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GEC for XFMR secondary? Neutral for derived service disco?

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    GEC for XFMR secondary? Neutral for derived service disco?

    I'm working in a building that splits a 2-phase, 5wire service into (2) 1-phase, 3wire, 240v services using transformers in the end-box and with the (2) 1-phase wire pairs running through (6) fused disconnects. These two, '3wire' services share a common neutral.

    ** The Question(s):
    ** The 2-ph 5w also runs to a big transformer that appears to power a grouping of 3-phase HVAC equipment on another floor. The only grounding on either side of the transformer is via RMC conduit tapping the 4 hots from out of the bonded switch cabinets and troughs of the other service disconnects. Starting from the remotest branch of this system:

    ** 1) can the RMC raceway feeding 3-phase 3wire (no neutral conductor) from the xfmr's secondaries to the HVAC equipment, serve as its equipment grounding conductor?

    ** 2) does the quad-fused disconnect interrupting 4 hots of the 2-phase service, qualify as the only point at which the transformer system needs a direct grounding electrode connection? (since the xfmr is separated from the service disconnect by a wall, there is a safety disconnect in the room with the transformer, between it and the service disconnect; only RMC raceway interconnects grounding path between all of these components: fused disco to unfused safety disco to xfmr).

    ** 3) I may want to insert a 3-ph loadcenter either on the load side of the transformer, or in the HVAC equipment room at the other end of the feeder (in RMC), on the load side of the xfmr. In the latter case, all loads would have to be line-to-line (no neutral), or else I would have to squeeze in a new neutral conductor (possibly after downsizing the existing HVAC loads and the feeder hot conductors) and somehow route it through the transformer (unless the xfmr has grounded terminals?). Does the transformer's line-side service disconnect qualify as the disconnect (and GEC connection point) for this entire system, or does the first disconnect on the line side of the xfmr constitute a service that needs a GEC (and neutral?)? Are there rules about how close the disconnect(s) on the transformer secondary must be to the xfmr?

    ** 4) Is it permitted to feed a 3ph distribution panel that has no grounded conductor (only equipment grounding, via RMC)? I could probably squeeze an insulated grounding conductor into the existing RMC feeder raceway without having to downsize the hots, if RMC doesn't cut it for grounding a remote distribution panel.

    5) My additional 3-ph loads would be refrigeration equipment for a community grocery. Maybe better than a new loadcenter would be to upsize the trough in the HVAC room, and just tap as many disconnects as I need equipment. Still, my intuition tells me there ought to be more connecting these loads to earth than some RMC, the walls of an xfmr box, and some more RMC (the RMC fitting connections at the cabinets and troughs being my concern, not the pipe itself).

    #2
    See this thread for all you want to know about doing what you want:
    http://forums.mikeholt.com/showthrea...hlight=2+phase

    The 2 phase transformer to 3-phase is called a Scott-T transformer and the link above has some wiring diagrams of it.
    Wayne A. From: N.W.Indiana
    Be Fair, Be Safe
    Just don't be fairly safe

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      #3
      Scott-T @ wikipedia doesn't describe grounding, only conductors.

      The Scott-T info gets me the right voltages for loads and conductors (although I'm new bough to transformers I can't tell if I answers my questions about neutral conductor connections). I really want to know about equipment grounding and system grounding, neither of which is discussed on the thread.

      By the way, EC&m magazine's most recent issue had a feature on xfmr grounding and bonding, that left me more confused than I had been before.

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