First time on forum

[Travis 1]

Question
i have a 1200a 480/277v MDP fed from the power co. Transformer it is bonded to building steel, 2-3/4 cu ground rods and to the water pipe. I have 3- 400a 277/480v sub-panels fed from MDP that each feed a 75kva transformer then to a 200A 120/208v panel
i have a 4th wire for ground going to all 3 panels. This is my question. Do I need to bond my 120/208 panels to building steel? If so, why?

 

[Dennis Alwon]

If the transformer is bonded to the building steel there is no need to bond the panels that it feeds to the steel.

 

[Travis 1]

Let me clarify
if my MDP is bonded to building steel do any of my sub-panels need to be bonded to building steel even if they go through a step down transformer

 

[Dennis Alwon]

Each transformer must be connected to the grounding electrode system. When this is done then the sub panel that is fed from the transformer does not need to be connected to the building steel.

Panels fed from a MDP also do not need to be connected to the building steel. This is only req. at the service or at each separately derive system (transformers)

 

[augie47]

As noted in the diagram your transformer connection to building steel can originate at the transformer or at the panel. It should be made at the same point the system bonding jumper is installed.

 

[electricalist]

By means of the supply side bonding jumper on the xfmr secondary, you will be bonded to bldg steel. If I'm correct.

 

[Dennis Alwon]

I agree with Gus and I omitted that concept since I usually see the bonding done at the trany. If the bonding is not done at the trany then everything can be done at the first disconnect just as you would do on a service. Grounding electrode system does not need to be connected to sub panels.