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subpanel grounding

Posted by blindmanbruce (My Page) on
Tue, Feb 14, 12 at 16:19

Existing wiring: 40A breaker from main panel feeds underground wiring to sub-panel in detached garage approx. 75' from the house. The garage is about 25'x40' in size. Previous owner ran 240V compressor from that location among other things. The sub-panel currently has 4 circuits...
I changed it a while back (it used to have 3 circuits, two 120v and one 240v); I removed the 30A Double pole breaker and installed 2 single pole circuits because I didn't have a need for a 240V circuit. (removed 240V outlet).

I wasn't having any electrical issues then or now.

This is what I was leading up to:
When I changed the above circuits I noted that there was NO ground wire feed from the main panel into the garage. It could be because they ran metal conduit underground but I can't be sure; but if it is then it's been there for over 25 years.
The only wires to the sub-panel were L1 (black), L2 (red), and Neutral (white)*[there were also two smaller wires that looked to be gauge 14 or less but they weren't connected to anything at either end (house or garage) and their color codes were faded -they didn't look red/black/white/or green.
Next: What I did was to install a separate ground bus inside the panel (NOT bonded to the neutral). I then installed a ground rod and sizing the circuit I installed the appropriate solid copper ground wire. I then attached all circuit ground wires to the ground bus (as I recall, before that most of them were not connected to anything.)

Was this correct?


Follow-Up Postings:

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RE: subpanel grounding

Sounds good other than the fact you need 2 ground rods not 1 and they should be at least 6 feet apart from one another. Make sure that the bond screw is OUT of the sub which it sounds like you did and yes all the grounds should be on the ground bar and nuetrals on the nuetral bar. 40 amp sub should be at least a #8copper so check that and i think you are in. In my area we are also required to bond any steel if the garage has any like a pole shed, from the risk of a lightning strike. good job.


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RE: subpanel grounding

Also be sure that there is continuity between the conduit and the panel, otherwise, there will be no path for fault current, as the conduit is serving as your EGC.


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RE: subpanel grounding

I thought perhaps I did need a second ground rod, I will install another one as soon as weather permits. Also, I do have #8 copper wire.
As far as the ground setup... The neutral bus bar is isolated from the ground bus bar. The ground bus is connected to the sub-panel housing which in turn is physically connected to the conduit. (and of course, the ground bus bar is connected to the ground rod.)
I thought that I may have misinterpreted the NEC. I was worried that a conduit ground if it does or still exists might have been compromised after all these years, so to be safe I should ground the garage sub-panel system. At first I thought the NEC wanted me to run a ground wire all the way to the garage from the main panel; I didn't like that idea because after I considered the length of the run it seemed it would provide too much resistance for a ground return path (I guess if the wire were uninsulated and buried it would be o.k., but it would cost a lot for 80' or more of #8- ouch). So, I opted for the installation of a ground rod and ground bus bar assy.

Thanks for the input ranger619 and electricalkid and anyone else who comes along.

One other question? I do not have any GFIC outlets in the garage. I think I'm required to have a least one. Do you forsee any problems for installing any GFIC at this location? I mean any problems with voltage drop or resistance issues?


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RE: subpanel grounding

What you did was revert a legal and proper installation.
It was not always required to run a separate dedicated equipment ground, the neutral was allowed to serve both purposes under certain circumstances.
The neutral bar WAS bonded to the panel box, just like a main panel.

What you did was isolate the grounding in the panel and sink ground rods, which serve a completely different purpose than an equipment ground, and are completely useless as they stand now in your installation.

Leave the ground rods, and re-bond the neutral. There was nothing wrong to begin with.


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To add...

My reply is assuming that there is not a complete metallic conduit run between the buildings.


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RE: subpanel grounding

"assuming that there is not a complete metallic conduit run between the buildings"

I know the wiring leaves the house basement below grade through conduit and it arrives in the garage via conduit.
Having lived in this old house for sometime now I've seen unbelievable things. The previous owners (and there have been many) have given all new meaning to the term jury rig), and although it would seem that there would be conduit the full distance underground, I am not altogether sure.

