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Separate building question


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Yes. As guru Katen would say, 'the requirements for the main panel in the house are again required for the panel in a detached structure', or something like that. That assumes there are no ground conductors or conductive conduits between house and detached structure.

Marc

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Because it is a separate building with more than one circuit it should have a ground rod, right?

And the only way to tie one in would be to have a sub panel, right?

It would have to be a 4 wire feeder because of the concrete patio that touches both foundations, right?

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Because it is a separate building with more than one circuit it should have a ground rod, right?

Except for some special circumstances, not more than one circuit or one feeder should supply the entire needs of a detached building.

The ground rod requirements kicks in on all detached structures that have a panel.

And the only way to tie one in would be to have a sub panel, right?

It would seem so.

It would have to be a 4 wire feeder because of the concrete patio that touches both foundations, right?

Three wire for 120/240 service. Don't need the ground wire.

If there's a continuous metallic path between house and separate building (such as a ground wire or metallic conduit) then don't join the neutral bar to the ground bar in the panel of the separate building because that might result in neutral currents in the metallic path. Ground rod connects only to ground bus.

If there's no continuous metallic path between house and separate building, join the neutral bar to ground bar, just like you would on the main panel in the house.

Look at the drawing in 250.32 (A) and focus on the sentence at the bottom of that drawing that says 'No continuous metallic paths between Building 1 and Building 2'. Clue: there's a continuous metal path between Building 2 and Building 3.

Concrete isn't a continuous metallic path but that ground wire is.

Marc

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. . .

If there's a continuous metallic path between house and separate building (such as a ground wire or metallic conduit) then don't join the neutral bar to the ground bar in the panel of the separate building because that might result in neutral currents in the metallic path. Ground rod connects only to ground bus.

But in that case, you need to have an equipment grounding conductor in the feeder. Otherwise you have no low-impedance grounding path back to the service panel. The pre-2008 rule was that if there was a continuous metallic path, you needed a 4-wire feeder.

If there's no continuous metallic path between house and separate building, join the neutral bar to ground bar, just like you would on the main panel in the house.

That was acceptable pre-2008. After 2008, you always need a 4-wire feeder and ground/neutral separation regardless of metallic paths.

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. . .

If there's a continuous metallic path between house and separate building (such as a ground wire or metallic conduit) then don't join the neutral bar to the ground bar in the panel of the separate building because that might result in neutral currents in the metallic path. Ground rod connects only to ground bus.

But in that case, you need to have an equipment grounding conductor in the feeder. Otherwise you have no low-impedance grounding path back to the service panel. The pre-2008 rule was that if there was a continuous metallic path, you needed a 4-wire feeder.

The existing 'continuous metallic path' is the low impedance path.

If there's no continuous metallic path between house and separate building, join the neutral bar to ground bar, just like you would on the main panel in the house.

That was acceptable pre-2008. After 2008, you always need a 4-wire feeder and ground/neutral separation regardless of metallic paths.

That's for sub-panels within the main structure.

Is the drawing in 250.32 (A) a mistake? I know it's not part of the code but I can't explain it any other way.

What's your cite? The 08' is the latest I have and the one I used.

Marc

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Multi-Wire Branch Circuit

250.32 (A)

...

Exception: A grounding electrode shall not be required where only a single branch circuit, including a multiwire branch circuit, supplies the building or structure and the branch circuit includes an equipment grounding conductor for grounding the normally non-current-carrying metal parts of equipment.

Marc

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[brThe existing 'continuous metallic path' is the low impedance path.

No, you can't rely on that for clearing faults. There's no way to know the impedance. Plus the metallic pathway usually isn't bonded to the electrical system. The concern is that a human can become part of that pathway - for instance if he were to touch the fence or the phone line.

If there's no continuous metallic path between house and separate building, join the neutral bar to ground bar, just like you would on the main panel in the house.

That was acceptable pre-2008. After 2008, you always need a 4-wire feeder and ground/neutral separation regardless of metallic paths.

That's for sub-panels within the main structure.

Look at 250.32. The entire section deals with separate structures.

Is the drawing in 250.32 (A) a mistake? I know it's not part of the code but I can't explain it any other way.

I'm not sure what drawing you're referring to. The NEC has no drawings of this. 250.32(A) has to do with grounding electrodes, not grounding wires in feeders. Look at the next section 250.32(B). It explains the need for the grounding conductor in the feeder.

What's your cite? The 08' is the latest I have and the one I used.

