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distance rule with sub-panels?


dtontarski
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This was a new one on me. I inspected a home the other day that was for sale by a home inspector. I always imagined that this would be loads of fun.

To the point - the sub-panel was not properly wired as a sub-panel - the grounds and neutrals were bonded. As he was right there, I pointed this out to him. He informed me that this was o.k. because it was close to the service panel. I, as tactfully as possible, explained I knew of no distance to service panel rule that made this o.k. This guy has years of experience on me as a home inspector and I always try to be as respectful as possible to everyone I come in contact with....my question is: Is there some such rule that I'm not aware of? I spoke a local electrical inspector and he told me I was right to call this, but I would rather trust the experience of the regulars on this site to review this and set me straight on this. Thanks!

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Hi,

Were the two panels connected by a steel conduit and a myers hub? I've always taken the position that unless there is a myers hub and a steel conduit joining the two within an inch or two of each other that the second panel needs to be configured as a sub-panel. I know that others here have always said that anything past the disconnect must be considered a sub-panel, and I agree with that, it's just that I think that the connection between the two steel boxes via the conduit makes them essentially one box.

About two weeks ago, I called a sub-panel that had been installed a foot from the main panel/disconnect but without a conduit and myers hub. The electrician who installed it must have figured it was a sub too, because he's isolated the neutrals and grounds from one another and left the bonding strap disconnected. That was a 30-year old sub-panel. Obviously, the idea of separation isn't new. Anyway, instead of a ground bus, he'd twisted all of the EGC's together into a single large stranded cable and screwed a single large ground terminal to the enclosure to terminate his mondo-EGC cable on. Other than that, it was a pretty nice sub. I'm told that there wasn't a peep out of the electrician that came out to correct it.

ONE TEAM - ONE FIGHT!!!

Mike

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Mike -

I'm not sure what a Myers hub is (couldn't find this in my Hansen book), but the panels were connected by a piece of steel conduit approximately 8" long (see photo). Feeder did not have separate grounding conductor and one terminal bar held all grounds and neutrals. (many together under single screws)

Other: what's the word on surface rust? The terminal bar screws in the main service panel had surface rust. How do you typically call this out?

Thanks for the advice.

Dave

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Hi,

I probably wouldn't have called that. The hub is the threaded object that connects the panels to the steel conduit. Let me ask you this; would you have called it if it were a split-bus panel and found all of the stuff on that left panel on the lower half of the a split-bus panel? You might if you'd seen that many breakers in one panel but you wouldn't have called to have the EGCs and GC's separated on the lower half, right? The two enclosures are joined together by a large conductive piece of metal, essentially making them one enclosure. The feeder is the same as the feeder from the top of a split-bus panel to the bottom half of a split bus panel. That's my take on it, anyway.

I already know that Jim and some others here will disagree with me on this. If I were you, I'd listen to them. After all, electricity is definitely my weakest area.

ONE TEAM - ONE FIGHT!!!

Mike

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Originally posted by hausdok

I know that others here have always said that anything past the disconnect must be considered a sub-panel, and I agree with that, it's just that I think that the connection between the two steel boxes via the conduit makes them essentially one box.

I'm not aware of anything in the NEC to support that idea. The enclosures have to be bonded whether the second panel is 6 inches or 60 feet away, in all cases. That point is often lost in the popular oversimplification "nothing gets bonded in a sub panel". For instance:

The electrician who installed it must have figured it was a sub too, because he's isolated the neutrals and grounds from one another and left the bonding strap disconnected.

It shouldn't have been disconnected. It should have been bonded on the equipment ground bar. That enclosure could easily shock someone if a hot wire got loose and came in contact with it.

Obviously, the idea of separation isn't new.

Douglas says the "separation" rule goes back at least as far as the 1920's in the NEC.

The "close by" idea is pure rot. He can't back that up with anything authoritative.

Brian G.

Da Rules R Da Rules [8]

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Originally posted by dtontarski

This was a new one on me. I inspected a home the other day that was for sale by a home inspector. I always imagined that this would be loads of fun.

