250.32 versus 250.24

[dbowling]

I have a project with electrical services for a multi-tenant building. Each tenant's space is separately metered with a main breaker (200A at 208 volt, 3 phase) service disconnect and meter located on a retaining wall (approximately 50 feet from the building) near the power company's transformer. Each service then feeds a tenant panel with main breaker within each tenant space. The neutral and ground are bonded together at each service disconnect with #4 ground from this point to a common grounding electrode system for all the tenant's services using #3/0.

The local inspector is viewing the retaining wall as a separate structure and stating (per 250.32) that the grounding system must generate from the tenant panels as well as a ground at the service disconnects. This seems to be in violation of 250.24(A)(5). Although the definition is vague in Article 100 for a Structure, it seems that if there are no power utilization items at the retaining wall then it should not be defined as a separate structure or building.

 

[augie47]

The definition of a structure could use some "tweaking"
What Code cycle are you under and was there an equipment ground run to the tennat space from the "retaining wall" ?

 

[raider1]

IMHO the inspector is correct. The retaining wall is a structure as defined by Article 100. So from the retaining wall to the multi-tenant building each supply would be a feeder and 250.32 would apply.

You would also have to comply with 225.30 and 225.31. So provided that you get special permission from the AHJ for multiple feeders to supply the building, you would have to have disconnecting means either outside the building or inside at the point where the feeders enter the building.

Chris

 

[dbowling]

Using the 2005 edition of the Code. Yes, there is an equipment grounding conductor between each service disconnecting means and tenant's panel.

The service disconnects are grouped together on the exterior at the retaining wall. Then each feeder is underground until coming up into each tenant's panelboard.

 

[augie47]

Possibly a gray area, (I'd vote with the inspector also) but seems like an easy fix. If I understand you correctly, all that would be needed is seperating the neutral and ground at the tennant panel. Since you ran both a grounded and grounding conductor, if they are tied together it sounds like you might have a violation of 310.4.

 

[dbowling]

The neutral and ground are only bonded at each service disconnect (located at the exterior retaining wall), not at the tenant panels.

If the Inspector is correct then do we bond the neutral and ground together at the panels and connect to the grounding electrode system in the building? What about the bonded neutral to the ground at each service disconnect? Should the ground at the tenant panels and the ground at the service disconnects be bonded or connected? Seems that we would have two grounds for each service the way the Inspector is asking for this to be installed with a ground connection for the tenant panels and a ground connection at the service disconnects.

 

[raider1]

The neutral and ground are only bonded at each service disconnect (located at the exterior retaining wall), not at the tenant panels.

This is correct.

If the Inspector is correct then do we bond the neutral and ground together at the panels

No, do not bond the neutral and the equipment grounding conductor together at the tenant panels. 250.32 requires that you connect the grounding electrode to the EGC terminal in the disconnect for the separate building. So you would just land the GEC on the equipment grounding conductor terminal at the tenant panels.

For the installation that you have there should be 2 grounding electrode systems.

The first system should be installed at the service on the retaining wall (First building/structure). This grounding electrode system should be connected to the service grounded conductor.

The second grounding electrode system should be installed at the multi-tenant building. This GES should be connected to the equipment grounding conductor terminal/bus in each tenant panel/disconnecting means.

Hope this makes sense.

Chris

 

[augie47]

IMHO, perfect explaination.

 

[dbowling]

Thanks for the explanation. Made revisions to the drawings illustrating what you explained and the Inspector had no objections and stated it was in compliance.

 

[raider1]

Your very welcome.

Chris

 

[TJjr]

250.24 vs. 310.4

250.24 vs. 310.4

I am going through the soars book on grounding and finding some irregularities on 250.24 vs. 310.4 when it comes to parallel feeders. 250.24 (C)(2) states sizing off the ungrounded conductors per race-way. 310.4 is referring me to the cir-mil calculations in Table 8 chapter 9. I am new at instructing the coarse of Service entry conductors and can not afford to teach wrong information. I need to know for certain with-out doubt that what I teach is correct. Any help on clarifying this is appreciated.