Ok, I confess... I've really confused myself now. :-?
For example... say we have 3-phase, 4-wire service with a 200A OCPD serving as a main cut-off. From there we have feeder conductors to three separate subpanels: one subpanel services a 3-phase 100A load, one subpanel services a 3-phase 60A load, and one subpanel services several (no more than 6) small single phase loads (less than 40A total). So for the 100A circuit, we use 1AWG for the branch. And for the 60A circuit, we use 4AWG for the branch. And for the single-phase subpanel, an assortment of 12AWB and 10AWG branches.
But what governs the size of the feeder to each subpanel? Is it the size of the branch load (100A, 60A, and 40A)? Or is it the size of the 200A OCPD that serves as the service disconnect? I suspect it's the latter. But don't the OCPD's in each of the subpanels also protect the "upstream" feeders (as well as the "downstream" branches)? For example, the 100A subpanel should never be able to draw more than 100A (because of the 100A OCPD inside), so why would that panel need a feeder larger than 1AWG? Of course there's still the possibility of a short in the feeders, but a short would trip the 200A OCPD.
If in fact it is the 200A OCPD that governs the size of the feeder, then that would mean running three 3/0 conductors from the cut-off to each of the three panels? And how should the neutral conductor to the single-phase subpanel be sized? Obviously, running nine hot 3/0 conductors plus a neutral and EGC's out of the cut-off would not be possible (let alone practical). So if the code requires 3/0 for these feeders, how would you wire this?
(Btw, why is it that feeders and branches must be larger than SEC's? For example, for 200A, the size for SEC's is 2/0, but for feeders and branches, it's 3/0.)
For example... say we have 3-phase, 4-wire service with a 200A OCPD serving as a main cut-off. From there we have feeder conductors to three separate subpanels: one subpanel services a 3-phase 100A load, one subpanel services a 3-phase 60A load, and one subpanel services several (no more than 6) small single phase loads (less than 40A total). So for the 100A circuit, we use 1AWG for the branch. And for the 60A circuit, we use 4AWG for the branch. And for the single-phase subpanel, an assortment of 12AWB and 10AWG branches.
But what governs the size of the feeder to each subpanel? Is it the size of the branch load (100A, 60A, and 40A)? Or is it the size of the 200A OCPD that serves as the service disconnect? I suspect it's the latter. But don't the OCPD's in each of the subpanels also protect the "upstream" feeders (as well as the "downstream" branches)? For example, the 100A subpanel should never be able to draw more than 100A (because of the 100A OCPD inside), so why would that panel need a feeder larger than 1AWG? Of course there's still the possibility of a short in the feeders, but a short would trip the 200A OCPD.
If in fact it is the 200A OCPD that governs the size of the feeder, then that would mean running three 3/0 conductors from the cut-off to each of the three panels? And how should the neutral conductor to the single-phase subpanel be sized? Obviously, running nine hot 3/0 conductors plus a neutral and EGC's out of the cut-off would not be possible (let alone practical). So if the code requires 3/0 for these feeders, how would you wire this?
(Btw, why is it that feeders and branches must be larger than SEC's? For example, for 200A, the size for SEC's is 2/0, but for feeders and branches, it's 3/0.)