# Where to ground and where not to ground? Please Help



## jbiancaniello (Apr 5, 2009)

Hey,

I have a unique job that I am working on and was wondering if anyone could answer the question of where ground and neutral should be tied together according to NEC. We are building a modular mechanical space that will but up against another building when completed. The power will be coming from the other building.The mechanical space has a 800 amp three phase disconnect that goes to a 480v 800 amp panel then has a floor mounted transformer that steps the 480v down to a 100 amp 3 phase 208/120 panel. So I am unclear if neutral and ground should be tied together in the other building or in the mechanical space and as well as the step down transformer. Please let me know your thoughts on this one. Thank You


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## manchestersparky (Mar 25, 2007)

Here are two links to a paper written (in 2 parts) by Ryan Jackson on the subject of transformer inspections. He tells about transformer grounding,overcurrent, sizing of grounds and such.

http://www.electricalknowledge.com/forum/topic.asp?TOPIC_ID=1371
http://www.electricalknowledge.com/forum/topic.asp?TOPIC_ID=1487


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## ESC2008 (Apr 9, 2009)

*Neutral Bonding*

The NEC specifies in 250.20(D) that all separately derived systems shall be gounded as specified in 250.30. The grounded conductor (neutral) must be grounded anywhere from the source to the first disconnect means. Your initial 480 volt service is NOT a separately derived system since it is grounded at the service. You will only need to ground the neutral on the secondary side of the new transformer and you can do that in the transformer or at the first means of disconnect.

That is my interpritation and I'm sticking with it!


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## vinster888 (May 3, 2009)

ESC2008 said:


> The NEC specifies in 250.20(D) that all separately derived systems shall be gounded as specified in 250.30. The grounded conductor (neutral) must be grounded anywhere from the source to the first disconnect means. Your initial 480 volt service is NOT a separately derived system since it is grounded at the service. You will only need to ground the neutral on the secondary side of the new transformer and you can do that in the transformer or at the first means of disconnect.
> 
> That is my interpritation and I'm sticking with it!


yeah yeah what he said ^


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## NevadaBoy (May 4, 2009)

I agree about only grounding the neutral on the secondary of XFMR. 

But something else related to your install you should read if you haven't already, read 250.32. Specifically 250.32(A) and 250.32(B)(1). This is in relation to the equipment grounding conductor and the grounding electrode your first disconnect requires. It's not exactly what you asked, but it gets screwed up all the time here. :blink:


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## brian john (Mar 11, 2007)

NevadaBoy said:


> I agree about only grounding the neutral on the secondary of XFMR.
> 
> But something else related to your install you should read if you haven't already, read 250.32. Specifically 250.32(A) and 250.32(B)(1). This is in relation to the equipment grounding conductor and the grounding electrode your first disconnect requires. It's not exactly what you asked, but it gets screwed up all the time here. :blink:


The modular structure with the 800 amp service will need to be 3 phase 4-wire with a EGC (5 wire) that is 3-phase/ungrounded conductors one neutral/grounded conductor and one Equipment grounding conductor (it does not need to be copper it can be the conduit). You need to drive a grounding electrode and connect it to the EGC in the modular. 

I would assume you have GFP upstream of the 800 amp on the main service, DO NOT BOND the neutral/grounded conductor as this will affect the operation of the GFP.


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## NevadaBoy (May 4, 2009)

brian john said:


> The modular structure with the 800 amp service will need to be 3 phase 4-wire with a EGC (5 wire) that is 3-phase/ungrounded conductors one neutral/grounded conductor and one Equipment grounding conductor (it does not need to be copper it can be the conduit). You need to drive a grounding electrode and connect it to the EGC in the modular.
> 
> I would assume you have GFP upstream of the 800 amp on the main service, DO NOT BOND the neutral/grounded conductor as this will affect the operation of the GFP.


What he said.


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