Question from 'Kimberly'

I've asked for more details. I think this is an interesting topic to discuss here.

Bill

It depends on the total calculated load of both dwellings. Your electrician should be able to figure this for you. For example: If the existing dwelling is 2000 sq.ft. and total electric I would guess a sub feed to another dwelling of any size would not be a good idea. You simply need to calculate the total load of both dwellings to determine if 200A service is adequate. The only reason I can think of that would definitely prohibit a second service is if the dwellings were to be connected and not have proper fire separation.

Down here (SoCA), you have to give the other dwelling a seperate address to have SCE provide another meter.
(I'm also interested in the details)

See the 1999 NEC, Section 230-40, Exception No. 3.

I think that, load aside, the decision to go with a new service or sub-feed brings up some interesting questions when there may be common utilities being used or coming from the first house. ie; a water pipe.

Bill

Joe,

Didn't see your post.
If that method was used, what would you do about a water line that runs from the first house to the second structure? How would bonding work so as not to create parallel paths?

Bill

There may be no definitive answer, only practical opinions. I, also wonder about the second address. Is this property sub-divided? Will it be possible to sell this home in the future? Or rent it out to someone? From a practical standpoint, I like the separate service, as long as the PoCo permits it.

Bill,
How is 2 services on one property with a common water line any different than 2 buildings next door to each other on a common water line?
Don(resqcapt19)

To bond or not to bond. That is the Question.

That pesky service disconnect neutral - GEC bond! I REALLY like some of the UK designs.

To Kimberly,

Sounds like the power company and local inspection in your area will allow the second meter but don't require it. IF your existing 200 Amp service load calculation with the new dwelling added to it is workable (your electrician does this calculation, ask for a copy and talk through it with him/her), then the choice boils down to how much the different installations cost, and how much you want to buy. The electrician may not want to take the time to do this, if so, talk to another.

Al

Don,

Normally houses (or structures) next to one another would be out on a 'branch' of the water main, wouldn't they? I was thinking about the service being bonded to the waterline out at the 2nd structure and it then running back through the first House. The waterline at the 1st house is bonded to service neutral also. Wouldn't there be current flow on the waterpipe all the time? One thing I'm thinking of is someone doing any plumbing work could be in danger, especially if one of the service neutrals went bad and no one knew about it.

Also, wouldn't/couldn't there be some difference in potential from this waterline to other conductive objects in contact with earth?

Bill

[This message has been edited by Bill Addiss (edited 11-08-2001).]

Bill,

IMO there is a current in the water pipe almost all the time anyway. The pipe between the old and the new house will be in intimate contact with earth, and, depending upon soil conditions, the earth will be a parallel return path to the xformer. If the neutral to the "new" house goes open and all the return current is forced into the water pipe and supplemental ground (presumably a ground rod), the "new" house will start showing voltage swings. The voltage swings in the connected load in the "new" house will be their greatest as the impedance to ground through the water pipe and supplemental reach their maximum. Kinda' creates a flashing alarm circuit, IMO. A great many of my customers are already worried about their systems when they can see the lights flutter when the refir or washer motor starts.

The plumber, poor guy, is given short shrift by the NEC. Case in point.

There is an island in the Mississippi that has had housing on it since the late 1800s. The soil of it is just deep enough to dig a basement before hitting limestone. The limestone is about 12 feet thick and sits on top of a deeper layer of sandstone, all of this above the normal water level of the river. When water and sewer were added to the island (about 40 now historic buildings), it was done by digging a brick lined tunnel through the sandstone big enough for a man, hunched over, to walk through. Water and sewer lines were then installed side by side on the floor, the man now forced to straddle the lines as he walks. In order to cut in or replace a water service, procedure requires the use of clamped on cable jumpers, and woe to the guy who doesn't.

Al

Al,

If the neutral was lost at the 2nd house I doubt they would even notice because the unbalanced current would flow through the waterpipe underground, and through the interior of the first house to the point where that service is bonded and then out on that Houses' neutral back to trans.

Is this making sense?

I also figure that current would regularly be flowing back and forth on this pipe (as a parallel pathway) in quantities and 'direction' dependant on the differences in load between the structures at that time.

Bill

Bill,

I agree with your points. The new house, #2, wouldn't notice the missing neutral at its service. . .that is until the plumber (that you mentioned in the post before) breaks the continuity of the pipe heading out of the old house, #1, towards the new house.

