Misleading Ground Wire in Datacenter

Status
Not open for further replies.
We have an existing 800A feeder (2) 4"C EMT 3 500kCM & 1#6G all CU paralleled 2 per phase and ground on the secondary of a 225kVA transformer 480-120/208Y. There is 400A CB on the primary and an 800A CB downstream, but obviously the issue is an undersized ground wire. My firm is responsible for recommending a solution to this.

My initial response was that the ground wire needed to be at least 1/0 per 110.122, to pull out the #6 and install the 1/O. Someone else on my team suggested that the ground was not required as the metallic conduit could act as a ground. We agreed this was true per code provided the conduit was installed with appropriate grounding bushings and jumpers. I added that this was not a good idea as this was a large datacenter where as far as we knew every other conduit had a ground wire providing the ground path - my thinking was that someone working on the system would not expect the conduit to be the ground path, especially when there was a green insulated wire present, which I argued would be misleading and therefore unsafe. And I added that if the conduit were to be the path the #6 wire should be removed or at least be disconnected and taped off, for example with black tape.

And that was the formal recommendation we made, with some disagreement in-house remaining as to whether the #6 ground could remain. My question is is there anything in code that allows the #6 green insulated wire to remain when the ground path is (verified to be) the metallic conduit?

All comments appreciated ... on this particular (and all) matters I would like to be right about my point of view! I relate to it like in a fire alarm system where you cannot have a device like a smoke or a pull station or any other device that does not work as it is supposed to.
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
I don't know if you must pull the #6 out but I am sure you cannot hook it up to anything. If it were me I would pull it out or at least cut it off flush with the pipe.

Also, as long as you have proper fittings on the conduit you don't need any special jumpers or bushings.
 

__dan

Senior Member
I would not pull the #6 out. I would be afraid of damaging the other conductors by pulling one and not the others.

I would absolutely leave the #6 connected. In the scenario given where the EMT is the code required grounding path, (for code) I would see the #6 as an unintended and additional grounding/bonding path that is too small to meet code but not prohibited (and certainly beneficial in its own way). The #6 would be protected from overheating and damage from fault currents by the EMT, while sharing its duty.

Perhaps signage and tagging would be advisable where the #6 could be taped green and warning tagged / labeled as too small to meet code for fault current grounding, as a warning to anyone who would otherwise expect that such a wire meets the normal industry standards and expectations.
 

ron

Senior Member
We have an existing 800A feeder (2) 4"C EMT 3 500kCM & 1#6G all CU paralleled 2 per phase and ground on the secondary of a 225kVA transformer 480-120/208Y. There is 400A CB on the primary and an 800A CB downstream, but obviously the issue is an undersized ground wire. My firm is responsible for recommending a solution to this.

My initial response was that the ground wire needed to be at least 1/0 per 110.122, to pull out the #6 and install the 1/O.

I agree the conductor you consider the EGC is undersized and should be 1/0AWG CU in each pipe. For clarity though, this "EGC" for the 800A feeders are located between the transformer secondary and the first overcurrent device on the secondary? If so, it is not sized based on 250.122 because it isn't really an EGC. It is a supply side bonding jumper sized from 250.66. http://www.jade1.com/jadecc/courses/UNIVERSAL/NEC05.php?imDif=13

Actually, the supply side bonding jumper can be sized based on the individual conductors in the pipe, rather than be fully sized per the largest ungrounded secondary conductor, since 250.30(A)(2)(a) sends you to 250.102(C).

So after all that, it still should be 1/0 CU (as you said) because of the 500kcmil ungrounded conductor in the pipe and 250.66 (instead of 250.122).
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I was going to say the same thing as Ron did about the supply side bonding jumper so I will move on to another issue.
The 500 kcmil conductors between the transformer and the 800 amp OCPD are too small. The rule in 240.4(B) that permits rounding up to the next higher rated OCPD does not apply to transformer secondary conductors.
240.21(C) Transformer Secondary Conductors. A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.

As far as the existing green wire, I don't see a need to remove it, after it is disconnected.

A couple of questions? Are there no line to neutral loads, and where is the grounding electrode conductor connected?
 
