I have a 30 amp 120v two wire plus ground feeding a subpanel in my steel detached garage. The garage has a re-bar for grounding but I assume I should use only the ground circuit from the supply line and not introduce another grounding source. Am I correct?
Grounding Detached Garage Subpanel?
- Thread starter Phil De
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2ManyProjects
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No.
Further, for clarity if nothing else, I have to at least somewhat question your description of the existing installation. Running ONLY 120V to a subpanel in a detached structure would be EXTREMELY unusual, at best. (Typically, if you REALLY only need 120V in a detached garage, such as to run a couple of lights and maybe a door opener, that's done sans sub-panel.) So... Are you ABSOLUTELY CERTAIN that you don't really have a 240V feed?
If the feed really is 240V, then it also needs to be FOUR-wire (two Hots, a Neutral, and a Ground).
In either case, the Neutral and Ground busses in the sub-panel MUST remain UNbonded. You then install a separate Earth-ground system (either UFER or two ground rods, spaced at least six feet apart) at the detached structure, and feed the Ground buss in the sub-panel directly to that. The Neutral gets fed back to the main panel, which is the ONLY place it gets bonded to the EGC.
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Welcome to GJ . . . your best chance for accurate answer is to provide plenty of detailed information on your end.
Thus, What COUNTRY and whether you are in city limits of your STATE that may have specific electrical code restrictions that are tighter than overall state?? Now would be a good time to Update LOCATION in GJ Profile.
What wiring do you plan on using?? What are long-term goals and equipment to use in the detached garage?? How many circuits do you expect to have in detached garage??
Read prior threads as detached garage can ONLY have 1 source of electricity, . . . AND . . . it needs to be a 4 wire feed with seperate ground at the detached garage.
Post up bunch of pics as they tell a thousand words.
Thus, What COUNTRY and whether you are in city limits of your STATE that may have specific electrical code restrictions that are tighter than overall state?? Now would be a good time to Update LOCATION in GJ Profile.
What wiring do you plan on using?? What are long-term goals and equipment to use in the detached garage?? How many circuits do you expect to have in detached garage??
Read prior threads as detached garage can ONLY have 1 source of electricity, . . . AND . . . it needs to be a 4 wire feed with seperate ground at the detached garage.
Post up bunch of pics as they tell a thousand words.
brokenknee
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Do you need to run an home run from each grounding rod?
Read prior GJ threads . . . . . current code requres detached garage to have 2 ground rods with continuous solid copper wire #6 (or larger gauge) that goes from detached garage subpanel ground (NOT bonded with neutral in panel). Prior threads even show the acorn nut that should be used to attach copper ground wire to the ground rods.
brokenknee
Well-known member
I have read many of the previous threads on grounding subpanels. Many have referenced the two ground rods, non that I recall referenced running the wire back to the panel. I am sure it is out there if you say so, I just missed it. I just did another search and found six pages on grounding subpanels, I did not read through six pages of threads but looked at five or six and did not see it referenced. I did see this illustration http://garagejournal.com/forum/attachment.php?attachmentid=293380&d=1384148722
but that only shows one grounding point.
Thanks for you answer.
6768rogues
Banned
I am not an electrician and you sound knowledgeable, so please explain my shortcoming in understanding. The subpanel will have its own grounding system and will not be bonded to the neutral. Ok, I understand so far. Also, it is my understanding that two grounding systems should not be interconnected. Why does it need 4 conductor wire providing grounding from the main panel if that wire cannot be connected to the ground bar in the subpanel? Where should that ground wire from the main panel terminate?
I installed my system many years ago and I know the rules constantly change. When I did mine, I had two options: 1: Run 4 conductor to the subpanel, unbonded neutral to ground, and no separate grounding system. 2: Run 3 conductor (hot, hot, neutral) and put in a separate grounding system, again with no bonding.
I am not saying you are in error in any way, I simply don't understand. A little knowledge is dangerous.
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I should have made it clear in my original post that the 120 volt two wire (hot, neutral and ground) feed was already there. It ran a single circuit for a light in the garage and the garage door opener. The underground supply emanates from a 30 amp breaker on the main panel at the house. I wanted to add another circuit for a couple of small shop tools and incorporate GFI's in each circuit. I thought a subpanel in the garage with circuit breakers for each circuit would make more sense than what is there. I'm in a rural area so no one is looking over my shoulder but I don't wish to do something dangerous.
It seemed to me that this is just another circuit like all of the others that service the house itself. The subpanel in the garage is a plastic box. There is a grounding re-bar at the service entrance. Am I to connect this re-bar to something? .... if so what? ... a plastic box? Why isn't the house ground system adequate for this new circuit? I am using armored cable so everything appears to be well grounded in that respect.