Yeah, right. Come to my house, you'll be inspecting for 8 hours straight.

To the point - the sub-panel was not properly wired as a sub-panel - the grounds and neutrals were bonded. As he was right there, I pointed this out to him. He informed me that this was o.k. because it was close to the service panel. I, as tactfully as possible, explained I knew of no distance to service panel rule that made this o.k. This guy has years of experience on me as a home inspector and I always try to be as respectful as possible to everyone I come in contact with....my question is: Is there some such rule that I'm not aware of? I spoke a local electrical inspector and he told me I was right to call this, but I would rather trust the experience of the regulars on this site to review this and set me straight on this. Thanks!

You're right. The other inspector is wrong.

Look carefully at the picture. During the normal, day-to-day operation of those panels -- not during a freak occurance where something goes wrong -- some of the return current from the sub panel is going to flow over the feeder's neutral conductor and some of the current will flow over the 8" conduit.

For ten points, who can tell me why this situation is bad?

- Jim Katen, Oregon

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Originally posted by Brian G

Originally posted by hausdok

The electrician who installed it must have figured it was a sub too, because he's isolated the neutrals and grounds from one another and left the bonding strap disconnected.

It shouldn't have been disconnected. It should have been bonded on the equipment ground bar. That enclosure could easily shock someone if a hot wire got loose and came in contact with it.

Ah, no Brian, the ground bus (in that case - terminal) was connected directly to the enclosure. The main bonding jumper in that circumstance was connected to the enclosure and only used to bond the neutral bus to the enclosure in the case of a main panel. In a sub-panel that neutral bus needs to be left unbonded.

The main bonding jumper in most of the older panels I see bonds only the neutral bus to the panel's enclosure and to the ground bus via the enclosure. Since the panel I looked at was a sub-panel, there should not have been a connection between the neutral bus and that enclosure.

Yeah, I've seen lots of new panels where the equipment ground bus is isolated from the enclosure and there needs to be a screw or strap connecting the enclosure to the ground bus but not in situations where the bus is mounted directly on the enclosure without an insulator. Those don't even come with a bonding stap or screw for the equipment ground bus; it's always on the neutral bus.

Obviously, the idea of separation isn't new.

Douglas says the "separation" rule goes back at least as far as the 1920's in the NEC.

The "close by" idea is pure rot. He can't back that up with anything authoritative.

Brian G.

Da Rules R Da Rules [8]

See? Toldja so!

ONE TEAM - ONE FIGHT!!!

Mike

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Originally posted by hausdok

Ah, no Brian, the ground bus (in that case - terminal) was connected directly to the enclosure.

Okay, I re-read the post and I've got you. No second bar, just an improvised terminal screwed right to the enclosure. My bad.

I swear, you'd think finding and installing an add-on equipment ground bar was a major ordeal. Every manufacturer makes them; every wholesale electrical supply worth 2 cents carries them in stock. Most have 2, count 'em 2, mounting screws, that fit in pre-drilled holes in the enclosure. *sigh*

Yeah, I've seen lots of new panels where the equipment ground bus is isolated from the enclosure and there needs to be a screw or strap connecting the enclosure to the ground bus but not in situations where the bus is mounted directly on the enclosure without an insulator.

I like the ones with two full-length neutral bars connected by a removable jumper bar, both with bonding screw holes. Take out the jumper and you have instant sub panel capability.

Brian G.

Versatility Is Good [:-thumbu]

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Originally posted by Brian G

I swear, you'd think finding and installing an add-on equipment ground bar was a major ordeal. Every manufacturer makes them; every wholesale electrical supply worth 2 cents carries them in stock. Most have 2, count 'em 2, mounting screws, that fit in pre-drilled holes in the enclosure. *sigh*[/navy]

Yeah,

Well, in that circumstance it was especially dumb because there was a ground bus there, screwed directly to the enclosure, and the electrician that installed the sub-panel chose not to use it, left it empty and improvised his single mondo-cable clamp.

