transformer bonding

[jmshiller]

My question concerns the proper bonding of a 480 volt transformer and a separately derived system: We have installed a 480 30 KVA Xmer feeding a 120/208 volt MB panelboard. The bonding occured at the transformer, building steel to case to XO. Therefore I believe the MB panelboard neutral should float. However the inspector is asking that because its a separately derived system it needs a proper sized bonding jumper, neutral to case. Any help with be greatly appreciated.

 

[augie47]

It does need a proper size bonding jumper however you need to make note of Exception 2 in 250.30(A)(1).
If you have a system bond at the transformer, you should not bond at the panel if it established a parallel path (such as thru a metallic conduit). You do need a bonding conductor from your transformer to your panel grounding terminal.

 

[don_resqcapt19]

If the bonding is at the transfomer, then you need to install a supply side bonding jumper between the transformer and the panel. This bonding jumper will connect to the panel and equipment grounding bus at the panel.