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Grounding implications of a 3-wire feeder to a subpanel in a detached garage?
Old 15th August 2015
  #1
Grounding implications of a 3-wire feeder to a subpanel in a detached garage?

First, let me describe my situation and then I'll move onto the questions. I have a garage which I am converting into a studio. I have a 3-wire feeder (two hots and a neutral, no ground) traveling about 70 feet from the main panel at the house, inside metal conduit underneath the house overhang and then underground to the garage and its subpanel. All three wires are 4 awg. I'm aware that a 3-wire feeder without a dedicated ground used to be allowed between two detached buildings, provided no other conductive paths like plumbing or a phone line exists between the buildings, which the metal conduit already violates. Though my situation is grandfathered, I'm aware that modern code now requires a dedicated ground (4-wire feeder).

I'd also like to make it clear that this conduit is not currently being used as a ground connection itself between the subpanel and main panel. Additionally, grounds and neutrals are present on both of the bus bars in the subpanel. These two bus bars in the sub panel are bonded together and various ground and neutral connections from receptacles are present on both busses.

So I have two options. My first option is to leave the system as is but I have questions about how this will impact my overall grounding scheme in my studio as far as ground loops and noise goes and I also am concerned about overall safety and the potential for frying my equipment if the neutral connection between the subpanel and main panel were to ever fail. My other option is to replicate what is required in new builds today and attempt to snake a dedicated ground wire down the conduit, make the appropriate connections and then separate grounds and neutrals in the subpanel onto separate busses, removing the bond between the two busses as a last step.

So what would you do? It would be nice to leave things as they are because it would cost no money or time but I do have concerns. Am I overly concerned here or is this just something that I really need to do to make sure I won't have any issues with noise related to grounding? If I were to do this, a 4 awg ground wire should be sufficent right? Or should I be looking to go bigger? Any other ideas? Any help would be greatly appreciated. Thanks.

Last edited by Quint; 15th August 2015 at 11:26 PM..
Old 16th August 2015
  #2
You could get a TOPAZ or other isolation transformer big enough to run all your equipment and put in a star ground to an outside ground rod for your equipment. Best to consult an good electrical contracting firm, you do not want to be in violation of any codes if something would happen and someone got electrocuted or shocked. I leave all my electrical work to a licensed electrician.
Old 16th August 2015
  #3
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As long as you have a local ground (which to the best of my knowledge is still a code requirement) then you're fine.

A ground IS required - but not a ground that runs back to the main panel.

Rod
Old 16th August 2015
  #4
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a] Is the conduit all metal and continuous from end-to-end?

b] The Safety Ground (EGC) Equipment Grounding Conductor should be the same size as the Hot & Neutral. Larger would be OK but hard to run.

c] Why not head over to the 'AC Power and Grounding' sub-forum and post the question there?
AC Power and Grounding
Old 16th August 2015
  #5
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By the way - just to make sure everything's clear........ when you have a sub-panel located in a separate building with an independent ground - you need to remove the bond between the neutral and ground in the sub panel.

The answer can be found in this thread at Mike Holt's website:

grounds and neutrals

Rod
Old 16th August 2015
  #6
Quote:
Originally Posted by Rod Gervais View Post
By the way - just to make sure everything's clear........ when you have a sub-panel located in a separate building with an independent ground - you need to remove the bond between the neutral and ground in the sub panel.

The answer can be found in this thread at Mike Holt's website:

grounds and neutrals

Rod
Agreed and that's exactly what I was planning on doing if/when I decided to add a dedicated ground back to the main panel. Also, the Mike Holt forum is great. I've always learned a lot there.

As for your previous comments Rod, I don't have a dedicated ground rod as far as I can tell for the subpanel. The metal conduit underneath ground may be good enough to act as one but I wouldn't count on it and, besides, it's not bonded to the grounds inside the subpanel, nor could it be as the ground wires from the receptacles are already on the same bus bar as the neutrals.

