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Thread: Generator sizing and Heat Pumps

  1. #1
    Join Date
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    Generator sizing and Heat Pumps

    I need to size a generator. Here are a couple of questions. The generator literature refers to AC units as being X number of Tons. Looking at the name plate of the outdoor unit I see no reference to TONS.
    Also, looking at the name plate, what figure states the actual usage in watts or amps?
    What I see is minimum circuit ampacity, min and maximum breaker etc.

  2. #2
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    Greg, RLA (Running Load Amps) for running, but starting current must be considered
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  3. #3
    Join Date
    Sep 2004
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    The tons is usually buried in the model number. A ton is 12,000 BTU. So a 2 ton unit will probably have an 024 in the model number. A 4 ton would probably have 048 in the model number, and so on.
    Mark
    Kent, WA

  4. #4
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    Quote Originally Posted by Greg1707 View Post
    I need to size a generator. Here are a couple of questions. The generator literature refers to AC units as being X number of Tons. Looking at the name plate of the outdoor unit I see no reference to TONS.
    Also, looking at the name plate, what figure states the actual usage in watts or amps?
    What I see is minimum circuit ampacity, min and maximum breaker etc.
    It's sometimes difficult to get running load out of HVAC manufactures. Your right, they give you a minimum circuit ampacity and that's it until the manuals are available. Even then it takes carefull reading to determine the running load. .

    I use the SEER rating to get close.


    SEER stands for Season Energy Efficiency Ratio. SEER can be from 5-13 I use 12 when I don't know

    SEER is defined as the cooling capacity, measured in BTU/Hr, of a continuously operating air conditioner by the electric power input, measured in WATTs.

    I Ton of AC= 12,000 BTUs/HR

    SEER= (BTU/hr)/Watts (definition of SEER )

    OR Watts=( BTU/hr)/SEER

    Or KVA= (BTUs/Hr)/(SEER*1000)*0.8 Assuming 80% Power factor.

    Comments and correction to this are welcome. I have ask mechinical engineers to check this but I don't think they did in great detail. :-?

  5. #5
    When I size generators for A/C loads I do it by the RLA printed on the nameplate. For starting current, I make sure that the LRA rating will not cause a 30% dip in voltage (for residential, 15% commercial).

    The manufacturer of the generator generally provides its dealers with the max starting capabilities of it's generators. I have the tables for Generac if that's what you're using. Just for instance, the 14kW unit can only handle a maximum spike of 102A without a 30% drop in voltage. Soft start equipment is a good idea if the starting current is what's making your generator size very large.

    I can also sell you generators tax free if you live out of state

  6. #6
    Join Date
    May 2005
    Location
    Richmond, Virginia
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    21,838
    Quote Originally Posted by IllinoisElectrician View Post
    I have the tables for Generac if that's what you're using.
    That would be great info to have.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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