Hi don_resqcapt19
I understand this part "First it would be very rare to use a wye/wye transformer for this application. It would normally be a delta/wye. You would bring an equipment grounding conductor with the 480 volt feeder conductors to the transformer and the EGC would connect the the non-current carrying metallic parts of the transformer."
Can you please confirm the following.
1.So what you are saying is that the 208/120Y Panel needs its own grounding electrode conductor to the existing building ground system?
This can also be run to the nearest grounding electrode that is also use by the 480 supply such as a metal structure of the building.
2.What's the difference between the "system bonding jumper" and the "supply side bonding jumper" that you mention?
Don has covered it already but to add some more info:
This is some wording changes to the 2005-2011 in 250.28 and .30 the wording "system bonding jumper" replaced the "main bonding jumper" between the grounded (neutral) and case of the transformer, or between the grounded conductor and the grounding bar at the first disconnect which can also be the bonding screw we install in a service panel or disconnect.
2002 just use the term "bonding jumper" the 2005 used both the term "main bonding jumper" and "system bonding jumper" and in the 2008 it was changed to just "system bonding jumper"
Supply side bonding jumper is any conductor used as an EGC ahead of any overcurrent protection but on the secondary side of the transformer, it is also the conductor that is used to bond the conduits to the cases, this conductor can also be the raceway that encloses the secondary conductors to the first disconnect, it is sized from table 250.66 same as a grounding electrode conductor except a GEC stops at a 3/0 these conductor must use the 12.5% rule after this point as found in 250.66
As above for the system bonding jumper, the 2002 used the term "equipment bonding jumper" this was used all the way up to the 2011 which changed it to todays "supply side bonding jumper" which some confuse with it meaning that this is some kind of bonding jumper on the primary side, but as said above it is any bonding jumpers that are on the secondary side of the transformer but at or ahead of the first disconnect.
3. If the 480V panel and the 208V panel both have a grounding electrode conductor to the same ground electrode they are essentially bonded.
Thanks,
EE_Intern_2
Yes the grounding of the secondary is required to be bonded to the grounding of the primary, otherwise if you have circuits that are off the secondary side that fault to something grounded to the primary side it might not cause a breaker to trip and you could have a voltage potential between the two systems that someone could come into contact with, so with the primary EGC bonded to the transformer case, and the secondary X0 is also bonded to the transformer case of grounding bar, you will have an effective fault path between the two systems, that doesn't include the GEC ran to the primary systems electrode.
Also just a heads up keep in mind that when you have two different voltage systems the wires must be identified to distinguish them to each system ( 210.5(C) ) as well as the grounded conductors ( 200.6(D) )