Service Ampacity / Grouped Circuit Breakers

[saladhawks]

I am familar with utility electric service requirements (esr) for service ampacity / short-circuit current rating when a main circuit breaker is included in the service.

How do these requirements apply to service installations with grouped meters? For example, a service with 6 - 60A circuit breakers. How do utilities determine the service ampacity (do they assume a coincident factor) and the short-circuit current?

My research shows that most utilities require that all grouped service installations contact the utility directly for short-circuit values rather than attempting to publish them for all of the possible different service scenarios.

 

[dpc]

The ampacity and short circuit capacity are not necessarily related. The short circuit capacity is almost completely a function of the size and impedance of the first upstream transformer.

The ampacity is just what ever load the utility assumes you require. In many cases, they take whatever service is requested (200 A, etc) and divide by 2. If they guess too small, they come back and put in a bigger service drop and maybe transformer.

So it depends on how many secondary services the utility wants to put on a single transformer.

 

[waross]

If there is a fault on the load terminals of any one of your 60 amp breakers, it will be required to interupt the full available fault current. This is usually the value determined by the size and percent impedance voltage of the supply transformer.
The available fault current is only less than this if the Utility tells you in writing that it is less. The utility may do this if the distribution system is unable to supply the full fault current of the transformer. They may not wish to do so because of the possibility of a future upgrade to the distribution circuit.
respectfully

 

[Marmite]

Utilities make use of the smoothing effect of aggregation when sizing everything upstream of the meter.The actual figures used will vary depending on the type of heating/cooling etc installed in the house, but as an example a typical house may only contribute 4kVA to the maximum demand on the service/transformer despite having a 200A breaker. The contribution per house decreases logarithmically as the number of houses increases. The figures are generally based on years of research and observation of real installations.
Regards
Marmite