ECN Forum Forums General Topics of the Trade General Discussion Area 3 wire vs. 4 wire after service disconnect?

I am having a little trouble( maybe my boss is) with the 3 wire vs. 4 wire after the service disconnecting means. We have done services with a meter/ disconnect on the pole,with a ground rod there, and run 3 wire to the house, drove a ground rod at the house. everything in the panel in the house is bonded together. wouldn't the disco on the pole be the service disconnect, which means the panel in the house should have been fed with 4 wire and the grounds and nuetrals seperated.does anyone have an argument for or against my argument?(not that we disagree here )

What type of wiring method was used between the meter/disconnect location and the house?
Don

I assume arial messanger cable to the building? Coming from a pole? Pole mount meter/disco not a common occurance where I'm at, but with grounds, and nuetral bond at both, I don't see a problem. In fact doing it otherwise may be a (local)violation. I would see the pole as a structure, and the house a seperate structure.

Covered in 250.32, provides two methods. One with both having a seperate ground, 4 wire no neutral bond at the house. And the second with both having a seperate ground, 3 wire with neutral bond at the house.

The second being more prefferable, due to possible damage to an arial cable. If neutral loss occurred, you have an over-voltage fire hazard. Opposite phase loads would re-connect at the damaged neutral!

Unless this was underground in steel conduit... The first method! Then you have parralel neutral path, a different story.

[This message has been edited by e57 (edited 06-27-2004).]

250-32 refers to a common service supplying 2 seperate buildings. a structure is" that which is bulit or constructed" I can't see the pole being regarded as a structure.the wiring method from the pole meter/ disco is PVC. 250-24 A(5) states there shall be no connection of the grounded and grounding after the service disco. So, my question is is the disco on the pole a service disconnect?

The pole is a structure.

This is the whole point of using the term structure.


The NEC could have named this section simply "Two or More Buildings Supplied from a Common Service."

They did not, they add the words "or Structures" this is to make clear that this does not only apply to "buildings" but anything built or constructed.

As a result of the broad definition of structure, exceptions had to be put in place for light poles. Light poles are structures and as such would be required to have a disconnect switch located on each pole.

These exceptions to 225.32 demonstrate that something as simple as a pole is a structure.


One thing I think worth stressing is this part of 250.32(B)(2)


You must be careful to think of all possible metallic paths, not just the raceway the conductors are in.

Water and gas pipes, phone and cable TV wires, it is possible even a metal fence might run between two metal structures creating a parallel path for neutral current.

Yes it is, and the conductors from that pole mounted service disconnect to the house are feeders run under the provisions of 250.32(B)(2)

I agree a temporary pole is a structure, and 250.32 applies

ok, I can see the interpretation of the utility pole as being a structure, because of the "constructed " part of the definition, but I am confused on the use of 250-32. when can you use part a or part b? also, in 250-32D, if the disconnect is located remote from the building(which is the case if the disco is on the pole), the the conditions say no grounding and grounded connection, and an EGC run to the seperate structure of building. am I confusing anyone yet?( Mostly myself

It is up to the installer to decide if they want to run a EGC or not between separate buildings and structures.

If the distance is long and you can be sure of no continuous metallic paths between the buildings or structures using the grounded conductor as the grounding means can save a lot of matrial. Less conductors perhaps a smaller raceway etc.

You also end up with a larger conductor for the fault path.

A 200 amp copper service with a EGC will have a 6 AWG for half of the fault current path, use the grounded conductor as the grounding means and the fault path will probably have at least a 2/0 copper all the way as the fault current path.

Bob

I did not know that about 250-32. good info, what about 250 32 d? the disco is on the pole, and then run underground to the house, shouldn;t this section be followed in this situation? I look at it like a tralier service, as it is set up exactly the same, and we run 4 wire for that .