I'm attaching two pics. One pic is of a three wire feeder system and is exactly what I currently have except for three things. One, the neutral/ground bus doesn't appear to be bonded to the subpanel housing itself. Two, there also doesn't appear to be a ground rod or any connection to one from the neutral/ground bus. Three is that, as indicated in the pic, you're not supposed to have a second conductive path back to the house or main panel but, in my case, I do in the form of the metal conduit. The second pic is of a four wire feeder and is exactly what I would be attempting to recreate if I went that route.
Attached Thumbnails
Grounding implications of a 3-wire feeder to a subpanel in a detached garage?-3-wire-feeder.jpg   Grounding implications of a 3-wire feeder to a subpanel in a detached garage?-4-wire-feeder.jpg  
Old 16th August 2015
  #7
Quote:
Originally Posted by Speedskater View Post
a] Is the conduit all metal and continuous from end-to-end?

b] The Safety Ground (EGC) Equipment Grounding Conductor should be the same size as the Hot & Neutral. Larger would be OK but hard to run.

c] Why not head over to the 'AC Power and Grounding' sub-forum and post the question there?
AC Power and Grounding
The conduit is all metal and continuous from end to end. At least I believe so but, IF there happened to be a separation in the underground portion or at one of the junctions, I would have no idea. I can tell where you're going with this, as far as using the conduit itself as a ground, and I had considered it myself but it seems less than ideal and makes a few assumptions about its overall integrity that I'm not sure I'm comfortable making.

4 awg is what I was planning on doing if I did add a fourth wire but pulling any wire over that distance through conduit that is already a little tight with the existing wires is going to be a bitch.

I'll try the prosound site as well. Though, I'm hoping some master Gearslutz will have all the info I need.

Last edited by Quint; 16th August 2015 at 03:32 PM..
Old 16th August 2015
  #8
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Quote:
Originally Posted by Quint View Post
Agreed and that's exactly what I was planning on doing if/when I decided to add a dedicated ground back to the main panel. Also, the Mike Holt forum is great. I've always learned a lot there.

As for your previous comments Rod, I don't have a dedicated ground rod as far as I can tell for the subpanel. The metal conduit underneath ground may be good enough to act as one but I wouldn't count on it and, besides, it's not bonded to the grounds inside the subpanel, nor could it be as the ground wires from the receptacles are already on the same bus bar as the neutrals.

I'm attaching two pics. One pic is of a three wire feeder system and is exactly what I currently have except for three things. One, the neutral/ground bus doesn't appear to be bonded to the subpanel housing itself. Two, there also doesn't appear to be a ground rod or any connection to one from the neutral/ground bus. Three is that, as indicated in the pic, you're not supposed to have a second conductive path back to the house or main panel but, in my case, I do in the form of the metal conduit. The second pic is of a four wire feeder and is exactly what I would be attempting to recreate if I went that route.
You need to add a grounding bar to your panel - you should not be tying both ground and neutral at the same location.

The you add a ground rod for the remote panel.

I completely disagree with the approach used in the 2nd sketch.

If providing a local ground for the remote panel - running an additional grounding conductor back to the main panel is not necessary.

Rod
Old 16th August 2015
  #9
And now that we've dived into this a little bit, let me say that my emphasis is on control of ground noise and potential ground loops. It's not that safety is unimportant to me but I do consider there to be a difference between things that are absolutely unsafe and those that are maybe just considered less than ideal such as three wire vs four wire feeder. I mean three wire feeders were allowed until the mid 2000's. Is there anything seriously unsafe about my current system besides maybe the lack of a ground rod at the sub panel? As for overall sound and ground noise issues, is there anything fundamentally different or better about the four wire system versus the three wire system I have?

Fyi, this all started because I was wiring up a couple of new dedicated 20 amp circuits to power a pair of large double conversion online UPS's which will power the entire studio. I employ a star grounding system by way of using the safety ground as my only path to ground for each piece of equipment. I lift shields on audio lines at all inputs and isolate gear in the racks with nylon washers.
Old 16th August 2015
  #10
Quote:
Originally Posted by Rod Gervais View Post
You need to add a grounding bar to your panel - you should not be tying both ground and neutral at the same location.

The you add a ground rod for the remote panel.

I completely disagree with the approach used in the 2nd sketch.

If providing a local ground for the remote panel - running an additional grounding conductor back to the main panel is not necessary.

Rod
But the local grounding rod at the subpanel is mainly just in the event of lightening. You still need a proper connection back to the power company's main lines. I totally understand why you don't want the grounds and neutrals bonded at more than one location when they each have their own separate path to both locations such as in the second picture (4 wire). It would cause all equipment to energize in the event of a failure in the neutral plus you would have parallel paths (ground and neutral) always carrying current back to the main service panel.

