Article 250.58 - Separate Services

[mgehringelcon]

Hello,

We have a job underway and I'm looking for a bit more clarification to verify that I truly see where the electrical engineer is coming from. The project is a school with two services, both fed from separate utility transformers (both primaries are primary metered). The high school service is existing and we are not scheduled to do any work, at the moment, on it. The middle school service is newly revamped.

When I asked for clarification on a note from the engineer on a note that requires a 3/0 conductor from the new middle school service to be routed and bonded to the existing high school service, their response included NEC 2011 Article 250.58 and a link to an article by EC&M. Now, looking at the information that they have provided, it seems that the article covers two separate services supplied by the same transformer. This is the part I'm unclear on. I understand that they may be attempting to mitigate potential differences between the two systems, but there is clear separation between the two services as they serve two buildings separated by a firewall. I may be missing the mark completely.

Another though I had just had is that this situation might be similar to a row of "main street" buildings that have been built to share a wall, but not necessarily a service served by the same transformer. Even if they were served by the same transformer, would I still be required to bond the services through the building? Certainly not especially if they were served by different transformers. Maybe I'm reading into it too much, but I've always tried to adhere to the rule of if it doesn't need to be energized, don't bond it.

I apologize for the lengthy post and appreciate your time and assistance on this.

I also apologize if this discussion had been covered in a previous post, there is a lot of great content available on this forum.

 

[Dennis Alwon]

If you truly are looking at 2 structures (buildings) and they are fed from 2 transformers or even from the same transformer I do not see the need for this 3/0 being run between the buildings. In fact, it may be a bad idea. Let's see what others have to say

 

[texie]

Assuming this is one building he is correct. All grounding electrodes in a building must be connected as one.

 

[mopowr steve]

Assuming this is one building he is correct. All grounding electrodes in a building must be connected as one.

I’ve got a feeling this is what they’re referring to also.
May help us to know what size the service conductors are in relation to the 3/0 requested to be ran as a bonding jumper though.

 

[Dennis Alwon]

Assuming this is one building he is correct. All grounding electrodes in a building must be connected as one.

But the OP states it is a separate building because of the fire wall

 

[mopowr steve]

But the OP states it is a separate building because of the fire wall

Gotch ya,
Maybe the engineers are unclear of that fact. Or maybe they know something the OP doesn’t about the structure.
Like maybe there are communication cables running thru the fire wall of buildings that may have a grounded shield which they may be mitigating any bad effect of a fault by running a bonding wire between the two buildings. ( just throwing stuff out there to see if it sticks!)

 

[synchro]

Here's some thoughts although I don't claim to be an expert in this area:
If there is a possibility of telecommunication cables or other conductors being routed to both buildings then it would be desirable to keep the EGCs of the two buildings at substantially the same potential, which the 3/0 conductor should do. Also, if they're not bonded together with the 3/0 then they might get bonded together anyway (possibly unintentionally) by telecom systems either now or in the future but with probably smaller conductors that might not be up to the task.

...I just saw that mopowr steve mentioned telecom cables before I was able to post.
Also, the telecom cables wouldn't have to go through the firewall for this issue to occur. They could come from a common point or place outside of the buildings.

 

[hillbilly1]

Shopping centers do not require it, as they usually have multiple services, also separated by firewalls, so it must be something else like Mopowr Steve and Synchro said.

 

[Dennis Alwon]

Forget all of this and tell me if this is a good idea even if it were the same building. Two services- two transformers- one loose a neutral and now it is being carried over to the service via this 3/0 conductor. Doesn't sound like a good idea to me

 

[hillbilly1]

I had an inspector want two transformers because he was worried about that, one for the building, the second for the fire pump service. I told him good luck on having the poco set a second 480 volt three phase transformer on something that would only produce revenue when the building was on fire! LOL!

 

[electrofelon]

Forget all of this and tell me if this is a good idea even if it were the same building. Two services- two transformers- one loose a neutral and now it is being carried over to the service via this 3/0 conductor. Doesn't sound like a good idea to me

A few thoughts: This is just an unavoidable consequence of of using an MGN distribution system. I think neutral currents WILL flow between the two no matter what you do and better to have it on the 3/0 than less reliable things like pipes, communication cables, ducting, etc. Also, I always like to note that many parts of the country do not use MGN distribution and in those cases neutral currents will not flow between services on different transformers. I dont see any language in the NEC that discusses the same or different utility transformers being used.

Another more general comment: what is the purpose of 250.58? Isnt that essentially already covered by 250.50?

 

[mgehringelcon]

To all,

Thank you for all of your responses! After reading them all, I've come to the conclusion that I may not have thought my question all the way through (as is the case sometimes). The responses that appear to be the closest to the potential reasoning of the EE are those that mention communication cables.

The high school does contain an IT/Server Room that serves both buildings through the firewall. After realizing this, I agree that in order to limit the potential of bonding at an unintentional point via the communications cables (for example), it would be best to follow the engineers requirement.

Again, thank you all for taking the time to voice your thoughts on this subject!

 

[mopowr steve]

Forget all of this and tell me if this is a good idea even if it were the same building. Two services- two transformers- one loose a neutral and now it is being carried over to the service via this 3/0 conductor. Doesn't sound like a good idea to me

Nobody likes a open neutral!
And one would defiantly not like to see what happens when communication cables start carrying current.
I’ve seen first hand a couple times this happening. Caused a fire on one of my jobs and witnessed another just before it became a fire.
Would much rather see that 3/0 there than not.