On a new shed and breaker box, I’m coming from another building that has 220v, 100 amp service.
My question: If I add a new full length ground rod to the new shed, do I have to bring the Neutral from the other building or can I just bring the two legs of the 220v?
Thanks,
Bill
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Replies
Absolutely you need the neutral.
The typical ground electrode has fairly poor resistance to the ground and would make a louse electrical connection.
You need the 2 hots and neutral.
If you have no other metallic connections between the buildings you can use just those 3 wires and bond the neutral to the ground at the sub-panel. Typical metallic connections are metall water pipes, TV cable or phone cable.
Otherwise you need to run a separate EGC (ground wire) and keep the neutral isolated from the grounds in the sub-panel.
In EITHER case you need to have a local ground electrode system at the sub-panel.
If you use a ground rod for the ground electrodes then you either need 2, at least 6 ft apart or does a specialized test that shows the first ground rod is less than 25 ohms.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
You must bring the neutral. You don't need to bring a separate ground.
You should use a four-wire feed: two current-carrying conductors, a neutral and a ground to bond the subfeed panel to the service ground.
Since the neutral is a current-carrying conductor, the subfeed panel must NOT be bonded between neutral and ground. Only one neutral-ground bond is allowed in a residential electrical system, at the main panel or service disconnect.
Edited 1/21/2008 12:20 am ET by Riversong
Wrong answer. This is a shed -- a separate structure. Go back and re-read the code on that.
If your view never changes you're following the wrong leader
"Wrong answer. This is a shed -- a separate structure. Go back and re-read the code on that."
I believe the 2008 NEC requires 4 wire feeds to all subfeed panels, for either attached or detached outbuildings, whether or not otherwise connected by metal.
This is, in any case, a safer alteranative since it allows for future metallic (phone, cable, etc) connection to outbuildings.
We've kind of been around this bush before. However, this time we're having it in 2008. Guess what? Treating a detached structure as a 'separate service' is no longer allowed. You get to run all four wires, AND bang in a ground rod, AND keep the neutrals separate from the grounds .... per the 2008 edition of the NEC. Just another reason for this to be a job for a real electrician. Other reasons to hire a pro include: 1) burying the wires deep enough;
2) A bias in favor of conduit;
3) The likely need for an outdoor disconnect;
4) Remembering to run a pipe for the phone / data / TV; and,
5) Planning for future growth.
I agree that all four wires should be run to the subpanel anyway, but it should be noted that not all parts of the country adopt the latest version of the NEC as soon as it is released. I have a chart around here someplace that shows which version is in force for each state, some are using 2002 or even earlier editions. We're still on the 2005 version in Minnesota, I expect we'll go to 2008 later in the spring sometime.
Something else not yet mentioned to the original poster is that local codes may call for an Ufer ground buried in the building's footing instead of ground rods. They should check into this before construction begins.
Last year I check on one of the Chicago subburans cities and they where still on the 92, with many amendments to outlaw NM..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
That there are all manner of NEC editions, local amendments, and even local (non-NEC) electrical codes about, you are correct. Yet ... running a ground wire is, for a number of reasons, the "best practice." It seems a bit off topic to go off on that tangent - again. Suffice it to say that you can no longer make the case that the NEC allows you to treat the detached building as if it were a separate service. So, saying 'the code lets you' is a false argument. I'll return to a point I've often made: Don't use the code as a design manual. For everyone who is so quick to say "but the code...", I refer them to the very introduction of the NEC, which plainly states it is neither a design manual, nor an instruction manual. Good design comes first. Omitting the ground wire is no longer the "code." It's poor design. That it is poor design, and that the grounding section of the NEC has been confusing, are pretty well established views. A few have mentioned that there are other acceptable 'grounding electrodes' in addition to the ground rod. Some, like the "Ufer," are even clearly better than banging in a ground rod. I was under the impression that this was an existing building; kind of hard to install a "Ufer" after the fact. We don't even know if there is a cement slab. The point being ... the building needs BOTH a ground wire AND a grounding electrode (ground rod). The two, despite similar names, serve different purposes altogether.
Omitting the ground may or may not be "poor design". Depends on the quality of the ground system, the presence of ground currents from other sources, etc. Sometimes the code prevents you from using the best design.
If your view never changes you're following the wrong leader
I'm more than a little confused by your post. Quality of the ground system? What ground system? The ground wire goes from panel bussbar to panel bussbar .... where, at the main panel, the grounds are bonded to the utility neutral. If you have a problem there ... you have some major problems everywhere. This ground wire, combined with the bond to the utility neutral, is what allows breakers to trip when something goes wrong. Or, perhaps, were you referring to the connection between the ground rod and mother earth? That connection has absolutely nothing to do with clearing faults, and is there primarily for lightning protection. Indeed, much of the modern world gets along just fine without this deliberate connection to earth.
This lightning issue is why separate structures require grounding electrodes. Do not confuse a 'grounding electrode' with a 'ground wire.' The first is for lightning, the second is for clearing faults. Ground currents from other sources? Are we perhaps speaking of "stray voltage?" There's a classic problem, created when folks fail to properly connect their neutral to the utility neutral .... resulting in the elect5ricity trying to make its' way back through the dirt under our feet. Dirt is a very bad conductor. That's why we're supposed to bond everything ... plumbing, pipes, antennas, telephone, CATV, etc. to the same grounding system as the one used by the main panel. Oddly enough, one of the easiest ways to create this problem is to omit that 4th wire between the panels.
If you're confused then it's because you don't understand the finer points of grounding.
If your view never changes you're following the wrong leader
Would you care to elaborate on where code would prevent using the best design?
Romex, for one. But you'd have to get the blinders off to understand that.
If your view never changes you're following the wrong leader
"AND bang in a ground rod,"That is not right either..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.