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AC unit in a Custom VFD Enclosure

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    AC unit in a Custom VFD Enclosure

    How do the tap rules apply to adding an AC unit to a custom VFD panel, the tap rules can get very drawn out.
    The panel would have a fused disconnect with a threw the door operator handle the tapped conductors will be under 10'.
    1. Do we need a means of disconnect for the AC unit
    2. Since we are tapping the conductors of the VFD do we need to add fusing between the distribution blocks and the VFD
    3. Which rule applies to the conductor size feeding the AC unit.

    #2
    Originally posted by Jbro1110 View Post
    How do the tap rules apply to adding an AC unit to a custom VFD panel, the tap rules can get very drawn out.
    The panel would have a fused disconnect with a threw the door operator handle the tapped conductors will be under 10'.
    1. Do we need a means of disconnect for the AC unit
    2. Since we are tapping the conductors of the VFD do we need to add fusing between the distribution blocks and the VFD
    3. Which rule applies to the conductor size feeding the AC unit.
    The tap rules are actually pretty simple.

    If you are making the tap downstream of the cabinet disconnect switch you can use the cabinet disconnect as the disconnecting means for the A/C.

    The A/C probably needs some kind of OCPD. The manual or nameplate that came with it will tell you what the branch circuit rating should be. The A/C manual or nameplate will also likely tell you what size wire to use.

    Click image for larger version

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    Bob

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      #3
      Are we assuming that the A/C unit is going to need the same voltage as the VFD then? If so, then what Bob said holds. If not, and you have a separate source coming in for the A/C unit, then that needs to be either run through an aux contact of the main disconnect (if rated for the power involved) or you do need a separate disconnect. If you have a transformer for the A/C unit, then the primary of that transformer can be tapped off of the load side of the main with separate primary fusing just for it.
      __________________________________________________ ____________________________
      Many people are shocked when they discover I am not a good electrician...

      I'm in California, ergo I am still stuck on the 2014 NEC... We'll get around to the 2017 code in around 2021.

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        #4
        Additional Considerations

        Originally posted by Jbro1110 View Post
        How do the tap rules apply to adding an AC unit to a custom VFD panel, the tap rules can get very drawn out.
        The panel would have a fused disconnect with a threw the door operator handle the tapped conductors will be under 10'.
        1. Do we need a means of disconnect for the AC unit
        2. Since we are tapping the conductors of the VFD do we need to add fusing between the distribution blocks and the VFD
        3. Which rule applies to the conductor size feeding the AC unit.
        I want to throw in the caution that control cabinet Air Conditioning is a different animal with respect to compressor and system control and if set up incorrectly will lead to very early compressor failure. The main tenet is the system needs to be loaded correctly and " not " short cycled in the least.
        This typically requires Return Air Control along with constant evaporator fan and some kind of mixed air strategy. If the surrounding ambient is fairly constant it simplifies some of the challenges.

        Star
        Microwave Radiation Dangers should be openly discussed

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