However, in the three wire system like I currently have, the safety grounds from receptacles in the garage have to connect somewhere. A ground rod and the earth are too high in resistance to trip the breaker in the event of a fault so the neutral and safety grounds have to bond together in a three wire subpanel so that there is a path sufficiently low in resistance to trip the breaker in the event of a fault. I'm not following what the objection to this is?

Last edited by Quint; 16th August 2015 at 04:30 PM..
Old 16th August 2015
  #11
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Quote:
Originally Posted by Quint View Post
But the local grounding rod at the subpanel is mainly just in the event of lightening. You still need a proper connection back to the power company's main lines. I totally understand why you don't want the grounds and neutrals bonded at more than one location when they each have their own separate path to both locations such as in the second picture (4 wire). It would cause all equipment to energize in the event of a failure in the neutral plus you would have parallel paths (ground and neutral) always carrying current back to the main service panel.

However, in the three wire system like I currently have, the safety grounds from receptacles in the garage have to connect somewhere. A ground rod and the earth are too high in resistance to trip the breaker in the event of a fault so the neutral and grounds have to bond together in a three wire subpanel. I'm not following what the objection to this is?
It would not be code compliant if it was installed today.

If you simply add a local ground rod and properly ground the remote panel there is no issue as long as the neutral and grounding bars are not bonded.

I have to believe that it would be a whole heck of a lot less expensive for you to add a buss bar and a local ground than it would be for you to run a ground wire all the way back to the main panel.

Having said that - if you have an existing electrical system that was compliant at the time it was installed - and it works for you - then leave it alone and don't worry about it. Unless you are changing it you are not required to bring it up to code.

Rod
Old 16th August 2015
  #12
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Quote:
Originally Posted by Quint View Post
But the local grounding rod at the subpanel is mainly just in the event of lightening. You still need a proper connection back to the power company's main lines. I totally understand why you don't want the grounds and neutrals bonded at more than one location when they each have their own separate path to both locations such as in the second picture (4 wire). It would cause all equipment to energize in the event of a failure in the neutral plus you would have parallel paths (ground and neutral) always carrying current back to the main service panel.

However, in the three wire system like I currently have, the safety grounds from receptacles in the garage have to connect somewhere. A ground rod and the earth are too high in resistance to trip the breaker in the event of a fault so the neutral and grounds have to bond together in a three wire subpanel. I'm not following what the objection to this is?
Listen - you came here to ask a question........ and I've answered it.......

If what you want is to argue that you don't need to change anything because you like things the way they are - then why bother to ask the question in the first place?

As I said above - if what you have was code compliant when it was installed - and you want to leave it - then leave it the way it is.

If what you have was installed after code changes were made and is therefore not code compliant - then make it code compliant.

If the question is "why should I follow code when I think it was OK the way things used to be?" then it's pretty much useless continuing the conversation.

Rod
Old 16th August 2015
  #13
Quote:
Originally Posted by Rod Gervais View Post
It would not be code compliant if it was installed today.

If you simply add a local ground rod and properly ground the remote panel there is no issue as long as the neutral and grounding bars are not bonded.

I have to believe that it would be a whole heck of a lot less expensive for you to add a buss bar and a local ground than it would be for you to run a ground wire all the way back to the main panel.

Having said that - if you have an existing electrical system that was compliant at the time it was installed - and it works for you - then leave it alone and don't worry about it. Unless you are changing it you are not required to bring it up to code.

Rod
I think we have a misunderstanding here. Possibly due to terminology. I agree it wouldn't be code compliant if built today. What I'm asking is how unbonding the neutrals and safety grounds in the subpanel and, instead, bussing all safety grounds to a ground rod, as you advise, will allow the breaker to trip if a fault occurs or how this would meet the code standards of today? If a fault occurs, the safety ground is supposed to be there to provide a low resistance path for electricity to alternatively flow and flow enough to trip the breaker. I'm not seeing how sending all safety grounds to just a local ground rod will provide this capability? The discussion in that Mike Holt link you posted backs up the very question I'm asking. Are you sure you're not mistaken? I'm not here to argue but I am asking for a little clarification.

