sub buildings and transformer bonding

[rjdad82]

I am feeding a secondary building with a 480/277 volt panel fed from my main panel in the main bld. In the sub bld. there is also a sealed step down transformer 480/277to 120/240v that is feeding a load center panel for misc lights and outlets. I know by code I have driven a rod electrode outside the building and tied it into the 480v panel and also bonded the steel of the building with the same system and also pulled a grounding wire in the raceway with the feeders along with a neutral from the main bld panel and did not bond the neutral at the 480v panel. how do I tie the transformer in to the bonding system without creating a parallell installation or am i looking into this to much???

 

[E.C.]

Man I swear I wrote on this already. rjdad82 from reading the forums i surmise the following. The 480/277 panel with separate egc is a sub and should not be tied to the ground rod. Unless there is no metallic connection to the main building including shielded telecom cables. In that case the 480/277 becomes the main for that building. Otherwise it is a sub panel an the 120/240 panel becomes a separately derived system with the NG bond to be formed in the new 120/240 panel and the 277/480 would have to have a Isolated neutral as a sub panel. I prbably missed something but I just wanted to respond. Read the posts. It's all there.

 

[RUWired]

. how do I tie the transformer in to the bonding system without creating a parallell installation or am i looking into this to much???

Are you concerned about creating a parallel ground path with the steel and the ground wire or do you think you will be creating a parallel neutral path? Creating parallel ground paths is ok and not a concern. Creating a parallel neutral path will not happen, the neutral currents will stop at the secondary of the transformer.
Rick

 

[tryinghard]

The rule is do not bond the grounded conductor (neutral, return) anywhere past the service disconnect 250-24(A)(5) & 250-142(B).

In your case you need to go with 250-32(B)(1) or (2), if you’re application meets all the criteria of 250-32(B)(2) you only need a 4W feeder - 3 phase conductors and 1 equipment ground conductor, careful though you cannot have any metallic paths between structures because this would cause a parallel neutral return path and this includes water/air/cable… If you cannot meet all this criteria you have to install a 5W feeder – 3 phase conductors, 1 grounded neutral conductor and 1 equipment ground conductor – and do not bond the neutral.

All of your electrodes must connect and your sub building - along with its transformer - will bond through the EGC

 

[RUWired]

In your case you need to go with 250-32(B)(1) or (2),

Just for a 2008 update, There is no longer a b-2 option. Only an exception to existing buildings.
Rick

 

[tryinghard]

Just for a 2008 update, There is no longer a b-2 option. Only an exception to existing buildings.
Rick

Kool! :grin: Actually I'm glad to see this, it will help eliminate paralleling returning current from 250-32(B)(2) being miss-applied.

 

[tryinghard]

...also a sealed step down transformer 480/277to 120/240v that is feeding a load center panel for misc lights and outlets...how do I tie the transformer in to the bonding system without creating a parallell installation or am i looking into this to much???

Your transformer is a separately derived system (no physical connection between high and low side) and it is a grounded system (needs a neutral for secondary feeder circuit). Because it is a separately derived system you'll need to re-establish a ground fault path - shorts (ground faults) are only trying to return to their source not to earth.

You can bond the neutral at the transformer XO (most common) or at the first disconnect but the bond must take place where the electrode conductor exists, see 250-30. The electrode for your transformer is usually building frame 250-30(4) but it sounds like your case you may be driving a rod either way the frame or rods must all connect.