Hi Everyone,
This week I installed a 300kVA 3 phase wye-delta 120/208-277/480 transformer in a mall to be used for about 6 months to feed a new parking garage. The transformer primary is connected to a new 600 Amp 120/208 main panel which is tapped onto the existing adjacent 2000 amp service buss. The secondary side feeds a 300 amp 277/480 fused switch, The new main panel, transformer and switch all have less than 12" between them.
I installed a new 4/0 water bond for the 600 amp main panel, ran a neutral and equipment ground to the transformer. I landed the neutrals from both the main panel and the switch to the HO buss and the equipment grounds to the transformer case. There is no bond between the HO buss and transformer case.
The inspector looked at it today and asked that the neutral and equipment ground be removed between the Main panel and the transformer, and install a second cold water bond for the transformer HO terminal and bond between the HO and the case with the equipment ground.
I understand that the inspector has asked to have this setup as a separately derived system but I opted not to go that route because:
1) The transformer is the only load on the new main panel. It will have no neutral returning to the switch with a 4 wire service.
2) The second cold water for the transformer bond will be within 10' of the main panel water bond.
3) There would be no neutral or equipment ground in the 4" seal tight between the main panel and the transformer.
From what I know both installations are correct but why would you create a separately derived system within 10' feet of the main panel?
Thanks in advance for your comments.
Mark
