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

  1. #21
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    Quote Originally Posted by electrofelon View Post
    But unless one is heading toward 220.87, I don't see how actual load comes into play?
    If you have multiple units isn't your minimum feeder/service load calculation 125% of largest load plus all others? If you used MCA of each individual unit you will come up with higher overall ampacity needed then if you only went with 125% of largest compressor because 125% of the largest compressor in each unit is already factored into MCA.
    I live for today, I'm just a day behind.

  2. #22
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    Quote Originally Posted by kwired View Post
    If you have multiple units isn't your minimum feeder/service load calculation 125% of largest load plus all others? If you used MCA of each individual unit you will come up with higher overall ampacity needed then if you only went with 125% of largest compressor because 125% of the largest compressor in each unit is already factored into MCA.
    Yup you are correct, I forgot about that scenario.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  3. #23
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    probably there are other loads inside which is not listed on nameplate. I would use MCA divided by 1.25 for expected current and for load summary of the project.

  4. #24
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    Quote Originally Posted by victor.cherkashi View Post
    probably there are other loads inside which is not listed on nameplate. I would use MCA divided by 1.25 for expected current and for load summary of the project.
    I think the OP presented that already. My calcs arrived at an exact MCA of 31.9A using his figures, the name plate must be correct.

  5. #25
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    Quote Originally Posted by topgone View Post
    I think the OP presented that already. My calcs arrived at an exact MCA of 31.9A using his figures, the name plate must be correct.
    Quote Originally Posted by topgone View Post
    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
    Why did you multiply total RLA by 1.56?

    I also question the added 115% but you at least told us why you did it.
    I live for today, I'm just a day behind.

  6. #26
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    We went a similar post a year or so ago...still not clear on why this sometimes is not clearly laid out by the manufacturers.

    I called the manufacturers tech line....they called back and stated the following.

    The calculations are done by engineering in Japan and the methods are "Proprietary".

  7. #27
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    Quote Originally Posted by topgone View Post
    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
    ???
    what is the 1.56?
    what do the ol's have to do with it? rla x 1.15, not divided, 17.6 is not their settng

  8. #28
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    Quote Originally Posted by kwired View Post
    Why did you multiply total RLA by 1.56?

    I also question the added 115% but you at least told us why you did it.
    Sorry for the late reply.
    Hermetic compressors are provided with thermal protectors. The code provision is for 0-9A compressors to never exceed 170% of the load amps; for 9.1 up to 20A compressors to never exceed 156% of load and for 20A up to never exceed 140% of load (please refer to 430.32(A)(2)-2011). As mentioned, manufacturers of compressors for HVAC units are required to determine the maximum continuous current (MCA) of their units to qualify for UL listing. Then, the RLA is computed from this value, which the divisors are provided for in 430.32(A)(2).

  9. #29
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    Quote Originally Posted by topgone View Post
    Sorry for the late reply.
    Hermetic compressors are provided with thermal protectors. The code provision is for 0-9A compressors to never exceed 170% of the load amps; for 9.1 up to 20A compressors to never exceed 156% of load and for 20A up to never exceed 140% of load (please refer to 430.32(A)(2)-2011). As mentioned, manufacturers of compressors for HVAC units are required to determine the maximum continuous current (MCA) of their units to qualify for UL listing. Then, the RLA is computed from this value, which the divisors are provided for in 430.32(A)(2).
    I see how you arrived at that. I don't know if I agree with that being how it is determined.

    What doesn't make sense is if you have 156 or 170% motor overload protection, didn't you essentially increase the RLA that can be seen without tripping anything whether the windings can take that extra heat or not?

    Kind of like taking a 100 watt incandescent lamp but increasing voltage from 120 to 150 to get more light out of it. Will work but if you don't do something to manage the extra heat - it won't last as long as it would at rated input.
    I live for today, I'm just a day behind.

  10. #30
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    Quote Originally Posted by kwired View Post
    I see how you arrived at that. I don't know if I agree with that being how it is determined.

    What doesn't make sense is if you have 156 or 170% motor overload protection, didn't you essentially increase the RLA that can be seen without tripping anything whether the windings can take that extra heat or not?

    Kind of like taking a 100 watt incandescent lamp but increasing voltage from 120 to 150 to get more light out of it. Will work but if you don't do something to manage the extra heat - it won't last as long as it would at rated input.
    I guess that's how HVAC compressors are made-->the compressor motor is lubricated by oil and the lube oil removes heat at the same time.

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