Transformer grounding

[electricrw]

Grounding arrangement for a separately derived system in witch the grounding electrode conductor connection is made at the first disconnecting means or panel board.
After said that I am having a ruff time sizing the grounded conductor coming from the transformer XO to the nutral bus where everything is bonded together. I think you size the grounded conductor from the largest ungrounded service conductor and size from table 250.66 and if ungrounded service conductor is larger than 1100kcmil than multiply it by 12.5%. Is this correct and or is there more to the answer. based on the 05 code. thank you for the help

 

[crossman]

My take:

First, you must do a load calculation based on Part III of Article 240 and come up with the required ampacity for the grounded conductor.

Then you must compare the above computed ampacity/wire size with that required by 250.30(A)(8) which sends us to 250.66. If the phase conductors are greater than 1100 kcmil, then we have to use 12.5 percent of the phase conductors.

Now, choose whichever of the above is the largest.

Even though 250.30(A)(8) does not specifically tell us to use 220 for calculating the grounded conductor load, notice that we are dealing with a feeder and the rules for calculating feeder wire size will still apply.

That's the way I see it!

 

[electricrw]

transformer grounding

transformer grounding

Thanks for the last answer. I have another question
Given, grounding arrangement for a separately derived system in witch the grounding electrode conductor connection is made at the transformer. Now I am bonding at the transformer. I want to send a grounded conductor from the transformer to nutral bus in the panel and a system bonding jumper form the transformer to the grounding bus in the panel. How do i size my nutral and how do I size my system bonding conductor.

 

[augie47]

I would think your bonding wire would still be sized per 250.102(C) and your neutral per 220.61

 

[quogueelectric]

Grounding arrangement for a separately derived system in witch the grounding electrode conductor connection is made at the first disconnecting means or panel board.
After said that I am having a ruff time sizing the grounded conductor coming from the transformer XO to the nutral bus where everything is bonded together. I think you size the grounded conductor from the largest ungrounded service conductor and size from table 250.66 and if ungrounded service conductor is larger than 1100kcmil than multiply it by 12.5%. Is this correct and or is there more to the answer. based on the 05 code. thank you for the help

Let me get this straight. You are going to "wing it " with a 1200 amp 480 volt xformer feed without any supporting documentation by say an architect or engineer in the form of blueprints?