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Power supply to large HVAC Air Handlers

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    Power supply to large HVAC Air Handlers

    We sell large air handling equipment.
    the unit have Ul labeled control panels
    and the entire unit has an ETL listed.

    typically there is a single point 480 volt
    power connection and a separate 120 volt power connection for
    convenience outlets and lighting. This allows service
    people to have plugs and lighting while they work on the equipment with the 480 power lovked
    off.

    The is fed into a generic 100 amp max load center with two or three 20 amp breakers
    for the lighting and plugs. Plugs are GFCI.

    What size wire and 120 volt breaker has to feed this from the building panel?
    Two duplex plugs on one twenty amp breaker
    minimal amperage drawn on lighting 10 amps or less.

    #2
    Sadly, your description is neither complete nor clear. If you have a 100 amp load center, it will not be served at 120 volts. It will either have 120/240 volt single phase or 120/208 volt three phase, depending on what is available from the building's electrical distribution system. Even though that load center may be rated for 100 amps, there is no requirement to bring 100 amps of capacity to it (i.e., the ratings of the upstream breaker and the intervening wires). That breaker and those wires will be based on the total load, which you have not given us.

    I am curious as to why you are asking these questions. If you sell the equipment, it will be some other company (i.e., the electrical contractor) who will be installing it. There may also be an engineering company involved in the overall building design. These people will be making the decisions on the breaker and wire sizes. What then is your intended role here?
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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      #3
      Not sure what you are asking. Is there a feeder that you need to run to this panel that is 120/208? If so then it could be 20 amp or more depending on what the panel may be used for later. 20 amp would be the smallest size in a commercial building although 15 amp may be allowed.

      I am not quite clear of the setup
      They say I shot a man named Gray and took his wife to Italy
      She inherited a million bucks and when she died it came to me
      I can't help it if I'm lucky

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        #4
        Originally posted by Dchaisson View Post
        The is fed into a generic 100 amp max load center with two or three 20 amp breakers
        for the lighting and plugs. Plugs are GFCI.

        What size wire and 120 volt breaker has to feed this from the building panel?
        Two duplex plugs on one twenty amp breaker
        minimal amperage drawn on lighting 10 amps or less.
        It's really a design issue, you said that the 100 amp panel has 2 or 3 circuits so it really depends on what you anticipate the load to be. A single 20 amp circuit is probably all that you'll need for the two receptacles and the lighting. Typically we feed units like these with a single 120 volt, 20 amp circuit for lights and power. The 100 amp panel is overkill.
        Rob

        Moderator

        All responses based on the 2017 NEC unless otherwise noted

        Comment


          #5
          Originally posted by infinity View Post
          It's really a design issue, you said that the 100 amp panel has 2 or 3 circuits so it really depends on what you anticipate the load to be. A single 20 amp circuit is probably all that you'll need for the two receptacles and the lighting. Typically we feed units like these with a single 120 volt, 20 amp circuit for lights and power. The 100 amp panel is overkill.
          I mostly agree here.

          What load is expected to be supplied by the receptacle circuits? Good chance a single 120 volt 20 amp supply never gets overloaded if it is only used for servicing this equipment.

          100 amp panel is overkill, but unless you only go with say a two space loadcenter you probably don't find many rated less than 100 amps. QO has a 30 space indoor and 40 space 3R version with only two spaces, and I believe a three pole 60 amp three phase version, most everything else they have jumps to 100 amp main lugs/bus.
          I live for today, I'm just a day behind.

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