Yes this "EGC" is from between the transformer secondary and the first overcurrent device on the secondary. I have wrongly been considering the grounding conductor from the separately derived system transformer to the building electrode to be per 250.66, and all others to be 250.122.

And yes I now see this is a Supply Side Bonding jumper (which cannot be flexible metallic conduit) and when paralleled is sized on the wiring in each conduit and not the overcurrent device as you just wrote -1/O either way.

I just spoke to nfpa advisory and they confirmed what both of you wrote, something I thought I needed to do as this is news to me! Derrick raised the question where was the XO bonded. I believe at the transformer. He did not see anything in code to prohibit the #6 staying as long as it was terminated at both ends, it would be a sort of optional bond is how I followed the conversation? But he agreed that he thinks anyone looking at the installation would expect the #6 to be the required ground, in this case a supply side bonding jumper.

I think I would be remiss not to either remove the #6 (it is a short run, should not have had the opportunity to be twisted with other conductors), to cut and tape it off, or perhaps best as Dan suggests to provide signage.

Thank you for the education here. I will definitely have to visit more often.
 
Don, thank you for the reference to not using the next higher OC device rating over the wire size. This situation is actually in duplicate both are 225kVA transformers but one downstream is 800AT and the other 700AT. So I see one complies and one does not, and we can adjust the high breaker down.

There are no neutral loads or neutral. The ground electrode conductor is 3/O connected at each transformer.
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
.....I would absolutely leave the #6 connected. In the scenario given where the EMT is the code required grounding path, (for code) I would see the #6 as an unintended and additional grounding/bonding path that is too small to meet code but not prohibited (and certainly beneficial in its own way). The #6 would be protected from overheating and damage from fault currents by the EMT, while sharing its duty.

Perhaps signage and tagging would be advisable where the #6 could be taped green and warning tagged / labeled as too small to meet code for fault current grounding, as a warning to anyone who would otherwise expect that such a wire meets the normal industry standards and expectations.
While I skimmed over some of the details caught by the smart, literate guys I'm sure you are not allowed to do any of this.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I spoke to nfpa advisory and they said it was not prohibited to leave the #6 in place - go figure..
His opinion is not an official interpretation of the code rule and is no more valid that the opinions that you will find on this site.
 

coop3339

Senior Member
Location
NJ
The conductor is a supply side bonding jumper and is required to be 1/0.

I disagree, 250-A2 says the conduit can be used if not flexible and is metallic. The conduit is the supply side bond, the #6 is an additional, not required, equipment bond and can be left in place.
 

John120/240

Senior Member
Location
Olathe, Kansas
I disagree, 250-A2 says the conduit can be used if not flexible and is metallic. The conduit is the supply side bond, the #6 is an additional, not required, equipment bond and can be left in place.

I disagree. A recent thread had this same predicament. Yes you can use the conduit as your equipment grounding conductor. But once you try to supplement the conduit with a wire used as equipment grounding conductor it has to be sized per 250 something. Maybe somebody can cite chapter & verse or the previous thread.
 

coop3339

Senior Member
Location
NJ
if anyone knows where it says that in the code book I would like to know. I believe it but just would like to know for my own education.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
We have an existing 800A feeder (2) 4"C EMT 3 500kCM & 1#6G all CU paralleled 2 per phase and ground on the secondary of a 225kVA transformer 480-120/208Y. There is 400A CB on the primary and an 800A CB downstream, but obviously the issue is an undersized ground wire. My firm is responsible for recommending a solution to this.

My initial response was that the ground wire needed to be at least 1/0 per 110.122, to pull out the #6 and install the 1/O. Someone else on my team suggested that the ground was not required as the metallic conduit could act as a ground. We agreed this was true per code provided the conduit was installed with appropriate grounding bushings and jumpers.

The bonding conductor in the parallel secondary conduits are supply side bonding jumpers (SSBJ) and are sized according to the ungrounded conductors within each parallel raceway according to T250.66 not 250.122 for 800 amps. Since the secondary conductors should be 25' or less I see no problem with pulling out the #6 and replacing it with a #1/0 in each raceway.
 
Status
Not open for further replies.
Top