I'm feeding a 480 volt, 3 ph, 3 wire piece of equipment from a 208 volt primary by reverse feeding a standard 480-208/120 volt dry transformer with 3 ph, 3 wire 208. The inspector wants a ground potential on the secondary side and has requested either a ground monitor or a 480 volt wye secondary transformer. Any reason why I shouldn't be able to sell him on corner tapping the 480 as we would a 480 to 240 delta/delta transformer? Appreciate the input.
Reverse fed xfmr, 480 sec:bonding required?
- Thread starter kerajam
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winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Any switch gear, OCPD, etc. on the secondary side would need to be rated for 480V, not 'slash' rated for 480Y/277V.
Some equipment has ground referenced internals, eg. surge suppression MOVs. You would need to confirm with the manufacturer of the equipment that it can safely be applied to a corner ground system.
But with suitable hardware and if the equipment can tolerate it, corner ground delta is code compliant.
You might also consider a small 'zig-zag' grounding transformer to derive a neutral on the secondary side, especially if you already have the transformer and equipment in place.
-Jon
Some equipment has ground referenced internals, eg. surge suppression MOVs. You would need to confirm with the manufacturer of the equipment that it can safely be applied to a corner ground system.
But with suitable hardware and if the equipment can tolerate it, corner ground delta is code compliant.
You might also consider a small 'zig-zag' grounding transformer to derive a neutral on the secondary side, especially if you already have the transformer and equipment in place.
-Jon
brian john
Senior Member
- Location
- Leesburg, VA
It is SOOOOOOOO easy to get a 208 to 480/277 Delta/Wye. Why do this "BACKWARDS FEEDING" ? Yes you can do it, but as I have said in this forum before, the number of service calls we receive is directly proportional to the number of "BACKWARDS FED" transformers installed in our area. At least 3-4 times a year we get this call. It freaks out the average electrician, building engineer, facility service personel, ECT...
Avoid problems think of the next guy...makes it easier for the profession in general.
The 480 to 208Y transformers are in stock commodity items and the 208 to 480Y are special order at three times the price, which adds up on a 150 KVA transformer. I was trying to come up with a possible alternative which was more affordable. I agree on the "freak out" component. The inspector has suggested using the reverse fed transformer and installing a ground monitor.
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