I was finishing up a job last week and this came up. The job was to install a 225A 208/120 panel and a 75KVA 480Y to 208Y/120 transformer . there were already 3 of these transformers located in the utility room and I was adding a forth. The installation was such that the new transformer was going to supply an existing panel and then that transformer would feed the new panel. Well the day of the plant shutdown, to switch it all around, was last Wens. and that's when I noticed the original installation had 5 wires on the primary. Four #2 and one #4. There was the #4 for the bond and three #2 for the 480 plus, and this is what I do not understand, a #2 grounded conductor from the primary on the same lug as The #4/0 that is the grounded conductor of the secondary going to the panel. I immediately called one of our state inspectors (Maine) and we discussed the situation and the way we left it is the new transformer, not having a grounded conductor from the primary, is to stay that way. The disconnected transformer is to be connected to the new panel with this primary grounded conductor wire disconnected and the other two transformers will have the grounded conductors, from the primary, removed during the next shutdown. Some added iformation is that when I checked the grounded conductors on the two still operating transformers I get 0 amps load on the 4/0 and about 10A on the #2 on one transformer and the other has 3A on the 4/0 and 5A on the #2. It should be noted the demand for 120V is small on all of these transformers and the highest reading on any secondary wire was about 15A at the time. It appears the worst case scenario would be the loss of a 4/0 grounded conductor would place the entire demand a #2 grounded conductor of another transformer. Can it be explained why anyone would wire a transformer this way and how, in laymen terms, can I best explain to the customer why I want to remove the primary grounded conductor on all of their other transformers. I hope this reads right.
