To Ground or Not to Ground

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Zorch

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Okay, I'm puzzled. I've got two electrical inspectors saying it is legal to run just three wires (single-phase 120/240 service) from a "main panel" to a "sub-panel" at a different location. I thought we ALWAYS have to run that fourth (grounding) conductor.

Both inspectors say that as long as the sub-panel has a main breaker and two ground rods it's okay to just run two phase conductors and one grounded conductor - just don't bond the panel with that "little green screw" - that would be the main bonding jumper.

I understand isolating ground fault, but I still can't find where such a set-up is legal by NEC standards. Am I losing my mind?
 
Zorch said:
Okay, I'm puzzled. I've got two electrical inspectors saying it is legal to run just three wires (single-phase 120/240 service) from a "main panel" to a "sub-panel" at a different location. I thought we ALWAYS have to run that fourth (grounding) conductor.
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As long as it's to a separate building, and with no other conductive pathways between the buildings, the 3-wire feeder may be used prior to the 2008 NEC.

Both inspectors say that as long as the sub-panel has a main breaker and two ground rods it's okay to just run two phase conductors and one grounded conductor - just don't bond the panel with that "little green screw" - that would be the main bonding jumper.
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Again, in a separate building, the main (or a max of six handles) is required anyway. In the case of a 3-wire feeder, you must bond with a MBJ; with a 4-wire only, don't bond.

I understand isolating ground fault, but I still can't find where such a set-up is legal by NEC standards. Am I losing my mind?
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Could be. ;) I believe you're referring to having a separate EGC with the feeder as an isolation of fault current. Remember, fault current is (supposed to be) a short-duration event.

Keep in mind that the supply neutral is the "goal" of fault current anyway. The idea of the 3-wire vs. 4-wire debate is keeping normal, long-duration neutral current off of the EGC.
 
Is the three-wire set-up permissible because it's considered a "service"? I'm told that it is, but such a set-up doesn't appear to meet the definition of "service". In this situation the subpanel is just a panel mounted on a plywood structure in the middle of a field.

Neither inspector can point to a specific code passage that adequately covers this 3-wire installation. And by now I'm getting a headache trying to find it. The party who can't prove his stance has to buy a round tonight, so as you can see, it is very important that I find a specific section.
 
What the inspectors are telling you is OK was made a violation in the 2008 NEC.
It was only allowed in the 02 and on if there were no parallel paths for ground current.
 
IMHO there is _never_ a situation where it is permissible under the NEC to have an installation as you describe.

As I understand your description, the suggestion is that you can run two ungrounded conductors, plus a grounded conductor, but that you would _not_ bond the grounded conductor to the grounding electrodes or EGC at the detached structure.

(Note: limiting to single phase three wire services.)

In the case of a 'service', you have two ungrounded conductors and a grounded circuit conductor. In this case you are _required_ to bond one time only the grounded conductor to the grounding electrode system and to the equipment grounding conductors.

In the case of _all_ feeders (2008 code cycle), you must run a _separate_ EGC along with the ungrounded and grounded conductors. In this case, you must _not_ bond the grounded conductor EGC or grounding electrodes, with the exception of the main bond at the service.

In previous code cycles, as Larry noted, in the specific case of a _separate_ structure with no other bonded metallic paths, you could run a three wire feeder. In this case the 'subpanel' at that separate structure would be treated as a service. If you have such a three wire feeder installed, then you _must_ bond the EGC and grounding electrodes to the grounded conductor. You _must_ use the 'green screw' (or the equivalent) in this case.

If you do not have an EGC with the feeder, and you do not bond the grounded conductor at the 'subpanel', then you are depending upon the soil electrodes for your fault current path. Code does not permit soil/Earth to be considered an effective ground fault path. In most all cases the current flow though a soil path is not sufficient to trip ocpd.

-Jon
 
Jon,

I agree wholeheartedly with you on everything you say. We're both on the same page concerning grounding and bonding. I just need a code section that allows a three-wire service AFTER the first point of disconnect. It's gotta be here someplace, but I'll be darned if I can find it.

