another subpanel question

I'm installing a new subpanel for the new garage/shop. I decided to run a ground wire this time instead of trying to ground through the conduit. The subpanel has a bus bar for ground, but I'm not sure where to attach the ground wire in the main panel.
In the main panel, there are two neutral bus bars running down each side of the box. There's the two black mains lines comming in the top, looks like 1/0 gauge or something. There's also an equally big 200amp wire comming into the right neutral bus bar. It's black but with white tape to indicate that it's neutral. The two bus bars are bonded together at the bottom with a bar. At the top of the left bus bar, there's a smaller wire comming out and leaving. It's black with no tape. It looks like #4 to me. It's definitly smaller than the mains wires.
Is this smaller wire going to the grounding rod outside (or water pipe or whatever)? Should I attach the green ground wire from the subpanel to this? It's just the neutral, same as the white wire. Should I just attach the green ground wire to the main breaker box metal instead? Where should I attach it?
brian
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I just went through this exact situation. Mind you, what I'm about to tell you is what is (apparently) code for my locality. YMMV. My panel was approved by the electrical inspector last week.
In the main panel, the ground and neutral bus will eventually be bonded together. From my main panel, there are four wires going out to the subpanel: two hots, a neutral, and a ground. The ground wire is connected to the main panel box with a ground lug. It attaches to a similar ground lug attached to the case in the subpanel. The neutral should float in the subpanel (i.e. the neutral and ground should not be bonded together in the subpanel). In my case, because I have multiple circuits in my subpanel, I was required to install a ground rod near the subpanel. The ground wire connected to it is attached to a ground lug in the subpanel case. I've also heard that if you have more than 6 circuits in the subpanel, that a main disconnect is required per the current code.
todd
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todd wrote:
> I've also > heard that if you have more than 6 circuits in the subpanel, that a main > disconnect is required per the current code.
Absolutely, it is know as the "six hand" rule.
You must be able to disconnect all power with a maximum of six motions of the hand at any location.
Lew
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todd wrote:

I don't see how this is really possible in my main breaker box. There's no separate grounding bus bar. From what I was told on woodnet, the ground and neutral bars are bonded together in the main box, which matches what I saw. Based on that, I had planned to attach the ground wire to the neutral bus bar, since it has a ground wire attached to it in addition to the mains neutral. There might be a green lug there somewhere, but I didn't see it. I need to look again.
It sounds to me like we're running into code differences between different parts of the country.

I have 5. One 20 amp 220 for machines, two 30 amp 220s for the compressor and DC, 1 15amp 110 for the lighting, and 1 20 amp 110 for bench top power tools.
brian
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I got a bit lost in your detailed explanation but:
The bus bars are for the white neutral and for the bare ground wire. At your sub panel you should have a full length grounding rod driven into the ground and attach a wire from the rod to the sub panel box. This is to put the box at earth potential (zero volts). (Do not attach the grounding rod to the bus bars). If done as described it will satisfy codes and the insurance company.
Don Dando

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Don Dando wrote:

What if this ground rod is already attached to the neutral bus bar? It appears to be that way. And since it's new construction, it obviously passed inspection. Some people have said that this is the correct way to do things. Others have said that there should be a separate ground bus.
brian
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If you're talking about a subpanel, it must be attached to the *grounding* bus bar, *not* the neutral bus bar. The neutral bus bar must be isolated from the panel chassis *and* from the grounding bus bar.

Again, if you're talking about a subpanel, "passed inspection" means that a sloppy inspector missed a *clear* Code violation.

They're mistaken.
Even in a main panel, it's better practice to connect the grounding rod to the grounding bus bar (even though that bar is electrically continuous with the neutral bus bar), if for no other reason than to avoid exacerbating the confusion that many people already have between neutral and ground.
And in a subpanel, it's a Code violation.

