NEC 230.40 Exception (3)

[taschendoerfler]

230.40 Only one set of service entrance conductors per service drop.

Exception No. 3: A single-family dwelling unit and itsaccessory structures shall be permitted to have one set ofservice-entrance conductors run to each from a single servicedrop, set of overhead service conductors, set of undergroundservice conductors, or service lateral.

Can someone explain the rational for this exception. Why is it allowable, but only for single-family dwelling unit with accessory structures ?

For me this exception seems to potentially create conditions similar to a situation where one doesn't run a dedicated Equipment Grounding Conductor (EGC) from a single service entrance panel to every accessory structure.

Basically, now one has multiple points in the overall system, where EGC and Neutral are bonded (i.e., the various service panels at the accessory buildings). If any water pipes run between the buildings, a parallel low-impedance path back to the source exists.

What if only plastic water pipes are used ? I feel the danger here is that 30 years later during a new project a metal pipe connection is suddenly established and with that a potentially dangerous situation.

 

[electrofelon]

Perhaps the thinking was, that if not for this exception, one could just get two services which would likely be fed from the same transformer, and thus you end up with the same situation - so they just give you exception 3 and save you the extra meter fee and install cost.

 

[GoldDigger]

Perhaps the thinking was, that if not for this exception, one could just get two services which would likely be fed from the same transformer, and thus you end up with the same situation - so they just give you exception 3 and save you the extra meter fee and install cost.

I suspect that the other reason the exception is narrowly for residential is that commercial and industrial situations which do not want multiple meters and accounts would instead just run feeders from the main building to the accessory buildings.
The other primary location that I am aware of that historically has used multiple POCO drops from a single meter and pole has been the family farm.