ECN Forum Forums Code Related Discussion NEC & other Code issues 3-wire subpanel?? I goofed.

the pole/structure agruement did not originate with me guys, just thought i'd throw it in to spice it up here

most of you know that this thread can also venture down the 'why don't the poco just give us N-G isolation?' with interjections of seasonally acidic soil, aluminum siding, and NEC ref's re: the earth being unconductable


Steve, (aka Noodles~R~Us) sparky

[This message has been edited by sparky (edited 11-18-2003).]

Thanks for all the conversation fellas!

Let me throw in a few more details...I don't think they change anything.

The house sits 1000' off the roadway out in the boonies. Always wanted to build a log home out in the woods...I have the woods, the house is stick framed with log siding, hence the workshop idea initially (my kids will be too old I figure when I can afford to build what I want, so I best do something now!)

Anyways, I have a transformer sitting in the woods on a vault...from there the secondary service runs about 100' to a 6x6 (treated, so it's built! LOL) and plywood "panel". There's the meter, then a outdoor passthru panel. A 200A main breaker in that panel serves as the disconnect. From there, I ran the 3 wire feeder into the house (workshop). The idea was someday when I built the house, the "temporary" pole would be moved presumably into the real log house.

It was my understanding that I need only run the 3 wires into the workshop and treat it as the main panel, hence the 3 wire.

OK, there's no cable service out in the boonies...but Direct TV works just great (satellite).

There is a phone line that is in separate conduit down to the roadway...and the plan is indeed to mount the NIC (i think that's what it is called) on that same 6x6 pole.

That may change things...as it is a conductor...

If I ran the service straight from the transformer to the house (workshop), then I'd not have the disconnect on the pole, and I'd be just fine.

Everything is on and energized...I past my rough wire inspection with no questions.

BUT, I am uncertain as to whether the inspector really understands how things are. I told him I only ran 3 wires in and had ground rods at the panel in the house.

I don't care much about inspectors per se, I care about saftey. If there's a compelling safety reason to dig and run the fourth ground wire, I'm willing to pony up the cash and just do it. Yuck. If it is OK as is, then I'd obviously prefer to not sink in the cash.

Guess I should just get on the horn with the inspector and ask him specifically. But I do want to understand all that I can...and I'm not the type to listen to just one source. You folks are wonderful in all our your discussion, help, and information.

Thank you!

Bob, I notice that when I count the posts I've made on this site plus the other sites you and I frequent, there are well over 1,000 of them. The only reason I bring that up is that I hope by now you know that I NEVER make up the code as I go, and in fact people that do really anger me.

I would require the 4-wire service because in my opinion a pole is not a structure. I'm not saying that to be a PITA, I'm saying that because I am in the business of enforcing the law. 90.4 says that I can waive a specific requirement unless there is an alternative that provides an equal amount of safety. Is there? I don't know...I'm not as well versed in theory as I wish I were.

As far as going over my head, you certainly could, but my boss has never allowed it. In my city I am the highest authority in electrical. I would much rather discuss it with the installer and perhaps one of us could walk away a wiser man (probably me). There is seldom need to go over my head. I think I'm a reasonable enough guy.

[This message has been edited by Ryan_J (edited 11-18-2003).]

i would'nt sweat it brucepirger, we just love to split hairs here.

Ryan


I have great respect for you and have admired your insistence on following the letter of the code(s).

That is why I was surprised on your take on this.

I can not agree with you that a pole is not a structure, the NEC had to put an exception in for disconnects on light poles because they are structures. 225.32 exp. 3

Your friend Bob

Ryan,
If you would look at the code panel comments on the ROP that resulted in the exception that Bob referenced, you will find out that the CMP considered the light pole a structure. In my opinion the code supports a 3 wire feeder in this case.
How would the safety of the installation be changed if the pole was a very small building?
Don

Thanks guys, I'll look into that.

I followed txsparky's link and it was interesting, especially when they make it sound easy to add a EGC when it is not existing.


Problem solved

Well, if only it were that easy to pull in the ground wire...LOL...I'd do it.

Truth is, I passed my rough wire inspection about a year ago...not sure what that says about the inspector...but I have the sticker. So, I "could" just do nothing and have the final done and I suspect never worry about it again.

But I'm not the type to do that...and will bring it up with him, and see what he thinks. I have grown convinced there is no safety difference...so no real "need" to do this.

On that note, is it perhaps more dangerous? What if the ground wire on the 4 feed service were to break...then all my equipment grounds would be tied only to the ground rods...and then a hot fault to gnd would leave me in a bad situation. Would I ever notice a broken ground wire back to the main panel, until the hot to gnd fault?? I'm not sure I would.

If on the other hand a 3 wire neutral broke, I'd notice that very quickly, as nothing would be "right".

Isn't that correct?

Although I'm not justifying the 3 wire, although it does seem to make sense.

What's wrong with that "argument"??

Also, since the phone line is not connected to the grounding electrode of the electrical system (is it??), then I assume that is not an issue anyways.

Again, thanks for all the help gentlemen! I really appreciate the sounding board and all your time!

I don't intend to sound snotty here, but I suspect that it will come off that way, so apologies in advance:

It sure sounds to me like you've already convinced yourself to rip up the old 3 wire system and install a 4 wire system, and just want to be talked into spending the money

The reason that single phase sub-panels are normally wired using 4 wires (2 hots, neutral (or ground_ed_) , ground (or ground_ing_) ) is that if you were to connect the neutral and the ground at more than one location, a certain fraction of the neutral current would flow through the grounding system, eg. through your raceways and conduit connectors, etc. Inside a structure, this can pose a significant hazard. Even outside a structure it can pose a hazard, but it is generally not considered as much of a problem. In power distribution systems, the neutral is _regularly_ bonded to ground in multiple locations, in other words following the exact opposite rule (in your house you bond neutral to ground _once_ and only _once_, but the POCO bonds neutral to ground many, many times).

In the situation that you face, you are halfway between service in your own house and power distribution. You have two separate 'structures', or perhaps a structure and a pole (I'll let the higher ups make that call). You _don't_ have any other metallic pathways between these two 'structures', so the three wire system is possibly allowed, and would be very much like the three wire service that the POCO normally provides.

Given the choice, I personally would prefer the three wire system. My reasoning is that I would prefer to have the electrical system tied to ground in my house, rather than tied to ground at a point more than a hundred feet from my house. The longer the run to the bond between the neutral and the ground, the greater the impedance, and the greater the possible voltage difference between the electrical system and the building ground. In terms of safety issues, I believe that you are at the point of '6 of one, half a dozen of the other'; by not having the alternate metallic pathway between the two 'structures', you have eliminated the major safety issue of tying ground to neutral and multiple points, so now you have to balance the remaining safety issues of having neutral and ground tied in two locations versus the safety issues of having a much longer run to your neutral to ground bond.

I suppose that you could eliminate the 'where to ground' issue entirely by using a transformer. You would re-derive the neutral in your house, and bond it to your grounding system there. Seems like a pointless exercise to me, but I believe that it would be legal....

-Jon
I personally would prefer the thr