Transformer bonding

[kqresq]

In a multiple building complex, Bldg A feeds Bldg B with an underground 3 phase 480v circuit including a neutral conductor and an equipment grounding conductor to a 200 amp 480v 3 ph panel. This panel is used to supply the building lighting and a 75 kva transformer for receptacle power. The 200 amp 480v panel is connected to a concrete encased grounding electrode and the building water piping system, building steel and gas piping is bonded to this panel. The neutral is not bonded at the 200 amp panel. The 75 kva transformer is connected to the Grounding Electrode via a GEC from XO to the grounding bus in the 200 amp panel. The question that has come up is: Does the water piping, building steel and gas pipe need to be bonded in the transformer as well as the 200 amp panel that is mounted about three feet away?

 

[bennie]

Re: Transformer bonding

Draw a single line of the equipment ground circuit. Make sure there is a solidly connected fault path to the X-O.

[ May 15, 2003, 12:55 PM: Message edited by: bennie ]

 

[don_resqcapt19]

Re: Transformer bonding

The 75 kva transformer is connected to the Grounding Electrode via a GEC from XO to the grounding bus in the 200 amp panel.

What grounding electrode is XO connected to?
Don

 

[kqresq]

Re: Transformer bonding

The concrete encased electrode that serves bldg B

 

[don_resqcapt19]

Re: Transformer bonding

I see no need for any addtional bonding in this case, but I'm still not clear on the GEC from XO. Does it go directly from XO to the concrete encased electrode?
Don

 

[kqresq]

Re: Transformer bonding

The GEC runs from XO to the ground bus in the 480v panel that feeds the transformer. The ground bus is connected to the concrete encased electrode.

 

[kevin]

Re: Transformer bonding

Your installation of the GEC appears to violate 250.64(C) which requires a continuous run from XO to the grounding electrode. You have described a non-approved splice in the ground bus of the 480 volt panel.
250.30(A)(4) requires that the grounding electrode for a separately derived system be the nearest of either (1) an effectively grounded structural metal member of the structure or (2) an effectively grounded metal water pipe within 5 feet from the point of entrance into the building. Only if neither of those two exist may another type of grounding electrode, i.e., a concrete-encased electrode, be selected.
250.104(A)(4) requires bonding of water piping in the area served by the separately derived system to XO in your installation.
I am unaware of a requirement to bond gas piping [(250.104(B)] or exposed structural steel [(250.104(C)] to XO.