Last edited by Quint; 17th August 2015 at 03:03 AM..
Old 17th August 2015
  #14



Rod,

I've read up more on your recommendation and I have to say, it really does seem unsafe. Are you sure we are talking about the same set of connections? Watch the video link above. It confirms what I was already trying to explain earlier. Best as I can tell, you might as well not even have safety grounds if you're just going to run them to a local grounding rod. Everything I've read on the subject, especially at the Mike Holt forum, indicates that grounding rods really are for protecting against large surges like lightning. Otherwise, grounding rods just don't seem like they provide a path low enough in resistance to trip the breaker.

Last edited by Quint; 17th August 2015 at 12:59 AM..
Old 17th August 2015
  #15
And from:

Mobile Site Preview

"Grounding" has 2 main functions.

One is to provide a path to trip a breaker in the event of a 'short' as in the text above. That function relies on a "ground"-to-neutral connection required at services in the US (the "main bonding jumper"). The path is (branch circuit ground wire) to (N-G bond at the service) to (service neutral) to (utility power transformer).

This path *must* be metallic back to the power transformer to provide low resistance to trip a circuit breaker. This function will work even if the service is not connected to earth. And the NEC *does not allow* earth to be used as part of this path.

One reason is the resistance of an earth path is too high. Assume the earthing is only through a ground rod and the rod has a quite good 10 ohms resistance to earth. Further assume there is a 'short' connecting hot to "ground". The current to earth will be 12A. There is a good chance this won't even trip a 15A circuit breaker. If the circuit is loaded the breaker will trip, but after a significant time delay. In the mean time, the "ground" potential with respect to the earth away from the ground rod will be 120V.

Note that if you are using the earth as in the quote above, the path is not just into the earth. It is back to the power source, and also depends on the earth connection at the power transformer.

This would be better termed a *bonding* function.



So despite the apparent disagreements with Rod...

I reiterate my original questions. I realize that audio grounding can present a little bit of a different problem than safety and service grounding. My hopes all along have been to get any insight into potential AUDIO ground problems created by the SAFETY grounding I have in place at the moment (3 wire feeder system) and what if any AUDIO ground problems may improve or be eliminated by adding a dedicated SAFETY ground (4 wire feeder system) back to the main service panel from the subpanel in my garage. Anyone have any thoughts on this? That includes you too Rod. I think you must have just been cranky this morning.

Last edited by Quint; 17th August 2015 at 03:10 AM..
Old 17th August 2015
  #16
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Actually - no I wasn't being cranky.

I don't bother with trying to understand everything that goes along with code changes........ and the reason for this is multifold.

If all I was involved with was the electrical code it wouldn't be a big deal........ but I deal with (literally) hundreds of different code. From building to fire and handicapped..... with everything involved in them. I do that while working full time days (right now anyway) on a relatively large project that I didn't design; doing design review of the owner's engineers and architects as well as the designs/coordination drawings (along with product submittals) provided by the various subcontractors on the project.

When I'm not doing my day job I'm working on my own design projects. I also do quite a bit of code consulting for architectural/engineering firms and for various builders. And in the midst of all of this - I find some time to offer a tidbit here (and on a couple of other sites) So I have a limited amount of time to study the "why's" of the code.

For me it's pretty simple - the code has requirements - they are specific - why they exist is meaningless....... all that matters is that they exist. So with all of that - the only time I ever start digging into codes to understand "why" is when I read them and they are not (to me) crystal clear.

We have a code - if you choose to follow it, follow it - if not, then don't. But I truly see not logic to asking the question if you already have your mind set on what you want to do.

This is what the current version of NFPA 70A (which deals with 1 and 2 family homes) has to say on the matter:

Quote:
250.32 Buildings or Structures Supplied by Feeder(s) or Branch Circuits.

(A) Grounding Electrode. Building(s) or structure(s) supplied by feeder(s) or branch circuit(s) shall have a grounding electrode system installed in accordance with 250.5. The grounding electrode conductor(s) shall be connected in accordance with 250.3.2(B). Where there is no existing grounding electrode, the grounding electrode(s) required in 250.5 shall be installed.

Exception: A grounding electrode shall not be required where only a single branch circuit supplies the building and the branch circuit includes an equipment grounding conductor for grounding the conductive non-current-carrying parts of equipment. For the purpose of this section a multi wire branch circuit shall be considered as a single branch circuit.
Like I said - you can do as you wish...... but for me it all begins (and ends) with the code requirements.

