I understand Dwelling units Service entrance Ampacity can be applied using 310.15(B) 6, But can sub-feeder leaving the dwelling unit feeding, example: (Garages) on the premises, or would you have to apply 310.16.
I've seen it argued both ways. The text before the chart is what sways me. "....that serve as the main power feeder to a dwelling unit ..." but more importantly "For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the .... branch cirucit panelboard(s). If you were to work it out, that the garage panel originated at the main disconnect, then I think it is ok. But if it is a subpanel to the main panelboard, then use 310.16.
Re: 310.16(B)6#81710 09/14/0205:51 AM09/14/0205:51 AM
We have had this one before and to understand it as written I don't think the garage would be considered a dwelling unit. I wonder however if the garage, as part of the property could be considered as part of the dwelling unit. That may or may not be stertching it but I don't understand why these ampacity ratings are ok for the entire house yet not for a subpanel feeding only part of the house or a garage.
If a single set of 3-wire, single-phase, service-entrance conductors in raceway or cable supplies a one-family, two-family, or multifamily dwelling, the reduced conductor size permitted by 310.15(B)(6) is applicable to the service-entrance conductors, service-lateral conductors, or any feeder conductors that supply the main power feeder to a dwelling unit. This section permits the main feeder to a dwelling unit to be sized according to the conductor sizes in Table 310.15(B)(6) even if other loads, such as ac units and pool loads, are fed from the same service. The feeder conductors to a dwelling unit are not required to be larger than its service-entrance conductors. Exhibits 310.7 and 310.8 illustrate the application of 310.15(B)(6). In Exhibit 310.7, the reduced conductor size permitted is applicable to the service-entrance conductors run to each apartment from the meters. In Exhibit 310.8, the reduced conductor size permitted is also applicable to the feeder conductors run to each apartment from the service disconnecting means, because these feeders carry the entire load to each apartment
Re: 310.16(B)6#81712 09/16/0209:36 AM09/16/0209:36 AM
I was taught that the reason a higher ampacity is permitted for these service and feeder conductors is the intermittent nature of dwelling loads and their inherent diversity. The key to sizing the feeders is that they must carry the entire load of the dwelling unit in question in order to make use of the reduced sizing. There is a much lower likelihood of the load on a garage feeder being diverse enough to make any reduction in feeder size a safe practice. -- Tom
"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison