Detached Building Service: Double Check My Plans?

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Fasttrack

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Hi All - I'm a project manager and electrical engineer with more experience doing odd-ball power distribution and control stuff for science / defense industry. Oftentimes, the work is exempt from the NEC. We do our calculations and build accordingly. Every now and then I find myself paging through the NEC, either to inform a design choice that is not strictly dictated by the NEC or to double check compliance on a project that will have to be inspected. But residential / commercial wiring is not my bread and butter. Electricians know it way better than I do, which is why I'm here!

I've got a side project that will need to be inspected and I want to make sure I do it right from the beginning. Currently, I have a building with a 200 amp service with a 30 amp sub-panel in a detached building. The detached building is about 720 square feet and will be home to a small R&D machine shop. The 30 amp sub-panel needs to be replaced with a 200 amp service. The first thing I'm planning is to replace the 200 amp pan on the first building with a 320/400 amp pan with dual lugs. Here's where things get a little tricky...

Due to the location of the overhead drop, along with trees, pavement and some underground tanks, the shortest, easiest route to the second building is basically through the first building, which is prohibited (I believe) by article 230.3, presumably because there is no over-current protection on the feeder. So I'm trying to come up with a legal solution. Here's what I've got:

From 320A pan, run SER to first building's 200 amp MB panel. In parallel, run another length of SER to a 200 amp fused disconnect, located in close proximity to the first building's MB. From the 200 amp fused disconnect, run RHW through the first building and then underground (18" deep direct burial with ribbon at 12") to the second building. Since the second building is not connected to the first building in anyway, I would not run a ground wire - only split phases and neutral. The second building would then get two 10' ground rods with 6' separation. I've got the testing equipment to check ground resistance on those; there's quite a bit of ledge around here so getting a good ground rod in can sometimes be tough.

Does this sound kosher? The two things I'm particularly concerned about:

1) Two service disconnects in one building - Again, article 230 explicitly forbids a building from having more than 1 service, with a few exceptions. Does a disconnect that feeds a detached building count as a service? It's just a disconnect, no spaces where a midnight electrician might be tempted to plug in breakers and start feeding the first building.
2) Grounding - I still get confused with grounding requirements for detached building. Since this is detached from the first building with no water mains, gas pipes, etc. joining the two, I think it gets its own ground, which would then be bonded to the neutral at the panel. But... I've got a service disconnect inside the first building. Does that mean I have to run a ground from the 2nd building's ground system to the disconnect? Do I have to bond the neutral there and not at the breaker box? Then sizing the ground wire and where to place the rods? Ugh!

Or is this one of those cases where I just bite the bullet and go with a really long run of USE from the pan to the second building and be done with it? The cost of excavation and material will be significantly higher, though... :(

Thanks!
 
Fasttrack said:
Does this sound kosher? The two things I'm particularly concerned about:

1) Two service disconnects in one building - Again, article 230 explicitly forbids a building from having more than 1 service, with a few exceptions. Does a disconnect that feeds a detached building count as a service? It's just a disconnect, no spaces where a midnight electrician might be tempted to plug in breakers and start feeding the first building.

You only have one "service" and that's to your meter. From the merter you have two sets of service entrance conductors (SE cable..no need for SER)installed to meet 230.70(A)(1) to two service disconnects which need to be grouped (+230.70(A)(2)}.
2) Grounding - I still get confused with grounding requirements for detached building. Since this is detached from the first building with no water mains, gas pipes, etc. joining the two, I think it gets its own ground, which would then be bonded to the neutral at the panel. But... I've got a service disconnect inside the first building. Does that mean I have to run a ground from the 2nd building's ground system to the disconnect? Do I have to bond the neutral there and not at the breaker box? Then sizing the ground wire and where to place the rods? Ugh!
Grounding for your 1st building service can be from you meter or your disconnects. You do need a separate equipment grounding conductor to building #2 {250.32} and establish a grounding electrode system at that building which will connect to your panel and equipment grounding conductor. The nenutral will "float"

Or is this one of those cases where I just bite the bullet and go with a really long run of USE from the pan to the second building and be done with it? The cost of excavation and material will be significantly higher, though... :(

Thanks!
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Thanks, Augie47. Service = feed to the meter, not disconnect / MB :ashamed1: I should know better!