Take care,

Rod
Old 17th August 2015
  #17
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To code you connect the earth ground to the main AC ground a the Main panel OR meter ONLY, after that ALL subpanels with 240V NEEDS 4 wires, 2 Hot, neutral and Ground that is NOT bonded to the neutral...
You MUST have a "Ground fault current path" in the event of a short to ground..
Connecting an isolated ground (Ground rod) Only to a sub panel provides NO "Ground fault current path"...
Have seen master electricians & Inspectors MISS this before, and where there WAS
a short to ground, Did NOT trip the breaker like it should have... Had 120V on ALL grounds/chassis in the Garage/Studio...NOT GOOD...
Can call the guy and ask him if you Don't believe me...
Old 17th August 2015
  #18
Quote:
Originally Posted by nosebleedaudio View Post
To code you connect the earth ground to the main AC ground a the Main panel OR meter ONLY, after that ALL subpanels with 240V NEEDS 4 wires, 2 Hot, neutral and Ground that is NOT bonded to the neutral...
You MUST have a "Ground fault current path" in the event of a short to ground..
Connecting an isolated ground (Ground rod) Only to a sub panel provides NO "Ground fault current path"...
Have seen master electricians & Inspectors MISS this before, and where there WAS
a short to ground, Did NOT trip the breaker like it should have... Had 120V on ALL grounds/chassis in the Garage/Studio...NOT GOOD...
Can call the guy and ask him if you Don't believe me...
Exactly.
Old 17th August 2015
  #19
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Quote:
Originally Posted by Rod Gervais View Post
It would not be code compliant if it was installed today.

If you simply add a local ground rod and properly ground the remote panel there is no issue as long as the neutral and grounding bars are not bonded.

I have to believe that it would be a whole heck of a lot less expensive for you to add a buss bar and a local ground than it would be for you to run a ground wire all the way back to the main panel.

Having said that - if you have an existing electrical system that was compliant at the time it was installed - and it works for you - then leave it alone and don't worry about it. Unless you are changing it you are not required to bring it up to code.

Rod
As long as it was to code AND SAFE in the first place...read my earlier post...
Old 17th August 2015
  #20
Quote:
Originally Posted by Rod Gervais View Post
Actually - no I wasn't being cranky.

I don't bother with trying to understand everything that goes along with code changes........ and the reason for this is multifold.

If all I was involved with was the electrical code it wouldn't be a big deal........ but I deal with (literally) hundreds of different code. From building to fire and handicapped..... with everything involved in them. I do that while working full time days (right now anyway) on a relatively large project that I didn't design; doing design review of the owner's engineers and architects as well as the designs/coordination drawings (along with product submittals) provided by the various subcontractors on the project.

When I'm not doing my day job I'm working on my own design projects. I also do quite a bit of code consulting for architectural/engineering firms and for various builders. And in the midst of all of this - I find some time to offer a tidbit here (and on a couple of other sites) So I have a limited amount of time to study the "why's" of the code.

For me it's pretty simple - the code has requirements - they are specific - why they exist is meaningless....... all that matters is that they exist. So with all of that - the only time I ever start digging into codes to understand "why" is when I read them and they are not (to me) crystal clear.

We have a code - if you choose to follow it, follow it - if not, then don't. But I truly see not logic to asking the question if you already have your mind set on what you want to do.

This is what the current version of NFPA 70A (which deals with 1 and 2 family homes) has to say on the matter:



Like I said - you can do as you wish...... but for me it all begins (and ends) with the code requirements.

Take care,

Rod
I agree 100% about the codes. The only thing I've already made my mind up about is my intent to follow them. However, I think you're missing the point. I've never disagreed with you about putting a grounding rod at my subpanel. I intend to. Where we disagreed is that you suggested that I could just remove my safety grounds from the neutral bar they were on and put them on a new ground bus and attach that ONLY to a ground rod. Your suggestion causes one code to now be met but violates another in the process. Per code, it's necessary for your safety grounds to also have a properly conductive path, be it a dedicated ground wire in a 4 wire feeder or the neutral in a 3 wire feeder, back to the service pole so that your breakers will trip in the event of a fault. I stand by that statement and we can agree to disagree I guess.


