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    DC Bus Conductor Sizing

    I have a multiple VFDs set up powered from a common DC bus. Each has DC branch fuse protection (though not required in UL508A 31.3.2 exception#2). How would you size these conductors that connect the DC bus to the VFD? 430.122 mentions 125% of drive input (assuming AC input). The drives have rated drive input currents in AC, but not input into the DC bus. Would you calculate off 430.250, or nameplate and convert to DC, or off the DC fuse size?

    #2
    I don't have an answer about sizing but I do have one caution: Be sure the fuse and insulation are rated for the DC voltage.
    A full-wave rectifier with 480-volt input will have a 630-volt output, and many fuses have a DC rating that's significantly less than their AC rating -- or no DC rating at all.

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      #3
      Thank you, yes, we have 700VAC/DC rated fuses.

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        #4
        Originally posted by aNerdyEngineer View Post
        I have a multiple VFDs set up powered from a common DC bus. Each has DC branch fuse protection (though not required in UL508A 31.3.2 exception#2). How would you size these conductors that connect the DC bus to the VFD? 430.122 mentions 125% of drive input (assuming AC input). The drives have rated drive input currents in AC, but not input into the DC bus. Would you calculate off 430.250, or nameplate and convert to DC, or off the DC fuse size?
        First off, I would not add fuses where they are not needed, or required. It just adds complexity and someplace where something can fail. No reason to add a point of failure where it is not required.

        If you insist on adding fuses that don't really serve any purpose, I would be inclined to size the conductors off the DC fuse rating.

        It would make no sense to size them from 430.250 since that is for 3 phase current which will not be the same as the DC current.
        Bob

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          #5
          Originally posted by petersonra View Post
          First off, I would not add fuses where they are not needed, or required.
          Thanks. I somewhat agree. Due in part that on some of these smaller drives troubleshooting personnel like them to be able to take a drive out simply be removing easy-ejecting fuses. Do you know if NEC has any requirements or exceptions to the need for DC bus fusing?

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            #6
            Originally posted by aNerdyEngineer View Post
            Thanks. I somewhat agree. Due in part that on some of these smaller drives troubleshooting personnel like them to be able to take a drive out simply be removing easy-ejecting fuses. Do you know if NEC has any requirements or exceptions to the need for DC bus fusing?
            I don't recall the NEC actually requires any OCPD on the DC side of a VFD at all. It is required on the AC side of the DC power supply, but not on the DC side.

            I can see why they might want them, however I would point out that fuse blocks are generally not make/break rated.
            Bob

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              #7
              If the VFD is not running when you disconnect the DC buss, there will be little or no current to disconnect.
              But the filter capacitors inside the VFD will be charged, whether or not it was running recently, so be sure to have a plan for discharging them.

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