Assessory buildings - subpanel requirements

[Perion]

There seems to be no lack of confusion about the proper method to feed and ground subpanels in detached buildings. Here's an interesting quote in a San Bruno, Calif. electrical requirements document that adds a bit more confusion ;) . See:
http://www.ci.sanbruno.ca.us/Com_Dev/Building/pdf/California_Electrical_Code.pdf

A subpanel in a garage or other detached accessory building, which is fed by the main service at the house, must have a separate grounding electrode. If the feeder from the main service is only 3 wires (2 hots and neutral) the grounding electrode conductor at the subpanel must be bonded to the neutral. If the feeder has four wires (2 hots, neutral and ground) the grounding electrode conductor at the subpanel must be isolated from the neutral, even though they are bonded together at the main service. This seems odd, but there is a good reason for it. If you cut the neutral wire of an active circuit and hold one cut end in each hand, you will be electrocuted. If you do it to a properly installed ground wire, you will not. But when you bond the neutral and ground together, the ground wire from that point back to the beginning of the circuit shares the current carried by the neutral and can electrocute you. For that reason, the bonding together of these two wires must only occur at the origin of the ground wire.

Notice the bold "if ...." phrases. Does anyone really know the actual scoop on feeding and grounding subfed panels in detached assessory buildings? Is it really optional whether to run 3 or 4 conductors to feed the panel as this paper implies?

Regards,
Perion

[suemarkp]

See the thread titled "aticle" in this group for the requirements for using either method. Key things to realize in the quoted California document:

You have these choices ONLY if the building is detached. An attached garage must have a 4 wire feeder.

If you cut the neutral and grab each half in separate hands, odds are good you'll be electrocuted whether or not a separate grounding wire exists.

People who cut wires and grab both ends are probably not going to live long. Cutting a separate grounding wire can kill you too if there is a faulted item in the detached building. You need to remove all sources of power when cutting wires, whether bare, white, or colored!

I don't think their explanation of why the separate wire should be installed was very good. See the other article about the detached shop feeder to see my answer there (for what its worth).

[Wgoodrich]

Here is the low down as you say. Below is a copied section of the rule whether 3 wire or 4 wire. If you wish more explaination look at the top of this page in the green and yellow boxes being links to our main web site containing an article for a detached garage. Most of that article will pertain to any accessory building "Detached Garage" button;

Copied section of NEC 2002;

250.32 Two or More Buildings or Structures Supplied from a Common Service.

(B) Grounded Systems. For a grounded system at the separate building or structure, the connection to the grounding electrode and grounding or bonding of equipment, structures, or frames required to be grounded or bonded shall comply with either 250.32(B)(1) or (2).
(1) Equipment Grounding Conductor. An equipment grounding conductor as described in 250.118 shall be run with the supply conductors and connected to the building or structure disconnecting means and to the grounding electrode(s). The equipment grounding conductor shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded. The equipment grounding conductor shall be sized in accordance with 250.122. Any installed grounded conductor shall not be connected to the equipment grounding conductor or to the grounding electrode(s).

NEC HANDBOOK COMMENTARY;
If a feeder supplies another building from the same service and an equipment grounding conductor is run with the feeder, the grounded conductor (neutral) is not permitted to be connected to the equipment grounding conductor or to the grounding electrode system, as illustrated in Exhibit 250.18.

COPIED SECTION NEC 2002;

(2) Grounded Conductor. Where (1) an equipment grounding conductor is not run with the supply to the building or structure, (2) there are no continuous metallic paths bonded to the grounding system in both buildings or structures involved, and (3) ground-fault protection of equipment has not been installed on the common ac service, the grounded circuit conductor run with the supply to the building or structure shall be connected to the building or structure disconnecting means and to the grounding electrode(s) and shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded. The size of the grounded conductor shall not be smaller than the larger of
(1) That required by 220.22
(2) That required by 250.122

NEC HANDBOOK COMMENTARY;
Similar to the provisions of 250.30(A)(2), 250.32(B)(2) also eliminates the creation of parallel paths for normal neutral current on grounding conductors, metal raceways, metal piping, and other metal structures. In previous editions of the Code, the grounding electrode conductor and equipment grounding conductors were permitted to be connected to the grounded conductor at a separate building or structure. This multiple-location grounding arrangement could provide parallel paths for neutral current along the electrical system and along other continuous metallic piping and mechanical systems as well. Connection of the grounded conductor to a grounding electrode system at a separate building or structure is permitted only if these parallel paths are not created and if there is no common ground-fault protection of equipment provided at the service where the feeder or branch circuit originates.