That isn't exactly correct either: in a subpanel, there *must* be a separate ground bus. Not "should". Must.
In the service entrance panel, it doesn't matter electrically: the two are bonded together anyway. In a subpanel, it matters very much. In any 120V circuit, the neutral carries current. Cross-connecting the neutral and the ground in a subpanel causes the grounding conductor in the cable feeding the subpanel to become a second, parallel path for the neutral current returning to the main panel -- and the metal chassis of any grounded appliance becomes part of that parallel path. This creates a shock hazard for anyone touching such a chassis. It is widely -- and *mistakenly* -- believed that electricity always follows "the path of least resistance". This is NOT true. Electricity always follows *all* *possible* paths. That's why this is a hazard _even_if_ there is a properly connected ground rod at the subpanel: *some* current will return to the main through the grounding conductor anyway. How much of it does depends on the resistance of the earth between the ground rod and the subpanel. Depending on soil conditions, that could mean a few microamps, or perhaps a few *amps*. And as little as ten milliamps can kill, under the right (wrong?) conditions.
--
Regards,
Doug Miller (alphageek at milmac dot com)
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On Jan 23, 1:24 pm, snipped-for-privacy@milmac.com (Doug Miller) wrote:

In the sub panel, they're separate. There are two neutral bars running down each side, and a third grounding bar off to the left.

This is what I saw.

ok, let me repeat what you've said while applying it to my situation and using your (correct) terminology.
I've run four wires from my subpanel to my service panel: two blacks, a white, and a green. The white wire I attached to the neutral bus bar on the right side of the service panel. The bar on the left side is the ground bar (as opposed to the neutral bar). The white main wire coming into the service panel connects to the neutral bar (on the right), while a smaller #4 cable connects to the ground bar on the left and goes to the grounding rod somewhere. The two bars (left and right) are bonded together at the bottom of the service panel.
There are return neutral wires attached to the ground bar on the left side of the panel, which is ok since it's bonded to the neutral bar on the right.
Because they are bonded, I can attach the green ground wire returning from my subpanel anywhere on the neutral or ground (right or left) bus bar. This is also where the white neutral wire returning from the subpanel is attached.
The green and white wires are separate in the sub panel.
Does all this sound right?
brian
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All sounds good.

Electrically, this is fine. As I noted previously, it is IMO not best practice, because it tends to obscure the fact that ground and neutral are not the same thing. It's clear that you know the difference, though, so this is at most a minor nit.

Ditto.
As they should be.

Yes, exactly.
--
Regards,
Doug Miller (alphageek at milmac dot com)
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Ground and Neutral must be connected together at one place and one place only. The proper place to do this is in the main panel with a bonding screw that goes through the neutral buss into a threaded hole in the metal panel behind the neutral buss. This screw is green and is provided with the main electrical panel when the panel is purchased. Ground and neutral must not ever be connected together at any other place. The only exception to this is when the panel is of older design and it doesn't have this grouund-neutral bonding screw feature. Under these conditions the inspectors allow you to use a # 6 bare or green jumper wire between the neutral buss and a good solid ground termnal connection on the cabinet or the ground buss, but again, it's only allowed to be at one point. Everywhere else ground and neutral are to be considered as separate conductors. An additional ground rod at a sub panel must connect to the sub panel ground and not to neutral.
--
Charley

"brianlanning" < snipped-for-privacy@gmail.com> wrote in message
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I was under the impression that this wasn't required if the subpanel was in the same building as the service panel. That is, a ground wire can be run back to the service panel instead.
brian
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Whether this is correct or not depends on the author's intent. :-)
If the intended meaning is "Any additional ground rod which may happen to be present at a subpanel must connect to the subpanel ground and not to neutral", it's fine.
OTOH, if it's meant to suggest that a subpanel must use an additional ground rod, it's incorrect.

That's correct -- in fact, I think the Code may *prohibit* using a separate ground rod if the panels are in the same building, but I'm not certain.
--
Regards,
Doug Miller (alphageek at milmac dot com)
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On Jan 24, 6:43 am, snipped-for-privacy@milmac.com (Doug Miller) wrote:

Thanks doug. This helps a lot.
brian
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