It seemed to me that this is just another circuit like all of the others that service the house itself. The subpanel in the garage is a plastic box. There is a grounding re-bar at the service entrance. Am I to connect this re-bar to something? .... if so what? ... a plastic box? Why isn't the house ground system adequate for this new circuit? I am using armored cable so everything appears to be well grounded in that respect.
mrobins297aaa
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this is what I have to my 100 amp panel in my barn, no ground rods..........inspected and approved 2 years ago.
all the talk on this board about ground rods i haven't been able to figure out what there for?......anybody know?
JoeFin
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I'm pretty detached from residential work too
Seemed to me 25 yrs ago as long as the main service was less then 1000amps you could run to the detached residential outbuilding without a ground conductor and re-establish ground at the subpanel. If you had plumbing in the detached building it would have to be bonded as well
Now their describing adding to the existing service ground at the detached building - some thing most inspectors (in olden times) absolutely wouldn't allow
Seemed to me 25 yrs ago as long as the main service was less then 1000amps you could run to the detached residential outbuilding without a ground conductor and re-establish ground at the subpanel. If you had plumbing in the detached building it would have to be bonded as well
Now their describing adding to the existing service ground at the detached building - some thing most inspectors (in olden times) absolutely wouldn't allow
MrMark
Well-known member
they're just for lightning.
Kevin C
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As long as the neutral and the ground are not connected in the sub building. That was one of the few things my inspector checked.
Having an UFER to connect to made the install very neat and easy ( it was on the inside, right below the panel).
Charles (in GA)
Well-known member
What you currently have is a branch circuit from the house. The house ground is sufficient and allowed for this purpose (ref. NEC 250.32(A) exception). However, the moment you add a sub panel and use that circuit to supply other circuits, the branch circuit becomes a feeder circuit and the new garage circuits become branch circuits and additional grounding means are required. You then have to have the two ground rods separated by 6 ft min, or the Ufer ground in the slab. In either case, the wire connecting the two rods and the panel must be one single continuous, un-spliced wire. The only way to splice such a ground would be with permanent methods, one of which is exothermic welding (cad welding).
Charles
2ManyProjects
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No. As others have since explained, the connections to the ground rods must be made via ONE continuous AWG 6 (or heavier) wire, with no breaks between the ground bus in the panel and the last ground rod.
Well, not to be facetious, but is it not obvious that if they didn't connect back to the panel, they couldn't do any good?
I won't swear to this; but I'm about 90% certain that it should ALSO connect to the ground bus in the sub-panel. The thing to understand here is that we are talking ONLY about the EGC "Ground" (which, if all is right with the world, will NEVER actually carry current), and NOT the Neutral conductor (which is sometimes mistakenly referred to as a "Ground", probably because it DOES get tied to ground back at the main service panel). As such, it's primary purpose is to provide an alternate (i.e., "backup") path to ground in the event of a failure anywhere else in the system (particularly in the Neutral conductor). The point here being, to take all possible measures to ensure that the least-resistance path-to-ground is NEVER through a Human.
So, with a sub-panel in a detached structure, the EGC actually gets "tied to ground" (i.e., Planet Earth) in at least TWO places: Once at/near the main service panel, and again at/near the sub-panel (well, to be pedantic about it, near where the service enters the structure, in both cases). Think "belt & suspenders." And again, all of this is completely separate and distinct from the Neutral, which is exclusively fed from (and connected to "ground" ONLY at) the main service panel.
AFAIK, neither of those arrangements would meet code today; but the first one is closer (and far easier to make truly "code-compliant").
So, at least as of the moment, there is NO sub-panel in the garage, correct?
And I tend to agree with you. But adding that sub-panel brings with it some OTHER requirements, as "Charles (in GA)" explained quite well.
Even so, you want to do this RIGHT, or not at all.
HUH?!?
First, I thought we already established that there is currently NO sub-panel in the garage?
Second, I have NEVER seen a plastic subpanel. Ever. And while willing to be proven wrong, I seriously doubt such exists.
Are you sure that whatever it is you're (incorrectly, I'd wager) calling a "sub-panel" is really not just a junction box?
WHICH "service entrance"? If I understand the situation correctly, there is (currently) only one such place which really deserves the term -- i.e., back at the house.
To what, if anything, is that re-bar connected now? And is it REALLY "re-bar", or is it a proper copper (or copper-plated) ground rod, at least 8-10 feet long and driven FULLY into the Earth?
Because IF you have a sub-panel in the garage, you then have a "service entrance" for that structure, as opposed to simply one more (far-flung, but that is coincidental) branch circuit off the main service panel. And given that, you need a proper Earth ground for the EGC.
"Armored cable" has absolutely NOTHING to do with any of this.
That's certainly what it sounds like. But then he goes back to talking about ALREADY having a "sub-panel" in the garage (which I tend to doubt, but...) -- and a PLASTIC one, at that.
BINGO!
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justsam
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What you are describing is not a subpanel, and it is now fed with a branch circuit as has been pointed out. Think of it as a 30 Amp extension cord from your house.
If you establish a real subpanel, with breakers etc. than you will need to go to proper requirements for a feeder circuit, in your case due to the number of existing conductors it will be a 120V feeder, unless you add a fourth wire.