COPIED SECTION NEC 2002;

250.50 Grounding Electrode System.
If available on the premises at each building or structure served, each item in 250.52(A)(1) through (A)(6) shall be bonded together to form the grounding electrode system. Where none of these electrodes are available, one or more of the electrodes specified in 250.52(A)(4) through (A)(7) shall be installed and used.


Read the rules closely so you don't miss the differences between 250.32.B.1 and 2. This is where the key point is. Also be aware the last rule in 250.50 calls for a grounding electrode [commonly a ground rod] installed at EACH building.

Hope this helps

Wg

[Perion]

Thanks very much to both for your replies. For some reason I've always been confused and probably just plain dense on this but at least now I do see why. I misinterpreted the wording in 250.32.B.1 ["Equipment Grounding Conductor. An equipment grounding conductor as described in 250.118 shall be run with the supply conductors... "] as requiring that an equipment ground conductor must be run with feeders to the building. Where would I ever get such a stupid idea? ;) But then it didn't make sense that in 250.32.B.2 it turns right around and says "(2) Grounded Conductor. Where (1) an equipment grounding conductor is not run with the supply... " which indicates the equipment grounding conductor is not manditory. Somehow I missed the "...shall comply with either 250.32(B)(1) or (2)" in the initial paragraph. Anyway, thanks again. Glad I'm not a lawyer an have to deal with such wordings day in and day out.

Perion

[Wgoodrich]

Perion, the fourth wire [equipment grounding wire with feeder between buildings] is required only when a noncurrent carrying metallic path connects the two separate grounding systems of the two buildings. Then the two grounding systems required one at each buildng must be made definitly interconnected to ensure equal potential between the two structures groundings systems fed by a common meter base or service.

Hope this helps

Wg

[Perion]

Perion, the fourth wire [equipment grounding wire with feeder between buildings] is required only when a noncurrent carrying metallic path connects the two separate grounding systems of the two buildings. Then the two grounding systems required one at each buildng must be made definitly interconnected to ensure equal potential between the two structures groundings systems fed by a common meter base or service.

Hope this helps

Wg

I can understand the reasoning in requiring a separate e.g.c. in the case of a "noncurrent carrying metallic path" existing between the buildings. But, to be quite honest, I really don't see those metalic paths being declared as the governing factor of when an e.g.c. should be run anywhere in the wording of 250.32.b.1 / b.2. Maybe I'm missing something here. It's interesting that the section descriptions for b.1 and b.2 are "Equipment Grounding Conductor" and "Grounded Conductor" respectively. For b.1 Equipment Grounding Conductor all it says is that an e.g.c. shall be run - unconditionally. It makes no mention of the presense or absense of metallic paths between the buildings. Section b.2, dealing with the "grounded conductor" or neutral, only lists the situation of where an e.g.c. is not run as one of three conditions (another being the absense of metallic paths) for determining whether the neutral should be connected to the building structure, disconnecting means, and ground rod.... If the combination of b.1 and b.2 somehow mysteriously results in some governing factor for when an e.g.c. should be run it certainly seems like a very convoluted way to try to establish some rules for a fairly simple set of situations.

BTW - I hope I'm not being a pain-in-the-butt. I'm not trying to be difficult or argumentative. It's just one of those little nagging areas that bothers me. I like things to clear, concise, and logical and this section of code seems anything but that.

Perion

[Wgoodrich]

You are not a pain in any manner. When a person has trouble connecting the points of a rule they have trouble in the industry as well as anyone they talk to creating confusion. The more knowledge is applied correctly as you seek the better the industry is safe. Knowledge is the weapon you seek, that is a good thing.

Go back to the copied rules provided in this post. The look closely for the following part of that rule that you are missing;

COPIED SECTION NEC 250.32.B.2

(2) Grounded Conductor. Where (2) there are no continuous metallic paths bonded to the grounding system in both buildings or structures involved,

Hope this hits the point you are seeking

Wg