a puzzlement.
    I wonder if this a VRF system with multiple evaporator units. Note the label says the electrical characteristics are for the outdoor unit only. That might explain the discrepancy.

  2. #12
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    Quote Originally Posted by texie View Post
    I wonder if this a VRF system with multiple evaporator units. Note the label says the electrical characteristics are for the outdoor unit only. That might explain the discrepancy.
    Yes it is.

    Also the compressor rating is in RLA which is a useless number for calculating MCA. RLA is only used for calculating the compressors overload.
    Last edited by Mgraw; 07-07-18 at 07:26 AM.

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    FWIW, I usually find the actual load to be about 66% of MCA. I am not saying there are not exceptions, but most I have measured have been right about there.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  4. #14
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    Quote Originally Posted by electrofelon View Post
    FWIW, I usually find the actual load to be about 66% of MCA. I am not saying there are not exceptions, but most I have measured have been right about there.
    These VRF units usually draw less than that but manufacturers are required to calculate worst case.


    I should also note in my previous post I should have said RLA is just a percentage of compressor overload values.

  5. #15
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    Quote Originally Posted by Mgraw View Post
    These VRF units usually draw less than that but manufacturers are required to calculate worst case.


    I should also note in my previous post I should have said RLA is just a percentage of compressor overload values.
    RLA is a computed figure by manufacturers, it is the maximum continuous current (or MCA) of the compressor divided by the overload factor of the unit. Since refrigeration compressors are internally protected, the factors per code are 1.70 for loads up to 9 amperes; 1.56 for loads 9.1A to 20, and 1.40 for loads over 20A.

  6. #16
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    Quote Originally Posted by electrofelon View Post
    FWIW, I usually find the actual load to be about 66% of MCA. I am not saying there are not exceptions, but most I have measured have been right about there.
    Load on a compressor is effected by how much heat is being moved by the refrigerant, and conditions of the heat exchanger coils can have an impact. Plugged condenser coil can't get rid of heat - leaving the system running at higher pressure and loading the compressor even more then usual.

    Quote Originally Posted by Mgraw View Post
    These VRF units usually draw less than that but manufacturers are required to calculate worst case.


    I should also note in my previous post I should have said RLA is just a percentage of compressor overload values.
    VRF needs to be running at 100% before it will reach RLA. When you first start system up you might get 100% speed but still depends on actual refrigerant loading as to what the motors will draw. In heating mode you may see 100% speed more often then during cooling mode - but there isn't as much heat being moved during extreme cold conditions and it won't be as much load on the system.


    RLA is sort of same thing as FLA in a regular motor - it is what the thing will draw when running at it's maximum design output. If it isn't loaded to it's maximum design it will draw less current. Just because a unit is rated for a certain BTU output doesn't mean it puts out that amount of BTU anytime it is running - other factors go into that load.
    I live for today, I'm just a day behind.

  7. #17
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    Here are my calculations, if you will:
    RLA TOTAL = 7.3 + 10.3 = 17.6
    MAX. CONT. CURRENT = 17.6X 1.56 = 27.5
    FLA OF COMPRESSORS = 27.5/1.15 = 23.9 (115% OVERLOAD APPLIED)
    MCA = 1.25X23.9 + 2 = 31.9!
    MOP = 2.25 X 23.9 + 2 = 43.34, rounded to the nearest available protection = 40A

  8. #18
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    Quote Originally Posted by topgone View Post
    MCA = 1.25X23.9 + 2 = 31.9!
    So, is #10cu adequate, or must #8cu be used?
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  9. #19
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    Quote Originally Posted by LarryFine View Post
    So, is #10cu adequate, or must #8cu be used?
    Unless you must use 60C ampacity table, 10AWG is good.
    I live for today, I'm just a day behind.

  10. #20
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    Quote Originally Posted by kwired View Post
    How about for feeder and service calculations?
    But unless one is heading toward 220.87, I don't see how actual load comes into play?
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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