Your point about current flowing in the pipe under regular conditions is dead on, and, I think, agrees with Don(resqcapt19). The equivalent circuit of the water pipe between house #1 & #2 is a low series resistance between the neutral bars of the respective services. Each neutral bar will have three parallel resistances to ground (theoretical "zero ohms" ground), (1)one resistance being the supplemental ground rod to earth resistance, (2)another resistance being the pipe's own contact with earth, and (3)the resistance of the service entrance neutral + service lateral neutral + transformer ground wire + earth resistance at the transformer ground. This equivalent circuit is created, also, if the houses are connected by their own private water line, or by connection through the water utility. Each service, with unbalanced load, will have unbalance current that will return, divided through the three parallel resistances. This causes the reverse of a voltage drop. . .that is, each service neutral will rise above zero by the I x R where R is single resistance gotten by adding the three parallel resistances. The voltage on the neutral of one house, when different than the neutral voltage of the other house results in a current in the water pipe resistance.

This is just for two houses. The description gets absurdly complex when more houses are included, houses connected to a common metal water distribution system, and more than one transformer supplies the houses.

(I woke up still thinking about this one. . .I should draw a diagram. A.)

[This message has been edited by ElectricAL (edited 11-09-2001).]

Bill,

As I got into it, the next door neighbor's service unbalance current became very interesting! Especially when Kimberly's old and new houses get individual services. I draw below one of many hypothetical arrangements. I assume the PoCo transformer is at the back lot line of Kimberly's old house (#1) and that the water service is out in the street in front.



If the electrician that works for Kimberly does a good job of balancing the services in both of her houses so they have low neutral currents, and if the neighbor's service is running unbalanced, I suggest that the current path to the transformer neutral terminal through the street water main and paralleled service and lateral neutrals at Kimberly's will have a resistance equal to or even lower than the resistance in House #3's (the neighbor) single neutral to transformer connection.

This is by NEC and PoCo. I find it troubling, to say the least.

Al

(Didn't get the UBB right)

[This message has been edited by ElectricAL (edited 06-01-2002).]

Hmm... If house #2 is a sub-feed from the main panel in house #1, I think I'd be inclined to want a separate neutral & ground on the sub-feeder cable and no N-G bond at #2's panel.

A bad neutral on the sub-feeder might result in some 120V popping on inreased voltage, but it would prevent the neutral current flowing back via the water line. (And no dryer/range in house #2 to have its frame bonded to neutral, either!)

But, even if the panel at house #2 is technically a sub-panel, the NEC would require a N-G bond because it's the main panel serving a separate building, right?

While the resistance of the water pipe may be less than that of the service neutral, its impedance won't be. The inductive reactance increases with the separation between the conductors.
Don(resqcapt19)

But, even if the panel at house #2 is technically a sub-panel, the NEC would require a N-G bond because it's the main panel serving a separate building, right?

Paul, a sub-panel could be done either way, with a 3 or 4 wire here per 250-32 ( another highly debated code)


Ok, let's introduce some NEC hypotheticals for Kim's sparky here....

He is confronted by 250.6(B).......

( Alterations to Objectionable Current)
If the use of multiple grounding connections results in objectionable current, one or more of the following alterations shall be permitted to be made, provided that the requirements of 250.4(A)(5) OR 250.4(B)(4) are met:

I find the OR interesting here. A5 talks about a low-impedance path for ground fault current ,B4 talks about a path to operate a OCPD, both state the old 'earth shall not be the sole path' deal.
The GEC would not really be relied on for either situation in premisis wiring, i'm a tad surpised the NEC would elude to this here. Also, what constitutes 'objectionable' is not defined, is it a matter of ma????

(1)Discontinue one or more but not all of such grounding connections.

Ok fine, do i violate the rest of 250 in doing so, or does this overule all else??


(2)Change the locations of the grounding connections

I would think this should be done before #1, so i would question if this is an ordered approach here.

(3)Interupt the continuity of the conductor or conductive path interconnecting the grounding connections.

Ok, let's cut in plastic to all those UG municipal water lines ( i think the gas guys do a non-conductive union??) , so we end up bonded, but do not have the advantage of the low-ohm, miles long GEC we would have....it simply could not be an electrode at this point, could it.

