This panel is in a detached building. It has a 3-wire feed, and the neutrals and grounds are separated. The panel has a GEC to a UFER ground. The neutral bus is not bonded to the enclosure. The panel looks very professional and tidy (except for the missing ground wire of course).
I haven’t seen a 3 wire sub-panel with grounds and neutrals separated for a while, and I had to sketch it out to remember just why this is dangerous. Under normal condtions, everything works just fine, but in the event of a ground fault, the fault current returns to the ground bus in the sub-panel and then has no low impedance path back to the transformer. I believe the only fix is to pull a ground wire to the panel because 3 wire sub-panels are no longer allowed in detached buildings. Am I remembering this correctly?
Of course I don’t go into any of the details in the report. I just note that it is wrong and call for an electrician to fix it.
1.) if the building is existing and their are no metallic paths ( ie: phone line, metal water line, etc. ) between the two buildings then a 3 wire circuit is still allowed to a detached structure. Under the 2008 NEC it is still allowed if it is existing but the allowance for 3 wire has been removed in 2008…again except for existing applications.
2.) The grounded and grounding conductors must be connected together in this type of setup so they need a bonding jumper to connect them together and then a connection to the panel enclosure. The “grounded” conductor and “grounding” conductor both should be connected to the GEC in this application via the bussbars you have mentioned.
You are correct in that the return low impedance path is needed and in the 3 wire situation the “grounded” conductor is serving both functions…this is not typically a good thing and why ranges and dryers had it changed years ago…it is just a case of the detached structure catching up with those type changes.
Now as to being something you write up if you see a 3 wire setup that is allowed based on the minimum safety standards is a call you will have to make. Their is a reason why they no longer allow it so that can be your guide. However, be aware it will cause a combative nature from electricians who you defer to…so as long as you are ok with that…fine. But, in your case it is not just about the 3 wire setup. Your case is a situation where they dont have the bonds properly done as I stated before and they should at the least be corrected.
FYI- I would however ask if they have a “Back-Fed” hold down breaker device and since you are calling the other things out, get them to re-identiy the white conductors on the breakers. Also see what you can about the conduit that may be from one building to the other…but remember it could also be plastic and bushings would still be required in this case for abrasion so don’t assume it is metal conduit that is continuous between buildings.
Paul, I want to be sure I got this right. In a 3 wire system, as we have here, the grounding and grounded wires must be bonded together. In a 4 wire system, the two wires must be separate and the grounded(white) wires must be floating.
The set up in this detached building is incorrect not because of the 3 wire feed but because the grounded and grounding wires are separate and not bonded. To correct this, you can ether run a forth wire (grounding) to the panel or bond the wires together. Is this correct?
You are correct. However, running the “4th” conductor could present a problem in regards to running all the conductors together so if it is a 3 wire set up and their is no metallic path between the two structures. Install a bonding jumper, install a GES and call it day.( Under the 2005 NEC, 2008 NEC will not allow the 3 wire option except if it is already existing )
The OP said there was only a 3 wire feed. With no other metallic paths back to the other building under codes prior to the 08 it was acceptable to re-bond at a detached building.
The 08 has removed that option and requires a 4 wire feed. In this case the neutrals and grounds are separate.
Not if it complies with 250.32 and their is no metallic path between the two buildings. You would treat the panel in the detached structure the same as you would a typical panel. You have to understand the two differences…As jim said…prior to the 2008 NEC is was allowed but is not any longer in new construction.
Below is the 2008 Verbiage ( minus the exception for existing detached structures )
**[FONT=Times New Roman]size=2 Grounded Systems. **[/size][/FONT][FONT=Times New Roman][size=2]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).
[FONT=Verdana]Ok…so read the above as it pertains to the NEW requirement of 250.32 and below is the exception.
[size=2][FONT=Times New Roman][size=2]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
[FONT=Times New Roman][size=2][FONT=Times New Roman][size=2]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
[/size][/size][/size][/size][/FONT][FONT=Courier][size=1]
[FONT=Verdana][size=2]How this helps you understand a little better.
[/size][/size][/FONT][/FONT][/FONT][/FONT][/FONT][/size][/FONT]
So Paul,
In a detached garage with a phone and 3 wire srevice, the ground bar and neutral is separated, there is a grouding rod for the garage. The neutral and ground do not need to be separated from what I understand now, but in this case they are. Is this a safety issue?
In the situation you posted a 3 wire feed would not have been a compliant install since you had another metallic path, (phone line), back to the other building.
as Jim stated…the flaw in your question or situation is the phone line between the buildings. It is indeed a metallic path ( unless it is fiber optics…lol ) so a 3 wire setup is not allowed if the metallic paths exist. And in fact it is not allowed ANYTIME in new construction if your area accepts the 2008 NEC.