I can't anwer Jim's question. I'm just not "wired" to understand this stuff very well. To me, I don't see the difference between these two panels and a split-bus panel.

OT - OF!!!

M.

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For ten points, who can tell me why this situation is bad?

I'll expose my soft underbelly.

It gives anyone touching the enclosure the opportunity to be a current carrying conductor. That would be objectionable but unlikely.

By making the cabinet part of the circuit it's possible, maybe even probable for electrolytic degradation?

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Originally posted by Jim Katen

Look carefully at the picture. During the normal, day-to-day operation of those panels -- not during a freak occurance where something goes wrong -- some of the return current from the sub panel is going to flow over the feeder's neutral conductor and some of the current will flow over the 8" conduit.

For ten points, who can tell me why this situation is bad?

I'll give it a try since takers are few. As I understand it, the current that goes across the 8" nipple will then pass to the enclosure, allowing the current to continue moving on the ground wires bonded to the enclosure. Hence all of the grounded/bonded items in the house can have current flowing over them; stoves, washers and dryers, toasters, HVAC, you name it. Then all you need is for someone to touch such an item and be grounded somehow, and ZAP! You may not be on a train, but you are a conductor. [;)]

So...close, or not?

Brian G.

What Are These Points Good For, By the Way? [?]

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Originally posted by Chad Fabry

For ten points, who can tell me why this situation is bad?

I'll expose my soft underbelly.

It gives anyone touching the enclosure the opportunity to be a current carrying conductor. That would be objectionable but unlikely.

It's not unlikely at all; it's a certainty. Anyone touching the enclosure *is* a conductor if he's grounded or touching the service panel enclosure. The amount of current that passes through his body will vary depending on the resistance of his body and the resistance of every other possible path back to the transformer. In most cases, there won't be enough current traveling through him to cause any harm.

But that's not why the conduit's important.

By making the cabinet part of the circuit it's possible, maybe even probable for electrolytic degradation?

I kind of doubt that.

- Jim Katen, Oregon

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The reason that this particular panel is worse that most other sub panels that lack ground/neutral separation is because of that 8" conduit.

In most sub panels where the grounds and neutrals are joined, there's only one really practical, low-resistance path back to the service panel: the feeder grounding conductor. The typical issue with these panels is that return current is looking for any way it can find to get back to the transformer. It tries to push its way back on every grounded surface and through every person who touches one of these surfaces. Since the resistance of these paths is so high, very little power gets through and people usually aren't shocked. With these panels, the improper ground/neutral separation only causes a problem when something else goes wrong, like a bad neutral connection.

In contrast, on this panel, there are two really practical, low-resistance paths back to the service panel. One is over the feeder's grounding conductor. The other is over that 8" length of conduit. Both are practical paths so a pretty good amount of current is going to be flowing over each of them.

There are several problems with this.

First, the conduit was never intended to be a current-carrying conductor. (If you don't believe me, try looking up the ampacity of conduit.) The connections at either end could heat up or even arc. More important, if an electrician were to try loosening the conduit, he could be severely shocked if his body were to complete the circuit.

Second, when you split up the current that travels on a circuit, the magnetic fields around the wires no longer cancel each other out. This means that you'll get EM fields at the feeder, the conduit and the wires in the conduit. EM fields can cause interference with radio and TV reception but more importantly, they can induce current on nearby conductive surfaces and cause them to get hot. There's no way to tell what's inducing current on what here because it varies constantly as the loads on the system vary.

When you don't maintain proper ground/neutral separation on sub panels, it creates conditions that might not be obviously dangerous and that might not cause problems for years or for decades. But you really can't predict that. That's why we have a simple, straightforward rule about it.

The panel is wired incorrectly. If the home inspector who owns it doesn't understand that, send him here and we can discuss it.

- Jim Katen, Oregon

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Jim -

Once again - thanks! Knowing the rules is one thing - understanding why the rules exist and the safety issues around them is so much better. I appreciate your willingness to share your expertise and your efforts to make those of us who have found this great educational resource better inspectors.

Dave

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