So anyway, in hopes of moving on, I'll quote my previous questions below:

Quote:
Originally Posted by Quint View Post
I realize that audio grounding can present a little bit of a different problem than safety and service grounding. My hopes all along have been to get any insight into potential AUDIO ground problems created by the SAFETY grounding I have in place at the moment (3 wire feeder system) and what if any AUDIO ground problems may improve or be eliminated by adding a dedicated SAFETY ground (4 wire feeder system) back to the main service panel from the subpanel in my garage.
So, anyone?

Last edited by Quint; 17th August 2015 at 02:49 PM..
Old 17th August 2015
  #21
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Quote:
Originally Posted by Quint View Post
I agree 100% about the codes. The only thing I've already made my mind up about is my intent to follow them. However, I think you're missing the point. I've never disagreed with you about putting a grounding rod at my subpanel. I intend to. Where we disagreed is that you suggested that I could just remove my safety grounds from the neutral bar they were on and put them on a new ground bus and attach that ONLY to a ground rod. It's necessary for your safety grounds to also have a properly conductive path, be it a dedicated ground wire in a 4 wire feeder or the neutral in a 3 wire feeder, back to the service pole so that your breakers will trip in the event of a fault. I stand by that statement and we can agree to disagree I guess.
You can NOT use the Neutral as a Ground...

Let me add to my post #17 ; the subpanel did NOT have 4 wires from the Main panel, someone added a ground rod to the Sub panel as the Ground connected to the Ground bus in the Subpanel, this is NOT a "ground fault current path"....
Like I said Several electricians did NOT catch this, I did when the client was having strange problems with some circuits...
Even when the Master electrician came out to correct the problem he still did NOT understand what was wrong...He pointed to the ground rod out side the separate building and said " Its grounded"....It was NOT...Many do not understand this...

Last edited by nosebleedaudio; 17th August 2015 at 02:42 PM..
Old 17th August 2015
  #22
Quote:
Originally Posted by nosebleedaudio View Post
You can NOT use the Neutral as a Ground...

Let me add to my post #17 ; the subpanel did NOT have 4 wires from the Main panel, someone added a ground rod to the Sub panel as the Ground connected to the Ground bus in the Subpanel, this is NOT a "ground fault current path"....
Like I said Several electricians did NOT catch this, I did when the client was having strange problems with some circuits...
Even when the Master electrician came out to correct the problem he still did NOT understand what was wrong...He pointed to the ground rod out side the separate building and said " Its grounded"....It was NOT...Many do not understand this...
I agree. In the case of a 3 wire feeder, I was referring to what used to be allowed before the code changed. As you said, any new build or update would require, per current code, a dedicated ground as part of a 4 wire feeder.

Last edited by Quint; 17th August 2015 at 02:58 PM..
Old 17th August 2015
  #23
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nosebleedaudio's Avatar
Quote:
Originally Posted by Quint View Post
I agree. In the case of a 3 wire feeder, I was referring to what used to be allowed before the code changed. As you said, any new build or update would require, per current code, a dedicated ground as part of a 4 wire feeder.
OK, just wanted to make sure..Not trying to be a jerk..But it's VERY important for safety & code...
Old 17th August 2015
  #24
Quote:
Originally Posted by nosebleedaudio View Post
OK, just wanted to make sure..Not trying to be a jerk..But it's VERY important for safety & code...
I'm glad I read up on the subject. It sure would have been easier to do what Rod suggested but it also would have been very unsafe. Now I just need to decide how bad I really want to snake a fourth wire down 75 feet of 1" conduit. Besides adding a grounding rod at the subpanel, I'm tempted to leave it as is, at least for now. Which brings me full circle and back to the whole point of the thread, as stated in my questions quoted below:

Quote:
Originally Posted by Quint View Post
I realize that audio grounding can present a little bit of a different problem than safety and service grounding. My hopes all along have been to get any insight into potential AUDIO ground problems created by the SAFETY grounding I have in place at the moment (3 wire feeder system) and what if any AUDIO ground problems may improve or be eliminated by adding a dedicated SAFETY ground (4 wire feeder system) back to the main service panel from the subpanel in my garage.
So does anyone have any thoughts on this?

