I previously asked a question (Grounding subpanels located ~100ft from main breaker) where I was informed of NEC 225.30, which states that a building
[…] shall be supplied by only one feeder or branch circuit […]
It sounds like this code exists to ensure that each building has a single breaker or disconnect method that disconnects the entire building.
How does this relate to a building which has a main breaker panel that feeds multiple subpanels? For example, if I have a 200A main panel with two 100A breakers, each feeding a 125A subpanel (with no main breaker, only branch circuit breakers), do the main panel and subpanels all need to be on the same building, or can the main breaker panel be located close to the meter (on a separate building) with the subpanels on a different building?
Functionally, there is no difference between the two setups, except physical location of the panels. In both cases, there is a single breaker that will disconnect both subpanels.
Best Answer
Running two feeders to one building for electricity is silly -- you can get the job done with one.
There is basically no reason in this day and age to run multiple feeders to a building in the fashion you are describing. Even if you are using multiple subpanels, you can have all of them fed from a single feeder using the feeder tap rules in 240.21(B) -- both (B)(1) and (B)(5) apply to your proposed application.
In a typical setup for this, the feeder conduit runs up to an auxiliary gutter (if you can't find an aux gutter, it's really another name for a wireway -- think fiddle vs. violin). This gutter runs over the top of the panels physically and contains the tap splices (think screw lug terminals), while the tap conductors run through conduits down to their respective subpanels. You do need a sub-main in each subpanel to do this though, but consider it a small price to pay for only having to pull once.
As to wire size -- the taps can be sized to match the sub-main overcurrent protection devices provided they're more than 10% of the size of the feeder being tapped, while the feeder itself should be sized to provide the needed ampacity to both subpanels.
Also, the panels need to be next to each other as this setup technically falls under the "rule of six" in 225.33 (this is 225.34).
Don't tear out the other pipe if it's already there, though
However, if you already have the second conduit trenched in, leave it there! It's handy as a convenient place to stuff phone, network, CATV, or other sorts of low voltage wiring, and you can simply reroute its ends to go wherever they need to with some conduit work.