I have a customer with a pole mounted 400 amp service. From the meter enclosure, a 400 amp disco is fed thru a gutter. This is all grouped on a board at the pole. From the disco there are 3 500MCM which go underground about 100 feet to the main residence, where there is a 150A disco and MDP. Now they are building a shop which will have 100 amps run to it, also from the pole mounted service. The shop will probably have metallic paths to the main residence, but not to the pole mounted service. Question is, 3 wire or 4 to the shop? I feel 4 are required since in the event of an open neutral, the current would attempt to return thru the metal path to the main house where it would find the neutral at the bond there. Also, if the main house experienced an open neutral, the current would return thru the metal path to the shop then on to the service. I am 99% sure on this but would like some confirmation.
(P.S. The metallic path would be phone/cable. The water will be PVC and no gas)
Check out 250.32(B). It gives you two options: with or without an equip. ground. Option (2) is without the equipment grounding conductor, and the rules for this option include: "there are no continuous metallic paths bonded to the grounding system in each building or structure involved". Sounds like you are right. The reasoning isn't so much for the eventuality of an open neutral, but more for the fact of a definite parallel path with the grounded conductor, which is to be avioded. How would you like to disconnect the shield on your cable TV, and pull an arc? Or, work on the plumbing, and get rapped?
Re: Two buildings from a remote service#95722 09/30/0508:44 PM09/30/0508:44 PM
If he feeds the house and the shop from a maypole and then closes the loop with a wire between the house and the shop he is going to have objectionable currents, whether he does 3 or 4 wire. I would have the cable and phone companies set dmarks at both buildings and let them deal with stopping the loop current. At that point you don't have a metalic path.
Re: Two buildings from a remote service#95723 10/04/0507:42 PM10/04/0507:42 PM
Even with demarcs, you are going to have metallic paths! If both buildings have the smae phone line - now or in the future- they will have a common path. Cable will always have a common path, that's also bonded to ground at each building via the shield. The electrons don't care if they are going through a demarc or not.
4 wire is the only way to go with something like this. Even if you can prove there are no mettalic paths now, is the gas man going to repull wires when they or a future resident install gas down the road?
Re: Two buildings from a remote service#95724 10/04/0509:02 PM10/04/0509:02 PM
If you close the loop between buildings it is worse that the "metalic path" problem. You have made a loop and eddy currents will flow. Lightning will fry everything on that loop. The Dmark will surge protect these on the phone (there is no grounded phone conductor). I am not sure how they accomplish it with cable. I have to believe there is some kind of isolation.