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Subpanels in Detached Buildings


Brian G
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Lord Jim,

In Electrical Inspection of Existing Dwellings by Douglas H., et al, it says "The code allows a 3-wire feed to the seperate building, using the neutral as a grounding conductor, ONLY when there are no other continuous conductive paths between the buildings. A conductive path could be metal water piping, a concrete sidewalk, a metal fence, or even a telephone line. In practical terms then, a 4-wire feed is needed almost every time a subpanel is installed in a second building."

Capitalization and italics exactly as the text shows it.

What do you think about a PVC water line? Is the water itself a potential conductor, or is a metal pipe the only significant concern?

Brian G.

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Not the one I'm looking at. I suspect it may have copper pipe run to it, but I can't be sure. I see copper under the sink, but it's those skinny little supply tubes coming out of the floor. They may have run PVC from the house to the building.

Water is a poor conductor? I thought water was a very good conductor, hence the danger of water and electricity together....?

Brian G.

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Originally posted by Brian G.

Lord Jim,

In Electrical Inspection of Existing Dwellings by Douglas H., et al, it says "The code allows a 3-wire feed to the seperate building, using the neutral as a grounding conductor, ONLY when there are no other continuous conductive paths between the buildings. A conductive path could be metal water piping, a concrete sidewalk, a metal fence, or even a telephone line. In practical terms then, a 4-wire feed is needed almost every time a subpanel is installed in a second building."

Capitalization and italics exactly as the text shows it.

What do you think about a PVC water line? Is the water itself a potential conductor, or is a metal pipe the only significant concern?

Brian G.

While I'm sure you've quoted Douglas correctly, the actual NEC reference (at least in the '99 edition) uses the phrase, ". . . no continuous metallic paths bonded to the grounding system in both buildings. . . "

Note that it specifies "metallic" and "bonded to the grounding system in both buildings."

It seems to me that a PVC water pipe full of water satisfies neither of these criteria.

Now, the big question is, did the '02 edition of the NEC change this wording to make the requirement more restrictive. I haven't got an '02 edition, anyone out there have one? [250-32(b)(2)]

- Jim Katen, Oregon

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Originally posted by Brian G.

. . . Water is a poor conductor? I thought water was a very good conductor, hence the danger of water and electricity together....?

Brian G.

Like Mike said, it's not the water, it's the contaminants.

Ancient theatre lighting systems used to use salt water dimmers. They consisted of cells of salt water into which some poor sot would insert an electrode. If the electrode was only partially submerged, the lights on the circuit would glow dimly. As the electrode was submerged further, the lights grew brighter. Without the salt, they wouldn't have worked at all.

This system had one small drawback. As the electrodes sat in the water passing current, the water began to break down into its constituent parts, hydrogen and oxygen. After a time, they would sometimes ignite. The resulting unwanted conflagration tended to detract from the intended drama onstage and so salt water dimmers were abandoned. (Their replacements, resistance dimmers, were almost as bad. But I digress.)

I've attached some pictures of an experiment that I played with tonight. The glass contains water from my kitchen tap. The wires are connected to a 30 amp breaker (all I had handy).

The multi meter reads zero on the ampere scale and 120 on the volt scale.

- Jim Katen, Oregon

Download Attachment: icon_photo.gif Tapwater as Conductor Voltage.JPG

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Download Attachment: icon_photo.gif Tapwater as Conductor Current.JPG

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Hi to all,

here's the NEC 2002 section that Jim refered to:

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

(A) Grounding Electrode. Where two or more buildings or structures are supplied from a common ac service by a feeder(s) or branch circuit(s), the grounding electrode(s) required in Part III of this article at each building or structure shall be connected in the manner specified in 250.32(B) or ©. Where there are no existing grounding electrodes, the grounding electrode(s) required in Part III of this article shall be installed.

If a single service supplies more than one building, such as illustrated in Exhibit 250.16, and the feeder is installed with an equipment grounding conductor, 250.32(A) requires that a grounding electrode system be established, unless one already exists. The equipment grounding bus must be bonded to the grounding electrode system. The disconnecting means, building steel, and interior metal water piping must be bonded to the grounding electrode system. All non-current-carrying metal parts of electrical equipment are required to be grounded by connection to the equipment grounding bus.

Exhibit 250.16 A single service (grounded system) supplying three buildings, where each building is required to have a grounding electrode installed in accordance with 250.32(A).

Exception: A grounding electrode at separate buildings or structures shall not be required where only one branch circuit supplies the building or structure and the branch circuit includes an equipment grounding conductor for grounding the conductive non–current-carrying parts of all equipment.

If a building is supplied by only one branch circuit with an equipment grounding conductor, there is no requirement to establish a grounding electrode system or connect to one if one exists. See Exhibit 250.17 for an example of such a system.

Exhibit 250.17 An installation in which connection from the enclosure of the building disconnecting means to the electrode is not required because an equipment grounding conductor is run with the circuit conductors.

(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).

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.

Exhibit 250.18 An installation in which connection between the grounded conductor (neutral) and equipment grounding terminal bar is not allowed. A connection to the grounding electrode is required.

(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

regards

Gerry

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Originally posted by Jim Katen

Like Mike said, it's not the water, it's the contaminants.

I've attached some pictures of an experiment that I played with tonight. The glass contains water from my kitchen tap. The wires are connected to a 30 amp breaker (all I had handy).

The multi meter reads zero on the ampere scale and 120 on the volt scale.

Hmmmm...Based on that I would deduce that any ordinary water line might well be a continous conductive path, just not a metallic one. Wouldn't it be interesting if the line was copper at both ends but PVC between the buildings. Would it then carry "objectionable currents" across the gap via the water alone? Something for you to try on a really boring weekend sometime. Thanks Jim.

Brian G.

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Originally posted by Gerry Beaumont

here's the NEC 2002 section that Jim refered to:

(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

Well it looks as if the wording hasn't changed significantly. Man, who writes that stuff? Don't try to read all of that while operating heavy equipment, you're liable to fall asleep at the wheel. I appreciate the effort Gerry. Jim Morrison will buy you a pint on me down at the pub. [:D] [:-party]

Brian G.

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