If you just use this as a branch circuit, it must be maintained as a 30 Amp branch circuit, which means all wiring will need to be 10 ga.
I am assuming the present wire to the garage which is on a 30 Amp breaker is at least 10 gauge.
If you do establish a 120v subpanel, then feeder circuit and subpanel rules come into play. If you have more than six branch circuits in the panel you will need a main breaker in your subpanel, in your case 30 Amp max. Less than six, no main required. Standard ampacity rules for branch circuits apply, e.g. 14 gauge protected by 15 Amp breaker and 12 gauge protected by 20 Amp breaker.
In regard to detached building ground, I would check with municipality but would lean to using the existing Ufer ground. Your original post suggested concern with creating "ground loops" and that is not of concern here.
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Alchymist
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Just for the record, the sub panel CB does not have to be 30 A max - can be larger, the circuit is protected by the 30 A breaker in the main panel. No limit on the sub panel main breaker size, it becomes just a disconnect.
justsam
Well-known member
Absolutely right! I stand corrected!
6768rogues
Banned
I asked a theoretical question that came to mind but did not accurately describe the situation of the original post. My real problem is understanding why one would have to run a ground conductor to the subpanel and also ground it at the subpanel with ground rods. I thought it was one or the other. Since the earth has different grounding potentials in different places, two grounding systems connected can try to balance that difference resulting in a ground conductor becoming a current carrying conductor. Additionally, the problem with lightning is that if it strikes near the building, it can put a charge in the ground in the immediate area. If one grounding system is in that area it can pick up the charge; if it is connected to another grounding system the ground conductor will carry that lightning charge to the other grounding system possibly destroying everything in its path. Am I wrong?
Alchymist
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More than you ever wanted to know about electrical grounding systems:
http://w5ea.com/FILES/Grounding.pdf
http://www.energyedcouncil.org/pdf/Event Presentations/Grounding and Bounding.pdf
http://www.pfeiffereng.com/Principals of Electrical Grounding.pdf
2ManyProjects
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Well... I'll be da*ed. Learn something new every day, and all that. But HOWINHELL are they getting "20-Circuit"s out of a "4-Space" Load Center?!?
Alchymist
Well-known member
Pretty common in the RV industry:
http://rvpowerpartsplus.net/index.p...id=340&zenid=2a43f89b67dbfd310996c0b3d071c0e8
Well, all of this grounding business is quite confusing. The job is done. The plastic subpanel is installed and the feeder cirucit (120 volt) is treated as simply an extension of the house circuit to the garage. This feeder circuit eminated from a 30 amp breaker in the house main panel. At the garage "box" (call it what you may) the feeder circuit branches to two circuit breakers, each feeding a separatea GFI protected circuits. All of the outlets test correctly and everything seems to be working fine an happily grounded. I'd attach a picture of the plastic installation but this site does not appear to accept photos from personal files (http// ...????).
I never did figure out to what I should attach the rebar sunk into the garage slab next to the underground conduit that brings in the feed line from the house. Lacking any other inspiration, I bonded the rebar to the metal building itself and hope it will act as a lighting gound. It is probably also acting as a second ground of some sort as all of the circuits are grounded to the metal duplex boxes which inturn are intergal to the steel building structure.
I never did figure out to what I should attach the rebar sunk into the garage slab next to the underground conduit that brings in the feed line from the house. Lacking any other inspiration, I bonded the rebar to the metal building itself and hope it will act as a lighting gound. It is probably also acting as a second ground of some sort as all of the circuits are grounded to the metal duplex boxes which inturn are intergal to the steel building structure.
2ManyProjects
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I suppose it can be. But it's easy enough to noodle out, as long as you are careful to keep the context straight as you go, and follow a few simple rules.
So far, so good, even if a 120V "sub-panel" remains a rather unusual application.
In which case, that "plastic box" IS effectively a sub-panel, by definition.
Actually, from your later comments, I tend to doubt that. Please DO NOT confuse "Ground" and "Neutral". Even on a 120V (only) circuit, they are two different things. The fact that the outlets work indicates only that the Hot and Neutral conductors are functioning; it implies NOTHING about the Ground.
It can be (and is, regularly) done; but there are restrictions vis-a-vis file size, image size, etc.
From that description, it sounds like you have the makings of a Ufer Ground system (cf. http://en.wikipedia.org/wiki/Ufer_ground). If so, it could/would take the place of the two separate ground rods discussed previously. (Some folks even consider it preferable.)
Well, it MIGHT do that. But it DOES NOT take the place of properly bonding the sub-panel's Ground bus to the Ufer ground and/or to two properly installed Ground rods.
I should hope not. Or rather, if you are counting on the skinny little bare-copper wire connections inside those boxes to substitute for directly bonding the Ground bus to the Earth ground, you are at least potentially setting yourself up for a serious problem (and I would be utterly amazed if it passed inspection).