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Grounding rod for outbuilding branch circuit? #100152
10/23/06 06:57 PM
10/23/06 06:57 PM
S
SteveFehr  Offline OP
Member
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
250.32(A) Exception: A grounding electrode shall not be required where only a single branch circuit supplies the building or structure and the branch circuit includes an equipment grounding conductor for grounding the non-current-carrying parts of equipment. For the porpose of this section, a multiwire branch circuit shall be considered as a single branch circuit.

Question: Can that excepted branch circuit feed a panel, or is a grounded rod always required when a panel is installed? Article 210.2 specifically lists branch circuits feeding panelboards, so does a panelboard feeder count as a branch circuit?

[This message has been edited by SteveFehr (edited 10-23-2006).]

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Re: Grounding rod for outbuilding branch circuit? #100153
10/23/06 06:59 PM
10/23/06 06:59 PM
renosteinke  Offline
Cat Servant
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Joined: Jan 2005
Posts: 5,316
Blue Collar Country
In a word, Steve....NO.

There are "services." There are "feeders." And, finally, there are "branch circuits".

A wire supplying an overcurrent device is a feeder, not a branch circuit.

Re: Grounding rod for outbuilding branch circuit? #100154
10/23/06 07:07 PM
10/23/06 07:07 PM
S
SteveFehr  Offline OP
Member
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
Then why does 210.2 list switchboards and panels as specific kinds of branch circuits?

Re: Grounding rod for outbuilding branch circuit? #100155
10/23/06 08:21 PM
10/23/06 08:21 PM
G
George Little  Offline
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Joined: Jan 2004
Posts: 1,489
Michigan USA
I'm not seeing anything in 210.2 that identify or list switchboards or panelborads as being fed by a branch circuit. I would agree with Reno on the hierarchy - Service> Feeder> Branch circut


George Little
Re: Grounding rod for outbuilding branch circuit? #100156
10/23/06 08:26 PM
10/23/06 08:26 PM
S
SteveFehr  Offline OP
Member
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
Table 210.2 (referenced to by 210.2) near the bottom.

Re: Grounding rod for outbuilding branch circuit? #100157
10/23/06 09:33 PM
10/23/06 09:33 PM
G
George Little  Offline
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Joined: Jan 2004
Posts: 1,489
Michigan USA
I see what you are talking about now. I was looking at 210.2 instead of Table 210.2. The table references 408.52 having to do with a circuit(s) that would be used for protecting instrument devices associated with a panelboard. The panelboard or switchboard would have in addition to this circuit, a feeder or service conductors that would be the actual feed for the panel or switchboard IMHO


George Little
Re: Grounding rod for outbuilding branch circuit? #100158
10/24/06 07:00 AM
10/24/06 07:00 AM
S
SteveFehr  Offline OP
Member
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
ahh, that would be the difference then!

Next loophole. So, an electrode is required at the outbuilding- NEC 250.52(A)(5) and 250.32(G) permit a ground rod to be used, but do not set any restrictions on placement. (Other methods are similarly unrestricted) 250.58 requires seperate services and feeders to share a common electrode, explicitly allowing multiple services and feeders to share a common electrode. 250.64(C)(3) permits connections to busbar to be used vice a single continuous conductor. Would this not allow the outbuilding subpanel to ground to the main building electrode to satisfy 250.32?

[This message has been edited by SteveFehr (edited 10-24-2006).]

Re: Grounding rod for outbuilding branch circuit? #100159
10/24/06 08:20 AM
10/24/06 08:20 AM
G
George Little  Offline
Member
Joined: Jan 2004
Posts: 1,489
Michigan USA
Steve- In 250.32 it tells us a grounding electrode is required per 250.50. When we go to 250.50 it tells us that all the electrodes that are present at that building or structure are to be tied together and if non are present we need to install one or more. I take that to mean that the Grounding electrode must be at or in the building or structure it serves.


George Little
Re: Grounding rod for outbuilding branch circuit? #100160
10/24/06 09:10 AM
10/24/06 09:10 AM
renosteinke  Offline
Cat Servant
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Joined: Jan 2005
Posts: 5,316
Blue Collar Country
Steve, separate electrodes simply must be connected to the grounding network; supplimentary electrodes, however, require a full size grounding electrode conductor.

In other words, if your 'ground wire' were #6AWG, and run without splicing all the way back to the service disconnect, you MIGHT have a hook to hang the argument on.

BUT- such a 'hook' would be a creature of grammar, not engineering. Remember, the 'ground rod' is there for lightning, and not for clearing faults. A separate structure is a separate target. Wouldn't you want to keep the lightning out at the shed?

Now, we can split hairs all day about what is, or is not, a separate structure. Without trying, I can think of several layouts that blur the line. I think the code simply uses the 'one branch circuit' rule as a threshold, a way of saying 'more electric than that, and it is, without a doubt, a separate structure.'

Re: Grounding rod for outbuilding branch circuit? #100161
10/24/06 11:02 AM
10/24/06 11:02 AM
S
SteveFehr  Offline OP
Member
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
A 12x12' shed, in the shadow of a 2-story house and surrounded by trees? Seems as though the 12/2 feeder feeding a 2-circuit panel with two 15A circuits aught to be plenty with just the #12 Cu ground.

I do have a problem with this related to several real sites, too. In commercial/industrial construction, we generally have a power vault on a corner of the building, often with switchgear, generators and UPS. What 250.32 says is that this is fine so long as that CMU wall is shared, but move that power room 5' further away to create a fire gap, and suddenly, I've got a real grounding problem with every single one of my subpanels.

Granted, I prefer to use multipoint grounds as much as possible, but there are some cases when a 250.50 ground connection is impractical, otherwise unnecessary and very expensive.

[This message has been edited by SteveFehr (edited 10-24-2006).]

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