In the 2008 NEC, here's the entire section dealing with this issue:

250.32 Buildings or Structures Supplied by a Feeder(s) or Branch Circuit(s).

(A) Grounding Electrode. Building(s) or structure(s) supplied by feeder(s) or branch circuit(s) shall have a grounding electrode or grounding electrode system installed in accordance with Part III of Article 250. The grounding electrode conductor(s) shall be connected in accordance with 250.32(B) or ©. Where there is no existing grounding electrode, the grounding electrode(s) required in 250.50 shall be installed.

Exception: A grounding electrode shall not be required where only a single branch circuit, including a multiwire branch circuit, supplies the building or structure and the branch circuit includes an equipment grounding conductor for grounding the normally non?current-carrying

metal parts of equipment.

(B) Grounded Systems. For a grounded system at the separate building or structure, an equipment grounding conductor as described in 250.118 shall be run with the supply conductors and be 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).

Exception: For existing premises wiring systems only, the grounded conductor run with the supply to the building or structure shall be permitted to 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 where all the requirements of (1), (2), and (3) are met:

(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 each building or structure involved.

(3) Ground-fault protection of equipment has not been installed on the supply side of the feeder(s).

Where the grounded conductor is used for grounding in accordance with the provision of this exception, the size of the grounded conductor shall not be smaller than the larger of either of the following:

(1) That required by 220.61

(2) That required by 250.122

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Can someone illustrate this with color coding and block letters? Crayons maybe...(?)....

I don't have pictures, but I have three simple rules.

1: A separate building should have a grounding electrode system unless it's only fed by a single circuit or a single multi-wire circuit. Period.

2: Since 2008, all *new* sub panels need to have the grounds and the neutrals separated. Period. (That means that the feeder needs to have separate neutral and grounding wires - you can't omit the grounding wire if you separate the neutrals and the grounds.)

3: Before 2008, sub panels in separate buildings could be wired as in #2 above or, if there were no continuous metallic pathways between the two buildings, they could omit the feeder's grounding wire and use the neutral instead. (The grounds and neutrals would be connected together at the sub panel.)

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OK then. I got it. Actually, I always got it.

It's the dissertations referencing NEC and acronyms no one I know uses that confuse me.

I (more or less) ignore the pre-2008 part; it's dangerous, right? That's why it's no longer allowed, correct?

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It's the 'Handbook' edition I have. 2008

Click to Enlarge
tn_20159249380_IMG_2048.jpg

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Again..I'm aware that this drawing is not part of the code.

Continuous metallic path between building 2 and 3. No EGC between any of the buildings. Each establishes it's own grounding w/ground rod, etc.

Marc

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That drawing seems to show bonded ground and neutral in the 2nd subpanel. 3rd panel they're isolated. Why is the 2nd panel bonded?

Because there is no continuous metallic path leading to that building. Under that condition it's safe to bond the two.

I suspect Building 3 does indeed have a continuous metallic path so that bonding connection isn't present in that one. It would be unsafe to do so because then neutral currents might choose the alternate continuous path instead of the neutral conductor.

The central theme here is don't make a connection that connects the neutral to the EGC in two different locations. Keep the neutral currents where they belong...in the neutral conductor.

JMHO

Marc

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OK, so why is it OK to bond neutrals and grounds in a 2nd building but not subpanels in a single building?

It's ok to bond neutrals and grounds in a 2nd building only if there is no EGC or other continuous metallic path of any sort between the two buildings.

It's not ok to do it in a subpanel within the main building because there's a requirement elsewhere in the NEC that all EGCs in a single building bond together. Given that, any connection between any EGC in that building and the neutral that is downstream of the main panel affords the neutral an alternate path.

Marc

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It's the 'Handbook' edition I have. 2008

Click to Enlarge
tn_20159249380_IMG_2048.jpg

33.87 KB

Again..I'm aware that this drawing is not part of the code.

Continuous metallic path between building 2 and 3. No EGC between any of the buildings. Each establishes it's own grounding w/ground rod, etc.

Marc

The drawing is correct up to Building 2. The wiring to Building 3 is wrong and really quite dangerous - it was never allowed under any version of the NEC that I know of. Without a grounding wire between Buildings 2 and 3, the equipment grounding terminal at Building 3 has no way to clear a fault. You certainly can't rely on the connection to the earth or the continuous metallic path. This drawing is wrong in that regard.

That said, I think that the authors were not trying to illustrate the proper wiring between panels here. They were just trying to show where the grounding electrode conductors go in each case. At least that's what the caption seems to show. This doesn't excuse the error, but it explains it.

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OK then. I got it. Actually, I always got it.

It's the dissertations referencing NEC and acronyms no one I know uses that confuse me.

I (more or less) ignore the pre-2008 part; it's dangerous, right? That's why it's no longer allowed, correct?

I'd put it this way: Having a 4-wire feeder is always less dangerous than having a 3-wire feeder.

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