reversing transformer

[domnic]

I have a 480v delta to 120/208 3 phase transformer i want to run it in reversed where do i get the EGC on the 480 side load side?

 

[augie47]

You need to decide if you want to use your 480v secondary as an ungrounded system or a corner grounded system.
If you operate as "ungrounded" then you GEC is simply connected to the transformer frame; if grounded then it is connected to the grounded conductor.\

NEC 250.30(B) provides guidance.

Note if you run the secondary as ungrounded you need to comply with 250.21(B) for ground detection.

 

[ActionDave]

You either corner ground or go ungrounded and use detectors, 250.4 and 250.21 accordingly.

 

[domnic]

transformer reversed

transformer reversed

If i bond one leg of the 480v load side to the transformer housing and use it as a EGC the neutral of the 208v would be bonded to the housing also so the neutral of the 208 would be bonded to one leg of the 480v load?

 

[augie47]

You do NOT want to connect the neutral of the 108 system to the transformer in any manner.
(That has noting to do with the grounding)

If you elect to connect one leg of the 480 system to ground it will have no effect on
any other connections to ground as the transformer isolates the 480 system.

Might we ask what the 480v system is supplying. Certain loads such as drives don't care for ungrounded or corner grounded systems.

 

[domnic]

transformer reversed

transformer reversed

If the conduit from the 208v supply is bonded to the transformer housing then the 480v bond to the housing they would be bonded to each other ?

 

[ActionDave]

If the conduit from the 208v supply is bonded to the transformer housing then the 480v bond to the housing they would be bonded to each other ?

Yes they are and should be, look at 250.30 and to help get your head around it remember the primary and secondary windings in a transformer don't touch each other.