Last edited by Quint; 17th August 2015 at 03:18 PM..
Old 17th August 2015
  #25
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nosebleedaudio's Avatar
Quote:
Originally Posted by Quint View Post
I'm glad I read up on the subject. It sure would have been easier to do what Rod suggested but it also would have been very unsafe. Now I just need to decide how bad I really want to snake a fourth wire down 75 feet of 1" conduit. Besides adding a grounding rod at the subpanel, I'm tempted to leave it as is, at least for now. Which brings me full circle and back to the whole point of the thread, as stated in my questions quoted below:



So does anyone have any thoughts on this?
There is no difference in any GOOD correct ground...
Have you ever been shocked by an Ungrounded Gtr amp?
I have measured over 100V between the AC Ground and the chassis of the Ungrounded Gtr Amp....
Connecting THAT Ungrounded Amp to your Grounded Audio means Now you are routing that current thru your nice clean audio gear on its way to the AC Ground point....

I insist (On studios I was involved with) that ALL AC grounds are connected to the chassis of all gear for this and other reasons...NO chassis grounds are lifted...And all were clean and quiet...
Old 17th August 2015
  #26
Quote:
Originally Posted by nosebleedaudio View Post
There is no difference in any GOOD correct ground...
Have you ever been shocked by an Ungrounded Gtr amp?
I have measured over 100V between the AC Ground and the chassis of the Ungrounded Gtr Amp....
Connecting THAT Ungrounded Amp to your Grounded Audio means Now you are routing that current thru your nice clean audio gear on its way to the AC Ground point....

I insist (On studios I was involved with) that ALL AC grounds are connected to the chassis of all gear for this and other reasons...NO chassis grounds are lifted...And all were clean and quiet...
I'm wiring up a couple of new dedicated 20 amp circuits to power a pair of large double conversion online UPS's which will power the entire studio. I employ a star grounding system by way of using the safety ground as my only path to ground for each piece of equipment. I lift shields on audio lines at all inputs and isolate gear in the racks with nylon washers.

I was basically just wondering if noise or any other audio improvements might occur by adding a dedicated ground wire back to the main service panel, as previously discussed, versus leaving it as it currently is with the 3 wire feeder.

Last edited by Quint; 17th August 2015 at 04:41 PM..
Old 17th August 2015
  #27
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Rod Gervais's Avatar
 

Quote:
Originally Posted by Quint View Post
I'm glad I read up on the subject. It sure would have been easier to do what Rod suggested but it also would have been very unsafe. Now I just need to decide how bad I really want to snake a fourth wire down 75 feet of 1" conduit. Besides adding a grounding rod at the subpanel, I'm tempted to leave it as is, at least for now. Which brings me full circle and back to the whole point of the thread, as stated in my questions quoted below:



So does anyone have any thoughts on this?
Sorry - but what I suggested is not unsafe...... removal of the bond between ground and neutral at the sub panel is required in your case should you add a ground at the sub panel.

This from Mike Holt (not a general discussion of concepts - but (rather) his response to a question as to the requirements of the code as relates to this subject. Mike Holt is (in my opinion anyway) the best of the best when it comes to electrical code interpretation.

Quote:
(Question Q4. Grounding Remote Building and Structures

NEC 250.32 allows the grounded (neutral) circuit conductor supplying a second building to be bonded to the equipment grounding conductor at the remote building grounding electrode system of the second building. If the two buildings are electrically connected via a conductive path, such as the shielding of a coax cable or water pipe, this would provide an alternate path for neutral current to flow during normal operation (like resistors in parallel). Is this a trivial concern? Do you know of any problems that can arise from such a situation such as GFI relays in the main service? All those whom I have discussed this with have dismissed it as trivial. They site residential houses connected via a common water line. I would like to hear your comments.
;/quote]

His response:
A4. The practice of bonding the grounded (neutral) conductor to the equipment enclosure is only permitted by 250.32(B)(2) where

(1) an equipment grounding conductor is not run with the supply to the building or structure, and

(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, and

(4) the size of the grounded conductor shall not be smaller than the larger of:

That required by 220.22 (maximum unbalanced neutral load), or

That required by 250.122 (equipment grounding conductor size).


When an equipment grounding conductor is not run to a separate building or structure, the grounded (neutral) conductor must be used to provide the effective ground-fault current path required to clear any ground-faults (line-to-case faults) in addition to carrying any unbalanced neutral current [250.4(A)(3)].
[/QUOTE]

The bold underlined "must be used" is mine for clarity.