Let me see if I understand.... (for detached structures)
1.) So for structures with existing wiring and NO continuous metallic path between buildings (which includes no ground feed wire), just the hot and neutral only. The neutral bus would be bonded to the panel and also connected to the ground rod either directly or through bonding to the ground bus.
In this situation, would garage circuits originating from the sub-panel to outlets, switches etc., that are not in conduit or flex cable, but are NM insulated 2wire with ground... would the ground wires be used?

2.) If the wiring between structures HAS a continuous metallic ground path between buildings ie. metallic conduit, ground wire, plumbing etc. , accompanied by hot and neutral wires. Then a separate grounding system with a ground bus and ground rod are installed and are connected to the panel and are NOT connected to neutral. Neutral is NOT bonded to the panel.

In which case would GFIC breakers or outlets be allowable in the garage? (I'm guessing case #2) In which case would garage circuit wiring from the sub-panel have to be in metal conduit or doesn't it matter?


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RE: subpanel grounding

#1 - yes, this is the way the code used to read. You would still hook the equipment grounds to all the circuits up to the common ground/neutral bar in the sub panel.

#2 Correct.

If this is a garage at or below grade, then all the 120V 15 and 20A receptacles must be protected by GFCI. It makes no difference how the feeder is run or even if this is a sub panel or if the garage has its own service.


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RE: subpanel grounding

OK. That about sums it up. Thanks for the information it is appreciated.


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RE: subpanel grounding

2.) If the wiring between structures HAS a continuous metallic ground path between buildings ie. metallic conduit, ground wire, plumbing etc., accompanied by hot and neutral wires. Then a separate grounding system with a ground bus and ground rod are installed and are connected to the panel and are NOT connected to neutral. Neutral is NOT bonded to the panel.

"Plumbing, etc" has nothing to do with whether or not you bond, and everything to do with whether you need a legal ground path ("metallic conduit, ground wire").

If you don't have a legal ground path then you MUST bond.

If you also have a other illegal grounded metallic paths, then you MUST provide a legal ground path and do NOT bond.


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RE: subpanel grounding

O.K. thanks for the correction. (The terminology really throws me for a loop).
I realize I may be repeating what you've said, but I have to put it in my own words to make sure I understand.

So, besides acceptable legal paths there may exist other continuous metal paths between structures. These other paths are considered illegal because they can provide an alternate or parallel grounding path and represent an electrical hazard. All illegal ground paths must be connected to a legal ground path.

Q.(Would this apply to gas pipe as well?).

Alternate ground paths (illegal ones) can not come in contact with the Neutral. Since illegal paths have to be legally grounded, the Neutral is not bonded because it would then come in contact with an illegal ground path. The only time the Neutral is bonded is when no continuous metal path exists between structures (whether they are legal or illegal).


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RE: subpanel grounding

First time poster, long time lurker. Sorry to follow up on an old thread, but this one seems relevant to my current situation. We just bought a new/old house, so I'm in the process of sorting out 90 years of work (60 of which were great work...last 30...meh). I would appreciate input on my current predicament:

- 3 wire (240V, 40A, #6...black, red, white) ungrounded feed from grounded main to a semi-attached structure (Cabin). It appears properly (over?) sized and in good condition...approximately 80 foot run, well protected.
- Cabin shares copper H2O and galvanized boiler pipes with main structure (House).
- House H20 not grounded. Majority of old galvanized was replaced w/ CPVC (only copper left is the run to and w/in the Cabin)
- House main panel grounded w/ 2 rods (has no gas lines).
- Cabin has subpanel w 2 rods grounded from box.
- Cabin Neutral bus not connected to ground (can be w/ screw).

It seems to me that it would be proper to ground the Neutral in this Cabin subpanel. That seems the consensus of this thread above. Thoughts appreciated. Also, would it be proper to also ground what copper H20 that there is in the Cabin?

Cheers


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RE: subpanel grounding

The older code version used to allow 3-wire service to panels in separate structures, while also requiring an electrode system.

3-wire service to a separate structure has NOT been allowed for the last few code revisions.

4-wire is required to feed a panel in a separate structure, still with grounding electrodes.

You can use one electrode if you show it is less than 25 ohms to earth.
The test normally requires you to drive another electrode to measure against.
You may as well just drive two and be done with it.

Then no measurement is required.


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