double drop

[enteng14]

Hello elite engineers,

Recently, I have discussion with my boss about number of services and here is the "brief" contents..

Assuming there is a building with 2-occupancy (not separated by an approved firewall) and they are applying for a service with the utility company. Their service equipment are located in one location and proposing to have an individual service drop coming from a 230V 3-phase transformer bank. They have the same characteristics (voltage, frequency, phase) and the capacity requirements of each occupancy did not exceed 500 amperes each. The service equipments installations are accessible to both occupants.

In my initial analysis, the proposal will result to double service drop and a clear violation of NEC Art. 230.2 Number of Services. This service installation was only permitted when the service is coming from an underground sets of conductors, size 50mm2 and larger, but not connected together at their load end (not parallel conductors), shall be considered to be supplying one service.

1. Was my initial analysis right that this will result to double drop?
2. What is the rational by allowing individual service lateral for each service disconnects not more six and group in one location?
a. Was it considered because there were no service entrance conductors when underground service used (solely served service)?
b. Was it to limit the shorrt-circuit current at the service equipment and permitting lower interrupting capacity-rated equipment?

Thank you

 

[charlie b]

I don't know the basis for the rule. But I agree that to bring two separate services to this building would be a code violation.

 

[jtester]

Based on your drawing it seems that you aren't bringing a service to either unit. They are feeders which originate at the service equipment, unless there is no disconnect there.

There is also 225.30, even more strict than 230.2, which limits you to one set of branch circuit or feeder conductors to a building. I think you are being governed by it, and not 230.2.

Jim T

 

[RB1]

The drawing shows the service disconnecting means located remote from the building. Considering the "rack" or "stand" that supports the service disconnecting means a single structure, the two service drops violate 230.2. If the service disconnect for each occupancy were mounted on separate "racks" or "stands" the argument gets a little more complicated. In that case each "structure" would be permitted to be supplied by it's own service but you couldn't run a feeder to each occupancy unless there was no space available for supply equipment acessible to all occupants (225.30(B)(1)).

The provision for allowing six separate underground runs from a transformer was added to the code to reduce fault current levels by increasing the cable impedance from the levels found in large parallel arrangements.

 

[jbwhite]

The utility, from what I have seen, will not bring two seperate sets of lines from the pole to the building. They will bring one set of larger wires.

From there it splits to the two meters and then the two main panels.

Since the main panels are located side by side, there is no violatoin if each panel has only one main breaker or disconnect.

 

[enteng14]

Thank you for your immediate reply to my query. You are all adept in code.

The service equipment (circuit breaker) are installed in individual structure(pole). The building is not considered as more than one building as provision in Art. 225.30 because the two occupancy are not separated by an approved fire wall. If they would be considered as an independent building then, there would be no code violation.

NEC define "building" as, a structure that stands alone or that is cut off from adjoining structures by fire walls with all openings therein protected by approved fire doors.

The two occupancy in these case are adjoining structures but NOT separated by fire walls, thus not considered as individual building.

This is a simple yet a challenging problem..:-D

 

[jtester]

My reference to 225.30 was not because you have 2 buildings. Your electric service is away from the building, and so 225.30 allows you to bring only one set of wires to this structure, not two as your drawing shows.

That is what I believe is a code violation.

Jim T

 

[kingpb]

Why would you want to bring two seperate drops to the meters when they sit side by side? There is no advantage of doing this, and I doubt the POCO would consider it without some extraneous circumstances.

 

[allenwayne]

My reference to 225.30 was not because you have 2 buildings. Your electric service is away from the building, and so 225.30 allows you to bring only one set of wires to this structure, not two as your drawing shows.

That is what I believe is a code violation.

Jim T

I did a construction trailer complex in which there were 4 seperate trailers that each had a 100 amp panel in each,The ahj approved 4 meters each feeding a panel in each trailer( These trailers were modular in essense making up one big trailer)But they each had a seperate panel.The building dept had service cut off saying that there can be only one service to the mobile office complex and that it wasn`t a construction trailer but a commercial building and required exit lighting etc.We wound up installing a temp 400 amp service and feeding the panels from there.Also retro fitting exit lights and emergency lights as they required.

 

[enteng14]

Why would you want to bring two seperate drops to the meters when they sit side by side? There is no advantage of doing this, and I doubt the POCO would consider it without some extraneous circumstances.

The customer want to have a separate drops serving each meter. they cliam that it can lessen the available fault current at the service since the service drop conductors are not in parallel, thus resistance is larger compared to parallel. In result, they would have a smaller Ic rating for their Service Equipment.

In myself, i also disagree with the contention and thats the reason why i post in the forum so that i could gain ideas to further support my initial analysis.

Thanks