Regarding grounding, it looks like things changed in 2008. I have an old paper copy of the NEC that's my go-to reference but I really should ditch it for a modern copy. I just want to clarify a few things on the grounding:

1) I seem to recall reading somewhere that bonding the neutral and ground at the meter pan is not "best practice" because it's not legally accessible, or something like that (i.e. one would have to break the poco seal to check bonding) so I would plan on bonding the neutral and ground at the MB panel in the first building.

2) Is there any reason an inspector would balk at bonding the neutral in the MB panel for the first building? Electrically, it's the same thing as bonding in the pan but visually, it looks a little different because bonding in the pan appears to be "up stream" of the MB and disconnect. In my (limited) experience, inspectors are sometimes more ... um ... "visual" than electrical, if you know what I mean?

3) Grounding conductor size: Even though the conductor going to the ground rod only has to be 6 AWG Cu, I would size the ground conductor in the feeder to the second building according to 250.66, right? So for 200 amps I'm looking at either 4 AWG Cu or 2 AWG Al.
 
You may find that the neutral connection point in the meter base is bolted directly to the can in some meters.

As far a where you land the GEC, that varies from place to place whether that be in the meter or the 1st means of disconnect.

You should never run a Grounded Conductor and an EGC from the meter base to the 1st means of disconnect.

The Grounded Conductor serves both purposes for that part of the return path.

JAP>
 
Fasttrack said:
Thanks, Augie47. Service = feed to the meter, not disconnect / MB :ashamed1: I should know better!

Regarding grounding, it looks like things changed in 2008. I have an old paper copy of the NEC that's my go-to reference but I really should ditch it for a modern copy. I just want to clarify a few things on the grounding:

1) I seem to recall reading somewhere that bonding the neutral and ground at the meter pan is not "best practice" because it's not legally accessible, or something like that (i.e. one would have to break the poco seal to check bonding) so I would plan on bonding the neutral and ground at the MB panel in the first building.
As jab states and you verify, the location of that connection can be alter by POCO or local rules. IF you elect to connect your GEC to the panel, keep in mind a GEC must be connected to all service panels not just one. Methods of accomplishing this vary and thats why I suggested meter is allowable.


2) Is there any reason an inspector would balk at bonding the neutral in the MB panel for the first building? Electrically, it's the same thing as bonding in the pan but visually, it looks a little different because bonding in the pan appears to be "up stream" of the MB and disconnect. In my (limited) experience, inspectors are sometimes more ... um ... "visual" than electrical, if you know what I mean?
The neutral is required to be bonded in service panels. (NEC 250.24) and as jap mentions, it is likely factory bonded in the meter pan.

3) Grounding conductor size: Even though the conductor going to the ground rod only has to be 6 AWG Cu, I would size the ground conductor in the feeder to the second building according to 250.66, right? So for 200 amps I'm looking at either 4 AWG Cu or 2 AWG Al
Actually its an equipment grounding conductor and is to be sized per 250.122 (minimum)
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Additional question based on initial post.

Additional question based on initial post.

I see the disconnect at the service entrance, however, would you not also have a disconnect at the detached building. I have a similar installation with two detached buildings, primary service to 1st, once in building-split to pan with 300A main and separate run to 400A discon with 300A fuses. From the discon through the 1st building then underground to the second. Question is this: EE on site spec'd MLO panel as means of distribution in second building with no local means of disconnect. Is this install permitted by code or even by conscious? MLO panel fed with 4-350mcm and 1/0 ground. Service is 480/3, 4w. By the way, the panels/discons have already been ordered and are on site. :)

Gotta love some jobs. :happyyes:

Gotta love some EEs. :lol: At least the original poster has some working knowledge of what we do for a living. :D

Thanks in advance.
 
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