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#62330 - 02/14/06 11:29 PM grounding a sub...old topic..yet need clarification  
NJI  Offline
Member
Joined: Jul 2005
Posts: 33
Oneonta, NY USA
Soooo...here we are again. let me get this straight..sub panel, seperate building.
single phase
if i have to buy ground rods for these things reguardless of wether i run a gouning conductor or not...why buy the extra wire in the first place?

if i don't run is..i have to install grond rods

if i do run it i have to install ground rod, but now i have to take out any system that may connect the grounding conductor from the grounded conductor.

am i missing something?


I have worked as an apprentice and on to a journeyman in a general construction company. 90% of what I do is residential. I have an associats degree in electrical technology. I have started my own buisness this year and am with out the help of other electricians. this is why i find this site so helpful.

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#62331 - 02/15/06 08:27 AM Re: grounding a sub...old topic..yet need clarification  
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
boston, ma
To start, a recap on _most_ subpanels: For most subfeeds, you run an equipment grounding conductor that is separate from the grounded conductor (separate ground and neutral), and then at the subpanel you must have a _separate_ ground and neutral bus.

The reason for this is to prevent 'objectionable currents' from flowing on the equipment ground system. Essentially you want to prevent the equipment ground system from operating as a parallel conductor to the neutral wiring.

The one exception to this rule is for a separate building, with a number of other restrictions. Basically, if the only 'parallel conductor' would be the earth itself, then you are permitted to have this parallel ground path. This means that you can have no _bonded metallic path_ between the two buildings. If you have a metal water line between the buildings (which would be bonded to the ground systems), then this would form a metallic parallel path, and you would have to separate the ground and neutral bus. If you have a communications line with a grounded shield, or a fence attached to building steel at both ends, same deal.

Finally, there is an additional requirement: if there is ground fault protection on this subfeed (either on this specific feeder, or on the entire service), you must run a separate equipment ground to the subpanel. The reason is that depending upon the design of the ground fault protection, rebonding ground and neutral might either result in false tripping or desensitization.

But if all of the requirements for not running an equipment ground conductor to a detached structure are met, then you can either run an EGC and use a 'standard' subpanel, or you can not run the EGC and have a 'service like' panel. This is now a _design choice_.

_Personally_, for _short_ runs I would run the EGC and have separate ground and neutral busses; this allows for more flexibility in the future, and for _long_ runs I would just run the neutral and rebond at the detached structure, because IMHO this would probably be better for lightning protection. But those are just hunches, and the dividing line between 'short' and 'long' is quite fuzzy [Linked Image]

-Jon


#62332 - 02/15/06 08:55 AM Re: grounding a sub...old topic..yet need clarification  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Quote
But those are just hunches, and the dividing line between 'short' and 'long' is quite fuzzy


Reminds me of the "fuzzy arithemtic" in an old mathematical joke:

"One is greater than two, for large values of one and small values of two." [Linked Image]

Code issues aside, in basic electrical terms I don't see a 3-wire subfeed to a detached building which has a parallel path via a water line as being any different than a common xfmr feeding two or more separate homes which are joined via copper pipes to a common water supply.

It's an interesting aspect of the NEC that the latter is allowed but the former is not.




[This message has been edited by pauluk (edited 02-15-2006).]


#62333 - 02/15/06 10:51 AM Re: grounding a sub...old topic..yet need clarification  
TNSunny  Offline
Member
Joined: Dec 2005
Posts: 44
Tennessee
DETACHED BUILDINGS

Single circuits run to a detached building which provide a proper and adequate equipment grounding method does not require a separate grounding electrode. 250.32(A), Exception.

A feeder ran to a separate building that DOES CONTAIN a proper and adequate equipment grounding method (e.g. conduit or separate EGC):
• DOES require a separate grounding electrode.
• The neutral bus bar in the subpanel is NOT bonded in this case. (To do so would provide multiple paths for neutral current to flow to ground.)
• The grounding electrode conductor is sized according to Table 250.66, but does not have to be larger than the largest ungrounded supply conductor. 250.32(E)

A feeder ran to a separate building that DOES NOT have a metal path between the two buildings, such as conduit or a water pipe, and no EGC is run with the feeder, then:
• A separate grounding electrode IS required.
• DO bond the neutral bus and grounding bus together.
• The grounded (neutral) conductor of the feeder must not be smaller than listed in Table 250.122, but doesn’t need to be larger than the largest ungrounded conductor. 230.32

Kevin


Kevin

#62334 - 02/15/06 09:43 PM Re: grounding a sub...old topic..yet need clarification  
NJI  Offline
Member
Joined: Jul 2005
Posts: 33
Oneonta, NY USA
well, thanks for the input. stil alittle "fuzzy" but mostly on simply which situations call for a EGC to be run and which don't. in the service under question here..there is no metal path back to the house other than the EGC that i have just run. there for.. i need to continue on and disconnect the neutral bar from the cross over bar to the grounding bar in the panel. which i have. and finally i must through some rods in ( no big deal i get a chance to run my hammer drill...havn't in a little while now.) but mostly i guess i'm a little fustrated that i spent money on wire that is in no way nessassary. but thanks alll


I have worked as an apprentice and on to a journeyman in a general construction company. 90% of what I do is residential. I have an associats degree in electrical technology. I have started my own buisness this year and am with out the help of other electricians. this is why i find this site so helpful.

#62335 - 02/15/06 10:06 PM Re: grounding a sub...old topic..yet need clarification  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,104
Estero,Fl,usa
It looks like the 2008 code will make all that confusion go away. If it makes it through the coordinating committee we will be running an EGC to all subs.


Greg Fretwell

#62336 - 02/15/06 11:44 PM Re: grounding a sub...old topic..yet need clarification  
NJI  Offline
Member
Joined: Jul 2005
Posts: 33
Oneonta, NY USA
hopefulluy, yet it will take another three years for the boys up here to agree to adopt it or not. I don;t think there are any AHJ's carrying the new code now. all work from 2002. I'm not sure if they are aware of any of the updates.
thankyou for the info and the help


I have worked as an apprentice and on to a journeyman in a general construction company. 90% of what I do is residential. I have an associats degree in electrical technology. I have started my own buisness this year and am with out the help of other electricians. this is why i find this site so helpful.


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