We are operating under the 2005 code here.
 
I don't have convenient access to the 2005 edition.

In the 2002 edition:

250.24(A)(5) Load-Side Grounding Connections is the paragraph that prohibits any grounding of the grounded conductor past the service disconnect. Since you must have a fault current path, and the fault current path has to be bonded to ground, this paragraph implies that the EGC must be separated from the grounded conductor.

250.32(B)(2) Is the paragraph that spells out that the grounded conductor may be used for equipment grounding to detached structures.

-Jon
 
Per the 2005 analysis the 02 and 05 are the same. They did change some wording for clarification. The 08 changed this to require a grounded and a grounding conductor per 250.118 with an electrode connected at the other structure. Exception is for existing installations.
 
Jon, Never ?
Read the FPN to your quoted article (250.32(a)(5) )
I don't see where the OP stated WHERE the subpanel was located, but if it is in a separate building and meets the criteria of 250.32 (prior to the '08 Code) it would be allowed.
 
I hate to beat a dead horse, but it seems as if these sections are referring to an existing 3-wire (no grounding conductor) feeder. But I've got inspectors saying it's okay to install (new) that same set-up without a grounding conductor.

THIS is where I scratch my head. I still think I am obligated to run that fourth wire to the subpanel. This has been a subject of great debate lately, with no one actually being able to find a code section that allows the running of just three wires.
 
augie47 said:
Jon, Never ?
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No never. What never? Well, hardly ever... :)

I managed to edit out the line where I said that 250.24(A)(5) prohibits these connections 'except as otherwise permitted'.

As I did say, 250.32(B)(2) spell out one of these 'otherwise permitted'.

Zorch, 250.32(B)(2) (2002) doesn't say anything about existing installations. If you don' run a separate EGC, and if there are no continuous metallic bonded paths, and if there is no GFP at the service, then you are required to bond to the grounded conductor.

The only 'existing installation' issue is that the above is _not_ permitted to be installed under the 2008 code, but would be 'grandfathered'. If you are still legally the 2005 code, then what 2008 prohibits is not directly relevant.

-Jon
 
Reviewing these code sections, it still appears to me that they deal with existing installations. I still can't find anything in the '05 code that allows me to run just three conductors in a new installation.

This is really what I'm questioning.
 
Jon - sorry about that. I've been viewing this thread in a non-reloaded manner that didn't show my previous post.
 
Zorch said:
Reviewing these code sections, it still appears to me that they deal with existing installations. I still can't find anything in the '05 code that allows me to run just three conductors in a new installation.

This is really what I'm questioning.
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250.32 is the requirements or instructions on how to handle the grounding electrode at a different structure, the existing you see in (A) is only about if the building has an existing something that meets the grounding electrode requirement in 250.52, such as a water pipe, building steel, Etc... if not you have to install one of the ones listed in 250.52 so install two rods and its done, now it also tells us how it must be connected to the system, they are three different systems.

(B)(1) is if a EGC is ran and how it is to be connected, and the fact it says not to connect it or the EGC to the "grounded conductor" neutral.

(B)(2) is for when we chose not to run an EGC, we have to meet the requirements of this section which states:
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,
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If we meet the above requirements then we can run just the 3 wires to the building and bond the GEC and the EGC's of this building only to the grounded circuit conductor which is the neutral.

(C) is for ungrounded systems which don't apply here.

So the only thing you see that says "existing" is in (A) which is only requiring an electrode installed if there is not one already that might be a part of the structure.

If you chose 250.32(B)(2) as to run 3 wires then you must use the main bonding jumper just like you would on any service, and the GEC must land on the neutral bar.
As per:
250.32(B)(2) 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
(1) That required by 220.22
(2) That required by 250.122
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the above in red is the neutral not the grounding conductor.
 
Wayne,

Nicely said. That makes perfect sense. I've always been fixated a few sections back where it says a EGC must be run. Thanks.
 
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