Parallel service conductors

[jmjr55]

We were talking at work about a 4 unit apartment building. The building currently has a 200 amp service with 4/0 aluminum SE cable feeding five meters going to a 60 amp service disconnect. Each apartment has a 60 amp panel and the building owner has 60 amps for common areas. The service needs to be increased to 125 amps for each unit (the building stays at 60 amp) because the heat is going to change from oil to electric boiler, for each unit. The new calculated load comes out to 275 amps the owner will need to jump up to a 400 amp incoming service. Using the existing 4/0 aluminum SE cable and paralleling another 4/0 aluminum SE cable will give that capacity. But I believe the paralleled cable needs to be electrical connected at both ends (as per 310.4 ?joined at both ends?) other guys feel they can be joined at the service over head and split at the meters. In other words three meters on one and two on the other. Any thoughts on this divide.
Thanks,
John

 

[augie47]

In my opinion, you would need to calculate it both ways. Initially, playing with nubers, it looks like it might work either way, but I'm unsure of your exact calculations. Your calcualted load for two units should work out to be more than the average using four units, so that number would have to be determined.

 

[LarryFine]

. . . I believe the paralleled cable needs to be electrical connected at both ends (as per 310.4 ?joined at both ends?) other guys feel they can be joined at the service over head and split at the meters. In other words three meters on one and two on the other.

Either is permissable. As Gus said, this kind of thing needs to be figured both ways. Don't forget to include the work involved in doing it each way.

Around these parts, the POCO provides meter bases, so the cost of a 5-gang vs. a 2- and a 3-gang is less important than the cost of installing them.

 

[Lcdrwalker]

You are correct about the parallel conductors. They must be joined at each end. To have a seperate drop for tsome of the units there would have to be a fire wall separating the units so they would essentially become separate structures. Otherwise the two drops would be two services to a single structure which be in violation.

 

[Pierre C Belarge]

If you are following the 2008 NEC, you will no longer be able to use the 75C column for the final ampacity of SE cable, that will limit the 4/0 AL, cable to 300 amps.
The amperage should be good for your service of multiple disconnects as per 230.90Ex3, based on your desciption of installation.

 

[augie47]

You are correct about the parallel conductors. They must be joined at each end. To have a seperate drop for tsome of the units there would have to be a fire wall separating the units so they would essentially become separate structures. Otherwise the two drops would be two services to a single structure which be in violation.

I agree. But, it is not unusual here to have one drop supplying more than one riser at a single location. As Larry points out, often the meter base expense effects the method.

 

[dicklaxt]

Parrallel feeders are in essence still the same incoming phase system relationship you had originally and each phase should still see all connected loads equally as it did before.

dick