(4)Take other suitable remidial and approved actions
plow it, install a UFER???
??????? ?????

Kim's sparky enrolls in plumbers school...

A friend of mine runs a mechanic's shop. Whenever he is confronted by the unknown, and/or irresistible object vs. immovable object scenario his standard reply is "7".

77777777777777777777777777777

[This message has been edited by sparky (edited 11-10-2001).]

Don,

The total complex impedance in the three house circuit above seems to me to be "complex" House 3, the neighbor, will have a net unbalanced current in the secondary from the transformer to the house that equals, in magnitude, the magnitude of the current in the water pipe (plus the incidental current in the earth itself). The unbalance in the secondary phase conductor will create an inductive reactance in the phase conductor it is on, and the closer the neutral is, will create a leading inductive reactance in the neutral that closely equals the magnitude of the water pipe lagging inductive reactance. Other than phase shift effects, the differences in the reactive impedance of the wire vs. pipe paths, are only going to come from that current which actually escapes to travel in the earth itself. I am assuming that the overall earth impedance is 10 times greater (at least) than the impedance in the conductors (pipe or wire).

My point, in English, is the reactance of pipe and wire are almost the same.

Al

Al,
I'm looking at a table in the IEEE green book that tells me that if the phase conductor is 30" away from the grounding conductor, the impedance is 2.65 times higher than when the phase conductor and grounding conductors are 2" apart. I'm assuming that the water pipe will have a much greater spacing and the impedance difference will be even greater.
Don(resqcapt19)

Isn't this called "Stray Voltage"?

Hi rbiro;

maybe stray V also, but i've heard it as Guass before.

Don,

You are right. The closer the phase and return conductor are (to being parallel with no separation), the less reactance there is, to the level of essentially zero. The current (at an instant in time) that is coming from the source on the phase conductor equals the current returning on the return conductor. Each current has a magnetic field of the same strength, but a field of opposite direction, so the magnetic fields cancel each other. No magnetic field, no inductive reactance. The further the two conductors are apart, with equal and opposite currents, the less canceling of magnetic field occurs, hence, more reactance. The Green book is starting off from a point (2" conductor separation) where there is a small amount of reactance, because, although close together, the magnetic fields don't quite cancel. Moving further apart (30"), the canceling effect of the other conductor's magnetic field falls of with the inverse of the square of the distance (1/d*d). The canceling effect decreases very rapidly, at first, then becomes very gradual, but never quite going away. Between 2" and 30", reactance in each conductor rises to about 90% of its theoretical maximum. So, the increase over 2.65 times that of the 2" amount won't be much.

Here's the other thing that balances the water pipe impedance: The neutral from house #3 to the transformer has a reactive impedance created in it by the unbalance current equal to the current in the water pipe, but the current is in the phase conductor. It's like this:

Hypothetically, house #3 has a 100 amp load on one phase and no load on the other phase conductor. As a result, the return current at the service neutral bus is 100 amps. Let's say, then, that 50 amps goes on the water pipe, and 50 amps goes on the service neutral conductor (for the purpose of this discussion). I think we're agreeing on what happens in the 50 amp water pipe current.

The 50 amp service neutral current is the interesting one. It creates a magnetic field that cancels half of the field created by the 100 amp current that is in the phase conductor right beside it (between the transformer and house #3). The remaining magnetic field from the phase conductor has a direction that generates current in the neutral in the direction of the current of the neutral -- a leading inductive reactance that is almost the same magnitude as the lagging inductive reactance in the water pipe.

Both of these reactances add to the resistance in the pipe or wire (a vector addition) and, IMO, the magnitude of the total impedance in each path almost the same, just with a difference in direction. The direction difference cancels at the transformer neutral terminal.

It's the extra 50 amps (or whatever the amount might actually be) in one of the phase conductors to house #3 that creates a matching reactance in the neutral conductor.

Al

Here's a more common configuration. Three houses on three seperate adjacent lots.



It doesn't matter much what is going on in houses #1 & #2, one could shut off the service disconnect and remove the meter from the socket and there is still going to be neutral current.

This current flows when the house farther away from the transformer is unbalanced. The more the unbalance in #3, the more the current in #1 & #2.

If I live in House #2, under the NEC, how can I object to this and defend it to the AHJ?

Al

[This message has been edited by ElectricAL (edited 06-01-2002).]