Your installation does not meet those 4 criteria according to what you've stated here.

Rod
Old 17th August 2015
  #28
Rod, these are your quotes:

Quote:
Originally Posted by Rod Gervais View Post
You need to add a grounding bar to your panel - you should not be tying both ground and neutral at the same location.

The you add a ground rod for the remote panel.

I completely disagree with the approach used in the 2nd sketch.

If providing a local ground for the remote panel - running an additional grounding conductor back to the main panel is not necessary.

Rod
Quote:
Originally Posted by Rod Gervais View Post
It would not be code compliant if it was installed today.

If you simply add a local ground rod and properly ground the remote panel there is no issue as long as the neutral and grounding bars are not bonded.

I have to believe that it would be a whole heck of a lot less expensive for you to add a buss bar and a local ground than it would be for you to run a ground wire all the way back to the main panel.

Having said that - if you have an existing electrical system that was compliant at the time it was installed - and it works for you - then leave it alone and don't worry about it. Unless you are changing it you are not required to bring it up to code.

Rod
Again, you're missing the point. Yes, ground and neutral should not be bonded in a 4 wire feeder scenario. But you were suggesting, for my current 3 wire feeder, that it would be fine and safe to simply unbond the grounds from the neutrals in the subpanel and place these grounds on a separate bus bar which is ONLY connected to a ground rod. You are contradicting yourself by providing the Mike Holt quotes and reference to the code.

A ground rod will NOT, by itself, provide sufficient means to trip the breaker in the event of a fault. I'm sorry but this IS unsafe.

Last edited by Quint; 17th August 2015 at 04:25 PM..
Old 17th August 2015
  #29
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Rod Gervais's Avatar
 

Quote:
Originally Posted by Quint View Post
Rod, these are your quotes:





Again, you're missing the point. Yes, ground and neutral should not be bonded in a 4 wire feeder scenario. But you were suggesting, for my current 3 wire feeder, that it would be fine and safe to simply unbond the grounds from the neutrals in the subpanel and place these grounds on a separate bus bar which is ONLY connected to a ground rod.

This IS unsafe.
Sorry - read what Mike had to say again - you're missing it. Is says "only permitted when" - what part of that is difficult to understand?

You told us " I know you're not supposed to have a second conductive path back to the house or main panel but, in my case, I do in the form of the metal conduit"

You have this completely backwards.......

He says that you must meet those 4 requirements before it (bonding of the neutral and ground at the subpanel) is permitted.... if you don't meet all 4 requirements it is NOT permitted.

You do not meet those requirements - it is (therefor) not permitted.

Rod
Old 17th August 2015
  #30
Quote:
Originally Posted by Rod Gervais View Post
Sorry - read what Mike had to say again - you're missing it. Is says "only permitted when" - what part of that is difficult to understand?

You told us " I know you're not supposed to have a second conductive path back to the house or main panel but, in my case, I do in the form of the metal conduit"

You have this completely backwards.......

He says that you must meet those 4 requirements before it (bonding of the neutral and ground at the subpanel) is permitted.... if you don't meet all 4 requirements it is NOT permitted.

You do not meet those requirements - it is (therefor) not permitted.

Rod
I understand all of this. I have never said that my current situation meets all of the requirements you listed and corrections are definitely in order. We are not in disagreement there but it is beside the point. What we are disagreeing about is your proposed solution, which does NOT meet code and IS unsafe, in fact, even more unsafe than what I already have. I think you have a misunderstanding of what a ground rod does in an electrical system.

Quote:
Originally Posted by Rod Gervais View Post
When an equipment grounding conductor is not run to a separate building or structure, the grounded (neutral) conductor must be used to provide the effective ground-fault current path required to clear any ground-faults (line-to-case faults) in addition to carrying any unbalanced neutral current [250.4(A)(3)].
As per the code you quoted above, the neutral and safety grounds have to be bonded together when a 3 wire feeder is used. This is necessary to carry ground fault current away. Your proposed solution violated this by suggesting that I could just put all of the grounds on a separate bus and run them only to a ground rod. Breakers will not trip in your proposed solution. That is the definition of unsafe.
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