Grounding of separately derived systems

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mshields

mshields

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Boston, MA
I have a high rise condo that is a concrete structure; no building steel. To ground ordinary dry type transformers in my electrical room, I've specified ground bars on isolators in each of these rooms interconnected via a 4/0 conductors. Is this the best way to go?

Thanks,
 
Separately derived source grounding

Separately derived source grounding

The bars are to be interconnected with 4/0 all the way down to the main electric room where there will be ground bar that connects to the switchboard ground and that will be interconnected with water main and triplex ground rods outside.
 
Mike,
I don't think that complies with the rules in 250.30(A)(4).
... Tap conductors shall be connected to the common grounding electrode conductor in such a manner that the common grounding electrode conductor remains without a splice or joint.
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Also this section will permit the use of 3/0.
Don
 
Mike to avoid the problem Don pointed out you could use a product like the Blackburn SP8SL

Blackburn Mechanical Connector pdf

In that Adobe file look at Catalog page 51.

In my opinion if you specify these type connectors your should specify that two are used per location a minimum distance apart.

IMO if used singly any flexing of the 4/0 will tend to loosen the nut holding it to the bar.

I was trying to find a "U"bolt type connector suitable for connecting cable to bar. I saw u-bolt connectors that would have worked great but are not listed for this purpose.
 
Don

I am not 100% sure I understand Mike's installation, but I have a thought/question.

If he installs the common GEC (in his case, the 4/0), and it is not installed all the way between the different rooms.

But it is the common GEC, so he uses a tap GEC from the bus detail to the transformer in the room the GEC is installed to and then takes another tap GEC to the second transformer in another room... is this permitted or not? If not where in the NEC does it specify that the common GEC has to be located near/within the location of all the transformers it supplies?
 
mshields said:
I have a high rise condo that is a concrete structure; no building steel. To ground ordinary dry type transformers in my electrical room, I've specified ground bars on isolators in each of these rooms interconnected via a 4/0 conductors. Is this the best way to go?

Thanks,
Click to expand...

why does it need isolation?
 
petersonra said:
why does it need isolation?
Click to expand...

It is just the standard way of mounting the bars.

Ground%20Bars,%20Insulators%20and%20Brackets.JPG
 
don_resqcapt19 said:
Mike,
I don't think that complies with the rules in 250.30(A)(4).

Also this section will permit the use of 3/0.
Don
Click to expand...


I'm wondering how you would install this conductor in say a 50 story building in one piece.
 
infinity said:
I'm wondering how you would install this conductor in say a 50 story building in one piece.
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A big spool and a puller? :grin:

One would imagine the electrical rooms will be more or less stacked.

Perhaps one common GEC for each stack?
 
iwire said:
A big spool and a puller? :grin:

One would imagine the electrical rooms will be more or less stacked.

Perhaps one common GEC for each stack?
Click to expand...


I was thinking that exothermic welding or irreversible compression connectors should be permitted as they are on a service GEC.
 
infinity said:
I was thinking that exothermic welding or irreversible compression connectors should be permitted as they are on a service GEC.
Click to expand...

I agree.

And if you look at the wording Don posted it does not say that you can not splice it. It says you can not break it at the tap location.

I really don't know the intent but in most instances I don't see it as that hard to run unbroken.
 
Pierre,
But it is the common GEC, so he uses a tap GEC from the bus detail to the transformer in the room the GEC is installed to and then takes another tap GEC to the second transformer in another room... is this permitted or not? If not where in the NEC does it specify that the common GEC has to be located near/within the location of all the transformers it supplies?
Click to expand...
The only issue that I have with Mike's installation if the fact that the 4/0 is not continuous from the grounding electrode to the end of the 4/0. There is no limit on the length or number of taps permitted.
Don
 
Tevor,
I'm wondering how you would install this conductor in say a 50 story building in one piece.
Click to expand...
While this section does not specifically permit it, I would say that it is a grounding electrode and 250.64(C) applies. Note that I do not agree that 250.64(C)(3) permits the use of bus bar connections to splice grounding electrode conductors.
Don
 
don_resqcapt19 said:
Pierre,

The only issue that I have with Mike's installation if the fact that the 4/0 is not continuous from the grounding electrode to the end of the 4/0. There is no limit on the length or number of taps permitted.
Don
Click to expand...


Based on what you have posted, the common GEC can be installed 5 feet from the water pipe, mounted to a detail as Bob has been kind enough to post, and then one could run the tap GEC the rest of the way to all the other locations.
 
No calc is needed for the common GEC between SDS, the minimum is 3/0 CU or 250 AL so no calc needed on that one. The taps can be sized based on 250.66 so that one takes care of that as well.

(a) Common Grounding Electrode Conductor Size. The common grounding electrode
conductor shall not be smaller than 3/0 AWG copper or 250 kcmil aluminum.
(b) Tap Conductor Size. Each tap conductor shall be sized in accordance with 250.66
based on the derived phase conductors of the separately derived system it serves.

(3) Connections. All tap connections to the common grounding electrode conductor
shall be made at an accessible location by one of the following methods:
(1) A listed connector.
(2) Listed connections to aluminum or copper busbars not less than 6 mm ? 50
mm ( 1/ 4 in. ? 2 in.). Where aluminum busbars are used, the installation shall
comply with 250.64(A).
(3) By the exothermic welding process.
Tap conductors shall be connected to the common grounding electrode conductor in such
a manner that the common grounding electrode conductor remains without a splice or
joint.
 
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