C.T. ENCLOSURE GROUNDING

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pete grosso

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I recently designed a 400 ampere service entrance using a main breaker panel as my main service switch (typically, I would locate a fused disconnect on building exterior, but elected to save the owner a few bucks by utilizing the main breaker for the main switch, since the main breaker panel was only a few feet from an exit door). My question is this: since the service ground will be established at the site of the service switch (main breaker panel), am I required to carry a ground through to my C.T. enclosure located on the building exterior to act as an equipment ground for the enclosure itself? If so, should this ground be sized from table 250.122 (equip. gnd) or table 250.66 (grounding electrode conductor)?
 
Re: C.T. ENCLOSURE GROUNDING

I was made to size my egc for the ct enclosure as per 250.66.This was the POCO`S take on this 800 amp service.1 phase residential service.From main,to feed through ct`s to trough where 6 disc were mounted.

4 200`s for house
1 100 for garage
1 100 for l/s/l
Since nobody bitched all agreed and away we went IIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
 
Re: C.T. ENCLOSURE GROUNDING

The CT cabinet will be grounded by the grounded service conductor terminated within it.

If you run any conductor based on 250.66 or 250.122 between the CT cabinet and the service disconnect all you will have done is provide a parallel path for imbalanced neutral current.
 
Re: C.T. ENCLOSURE GROUNDING

Originally posted by raider1:
Check out 250.102 (C), also you might want to check out 250.92 (A) and (B).

Chris
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If you where to run a bonding jumper those would be the sections. :)

In this case I recommend taking advantage of 250.142(A).
 
Re: C.T. ENCLOSURE GROUNDING

Originally posted by iwire:
The CT cabinet will be grounded by the grounded service conductor terminated within it.

If you run any conductor based on 250.66 or 250.122 between the CT cabinet and the service disconnect all you will have done is provide a parallel path for imbalanced neutral current.
Click to expand...
I agree with Bob. Around here we are required to bond the neutral in the CT cabinet by the POCO. If metallic conduit is used it will have the same effect as the conductor Bob mentioned as a parallel current path.

[ November 23, 2005, 03:39 PM: Message edited by: infinity ]
 
Re: C.T. ENCLOSURE GROUNDING

Originally posted by infinity:
If metallic conduit is used it will have the same effect as the conductor Bob mentioned as a parallel current path.
Click to expand...
I agree, about the only difference is that the metal raceway may be needed for physical protection, the bonding conductor serves no purpose whatsoever.
 
Re: C.T. ENCLOSURE GROUNDING

I agree that the grounded service conductor is the best way to bond a CT enclosure. The POCO in my area will not let you bond the grounded conductor at the CT can. The POCO makes us run a bonding jumper or use the conduit for bonding.

Iwire I had in mind the same thing just didn't site the right code section (got in a hurry) sorry. :eek:

Chris
 
Re: C.T. ENCLOSURE GROUNDING

Originally posted by raider1:
The POCO in my area will not let you bond the grounded conductor at the CT can.
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Chris how do you 'not bond' the meter or CT enclosure to the grounded conductor? :confused:

I have never seen a CT or meter enclosure that did not have the grounded conductor terminal factory bolted directly to the enclosure.
 
Re: C.T. ENCLOSURE GROUNDING

Here is a link to the enclosures I am talking about http://www.b-line.com/whatsnew/wn_176.asp

When we have a CT enclosure that is installed before the main disconnect, then the POCO won't let us bond the grounded conductor to the can, we must use an isolation bus for the grounded conductor. They only want the grounded conductor to be bonded at the main disconnect.

We have argued this with them for a while and still have not been able to persuade them that bonding on the line side of the service with the grounded conductor is NEC compliant.

What I am refering to is for commercial installation that have a CT enclosure that is seperate from the service disconnect.

(edited to add last paragraph)

Chris

[ November 23, 2005, 05:17 PM: Message edited by: raider1 ]
 
Re: C.T. ENCLOSURE GROUNDING

Let me confirm what Chris is saying and add a bit to it. The power company here will not allow these separate CT enclosures to be bonded to the grounded service conductor. We insist on a bonding jumper from the service disconnect back to the CT enclosure because we can't see another way to bond the CT enclosure. We have argued with the power company that bonding to the grounded conductor is the preferable way. These seperate CT enclosures can be 150' or more from the service disconnect. When we pointed out to them that using a bonding jumper added greatly to the length of the fault clearing path, they suggested that a ground rod attached to the CT enclosure would suffice :eek:
 
Re: C.T. ENCLOSURE GROUNDING

Originally posted by raider1:
What I am refering to is for commercial installation that have a CT enclosure that is seperate from the service disconnect.
Click to expand...
Yes I understood that, here that CT cabinet would arrive with a factory bonded grounded conductor terminal with no way of 'un' bonding it.

I don't think the power companies in this area would allow an isolated connection.
 
Re: C.T. ENCLOSURE GROUNDING

since the main breaker panel was only a few feet from an exit door).
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that terminology interests me....why is this pertinent?
reminds me of an inspector we had in this area that enforced 230.70 to mean the disconnect had to be nearest the point of entrance to the building (not sure if the wording of 230.70 ever opened the door to that ruling..but it did for him)
 
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