I have a customer who is currently using a 120/208 3ph, 4w system. They have outgrown the service and we are installing a new 277/480 3ph, 4w system in a new addition. We will be feeding the old service via a 480 to 120/208 xfmr and have pulled a EGC with the 480 conductors. Can that ECG be used to connect the XO in order to derive the neutral? OR Would it be better to use the existing ground rods from the former service? OR Both? Exhibits 250.13 and 250.14 in the Handbook seem to apply to my situation (nearest GE is rods, due to wood structure) . PVC conduit on both sides of xfmr.
step down xfmr ground ?'s
- Thread starter sgl
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The answer would be (probably) Neither, not Both.
The stepdown transformer creates a separately derived system which must be grounded in accordance with 250.30. 250.30(7) says the grounding electrode for the separately derived system shall be the nearest of either: Metal water pipe grounding electrode... or Structural Metal grounding electrode.
If the metal water pipe or structural metal grounding electrodes do not exist, then you could look at using other electrodes listed in 250.52(A), such as ground rods.
The water bond is near (as are the grd rods) the xfmr location since it is currently used for the old (existing) service. Since it is impossible to get all the way to the water main due to building finish, our inspector is allowing us to bring a 2/0 conductor to this service, FROM the new service, REMOVE the existing water bond from the 120/208 panel, and attach the two together with a non-reversible crimp. So should we attach another wire to that water bond conductor and take that to the XO? (For info, the existing water bond is a 2/0 and my EGC from the 480 is a #6.) Then, would we just use the 480 EGC for transformer case bonding while the water bond would actually be my XO's ground reference? But in effect, all are going to be connected at the new 480 V service. I have always heard of the dangers of ground loops, but my theory knowledge on them is weak. Could we get in that situation?
dl, in re-reading your reply, did you mean to say "Either, not Both" rather than "Neither, not Both" ?
dl, in re-reading your reply, did you mean to say "Either, not Both" rather than "Neither, not Both" ?
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