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Thread: HVAC Calculations

  1. #1
    Join Date
    Aug 2007
    Posts
    26

    HVAC Calculations

    I am in a disagreement with an plan reviewer on how to properly calculate a load for an HVAC ( commercial) in doing a service or feeder calculation. the HVAC in question is a wall mount unit 3 ton with 10kw heat strip, no dehumidification. 208V 3 phase.

    Assumptions
    1. Calculate the higher of the two- heat (10KW) or compressor AC loads and use the larger of the two.
    2. Use 220.51 for the fixed heat strip at 100%
    3. If compressor load is larger follow 220.50 which will take you to 440.6 , AHJ is saying you have to use 1.25% on compressor plus total of fan motors., if you go strictly by 440.6 I do not see any indication that you use 1.25
    4. AHJ is also saying heat load is continuous and should be at 1.25%

    thanks,

  2. #2
    Join Date
    Feb 2003
    Location
    Connecticut
    Posts
    3,044
    Quote Originally Posted by gmayeux View Post
    I am in a disagreement with an plan reviewer on how to properly calculate a load for an HVAC ( commercial) in doing a service or feeder calculation. the HVAC in question is a wall mount unit 3 ton with 10kw heat strip, no dehumidification. 208V 3 phase.

    Assumptions
    1. Calculate the higher of the two- heat (10KW) or compressor AC loads and use the larger of the two.
    2. Use 220.51 for the fixed heat strip at 100%
    3. If compressor load is larger follow 220.50 which will take you to 440.6 , AHJ is saying you have to use 1.25% on compressor plus total of fan motors., if you go strictly by 440.6 I do not see any indication that you use 1.25
    4. AHJ is also saying heat load is continuous and should be at 1.25%

    thanks,
    See 440.33 rather than 440.6 for the A/C load.

    I think the AHJ is wrong about the heat...424.3(B) says that fixed electric space heating shall be considered continuous load for branch circuit sizing. It says nothing about feeder/service sizing.

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