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A/C Loads and generators

Motors on start up can supposedly reach 3 times their amp. rating for a short time. I would think that if that happened the fuse would blow if the generator was not capable of handling the start load. Capacitor should help with the start up load. Whatever amp breaker you are coming off for the units main feed would be that same amp. supply I would use from the generator or I would guess that anything less could give you some problems which would probably be continuos tripping..............Dan


  • TGO_54
    TGO_54 Member Posts: 327
    A/C Loads and generators

    When sizing a gererator for use with a CAC system, how do you figure for the draw on start? Are A/C componets likley to be damaged from being powered by a generator?

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  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380


    Sorry it took so long for me to respond, but I was looking for a magical method for sizing a generator and was unable to find one.

    As you very well know, emergency generators are rated in power (watts) and are typically not intended to power the entire structure in the event of a power failure (unless you have a really big generator).

    The power that is used (consumed) by a central air conditioning system can be found by multiplying the supply voltage by the amperage draw of the equipment times the power factor:

    Power = Voltage x Current x Power Factor

    This is from our old friend OHM, who is now, as Dan says, is on the other side of the lawn. Since the power factor is diifcult to calculate, you can use a watt-meter to determine the actual power consumed.

    To err on the side of caution, you can ue 0.75 or 0.8 as the power factor when determining the power requirements of the generator for the ac equipment.

    Example: You have a central air conditioning system that is connected to a 220-volt power supply and draws 20 amps.

    The maximum power is 220-volts x 20 = 4400 watts

    The power used for calculation is (0.8) x 4400 = 3520 watts.

    The power factor takes the shift betwen the voltage and current into account, as maximum power (voltage x current) is not realized since the voltage and current are not in phase with each other.

    Defintely check the specs on the generator as loads draw Locked Rotor Ampeage (PLRA) on startup, which is in the range of 5 to 7 time higher than the running load amps of the device. So a motor that draw 10 amps when up and running, will draw anywhere between 50 and 70 amps on startup. Of course, this is only for a short period of time.

    Keep us posted with what you come up with on this project.

  • TGO_54
    TGO_54 Member Posts: 327
    Thanks Eugene

    The info you provided will be quite helpfull. It seems that people are considering generators that will allow then to operate their houses the same as if the utility were on line. Will compressors be damaged by starting in a low voltage/amperage situation? The draw on start is what concerns me - as you mentioned it is substantionaly higher than the running load.



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