High Rise Electrical closet ground bars - Are they really needed?

[cipher31]

The following applies to a 30 story highrise building with medium voltage to low voltage substations (12kV - 480/277V or 12kV - 208/120V) at both the basement and penthouse levels.

I've been told that every electrical closet in a highrise building like the one described above should have a ground bar. The question that I have is what exactly connects to the ground bar?

Let's say that the electrical closet has a 208Y/120V busduct riser and no transformers. Is there a need for a ground bar? The busway, bus tap, distribution panel and panelboards within the closet are all grounded via the equipment grounding conductor run as part of the distribution feeders. In this case, what exactly would connect to the ground bar?

Let's say that the electrical closet has a 480Y/277V busduct riser and transformers every five floors. Since the transformers are not located on every floor, I would assume that the ground bar might only be necessary on the transformer floors for a convenient place to connect the GEC (grounding electrode conductor). Otherwise, the GEC could just connect to building steel if available (assuming it is in proper contact with earth and electrically continuous). In the case where building steel is not available, besides the GEC, what else would connect to the ground bar?

Besides for GEC connection purposes, are electrical closet ground bars needed? If GEC connections are not required within a particular closet, are ground bars a good design practice or a waste or client money?

Thanks in advance.

 

[don_resqcapt19]

This is not required by the NEC, but I see your location is listed as Chicago and it may be required by the Chicago code.
Don

 

[infinity]

If building steel is available IMO a grounding riser and bus bar combo is a waste of money. Having said that, we have installed them as per some contract drawings. We have also installed grounding bus bars in electric closets with a short length of conductor cad-welded to building steel. (IMO a more cost effective and practical installation). This was used as a convenient way to connect the transformer GEC's. What real purpose they serve is beyond me. Although it's required, will a GEC from a transformer on the 30th floor ever really need this connection?

 

[e57]

The following applies to a 30 story highrise building with medium voltage to low voltage substations (12kV - 480/277V or 12kV - 208/120V) at both the basement and penthouse levels.

I've been told that every electrical closet in a highrise building like the one described above should have a ground bar. The question that I have is what exactly connects to the ground bar?

Are you sure this ground bar is for 'electrical', and not Telco/VDV? Is ther also Telco equipment in those rooms?

250.94 Bonding for Other Systems.
An accessible means external to enclosures for connecting intersystem bonding and grounding conductors shall be provided at the service equipment and at the disconnecting means for any additional buildings or structures by at least one of the following means:
(1) Exposed nonflexible metallic raceways
(2) Exposed grounding electrode conductor
(3) Approved means for the external connection of a copper or other corrosion-resistant bonding or grounding conductor to the grounded raceway or equipment

Also EIA/TIA 607 requires something simular....
Heres an article: http://www.ecmweb.com/mag/electric_telecom_infrastructure_bonding/index.html

There is also more information here on page 98.... http://www.markhellerelectric.com/phoneclass.pdf

 

[e57]

Heres another one.... http://www.anixter.com/./AXECOM/AXEDocLib.nsf/(UnID)/FDA94AB8D8545A2C86256F88007935FE/$file/607.pdf

If it is a "specified" item on a bid, or engineered design, you should be carefull to follow it to avoid finger pointing. FYI these are not your 'normal' ground bars normally. They are often specified sizes, like 1/2"X4"X6" or larger. And conductors of 2/0~250MCM. You should check with the engineer, if thats what they are for, for full design and marking spec's.

 

[kingpb]

You say you have been told, curious as to whom told you it is required? If it was the AHJ, then you may not have a choice. You could ask for the "written" reference for this requirement, for future use. If it is not written, or published, then you will have to choose on whether or not it is worth a fight.

 

[cipher31]

Sorry. I am an electrical engineer in the building industry. Some engineers quote "Each electrical closet should have a ground bar." And this is in reference to the power side of things, not telecom. Just trying to base my design on technical reasoning instead of rules of thumb.

Thanks.

 

[e57]

Well then, (not to sound off hand) but you're the engineer, you tell us....

Anyway, 250.94 not only makes this accessible ground available for telco/catv/SAT/network that often DeMark every few floors, if not every floor, or other bonding not just at the MCOP. Having it available not just at the MCOP, when it is needed on floor 25 is a big bonus. As a design consideration, it could be available to to bond isolated piping systems, and if over-sized large enough, and installed in a method that would qualify it as a GEC in a 30 story building it would be a value added item over the life of the building that may (hopefully) outlast us all. Truthfully, if you design a system well, it should be about a week and a half before the end-user decides they need more.... Insurance.... For that matter 20 years down the line, having a GEC available could save extrordinary amounts of money by not having to find one, and prove it suitable. Especially in med-high rise. Power might be available at a nearby panel, but a GEC wont. Building steel could be encased, or fire-proofed, or unsuitable due to isolation, etc. (not all steel is "effectively grounded", even though it should be, blame that on your siesmic structural friends*) Having a marked common GEC makes it a no brainer, meaning you could add a tranformer(s) of any size up to what that GEC can handle. See 250.30. And if YOU, as the original engineer document the steel and its point of connection as a suitable GEC up front, some other engineer who has not been born yet wont have to prove it to himself to allow some un-born electrician to connect to it on th the 28th floor in the year 2100. And your stamp and name there for praise...

Whats that they say about 20/20 hindsight? Design for the future, not for the moment.

* I have a Civil Engineer buddy that can bore you to death about flexible steel connections. I brought this up with him once, and his opinion was "tough turd", he sees it as survivabilty of the structure as opposed to electrical convieniance.

 

[cipher31]

I do agree about designing for the future and whether or not the GEC is required is a matter of code. My apologies if I confused the issue but my original question regards the ground bar. Some enginees like to have a ground bar in every electrical closet. I only provide one if a GEC connection is required in that closet or floor. Just wondering what other people do and have seen.

Thanks for all the input.

 

[kingpb]

I see a lot of engineers put them into there design as standard practice, but don't really understand what they are for, and consequently they don't ever get used. It can be useful in the future, as some have pointed out, and therefore you may want to consider them. If the Owner starts to "value engineer" the project later, you can take them out.