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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.

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    Another reason for the reg is to reduce the probability of mixing up neutrals and grounds from different panels. For instance I have 2 main services located right next to each other, and had a big problem with past wiring crossing neutrals between the two services. – Harper - Reinstate Monica Mar 17 '17 at 20:00
  • That does also make sense (especially given some of the exceptions). In my case, there is one service entry, two meters, two main breakers (currently), and three panels. I believe I will have fewer problems mixing neutrals, especially if we consolidate to one service, but I see your point. An inspector might see it similarly as well. – Hari Ganti Mar 17 '17 at 20:26
  • Late coming -- are both subpanels on the same building? Rule of six + feeder taps might be your best bet here vs. trying to do something else... – ThreePhaseEel Apr 18 '17 at 23:25
  • @ThreePhaseEel Just noticed this got bumped. Yes, both are on the same building. Can you explain more in an answer? – Hari Ganti Apr 18 '17 at 23:51
  • @HariGanti -- sure will :) – ThreePhaseEel Apr 18 '17 at 23:51
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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.

  • Thanks. This is essentially the strategy I think we'd go with. We'd simply pull new wire through the existing conduit (it was never the plan to add another conduit, just to pull more wires through the existing one). – Hari Ganti Apr 22 '17 at 16:53
  • @HariGanti -- the idea is to pull fatter wires instead of pulling more wire -- you still have a single feeder, just tapped to feed both panels at the same time. – ThreePhaseEel Apr 22 '17 at 16:55
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    Yes, I understand that. Like I said, that's how we'd proceed anyway. New wires, same conduit. – Hari Ganti Apr 22 '17 at 16:56
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I think the code is quite clear. You cannot supply a building with more than one feeder or branch circuit. You'll have to supply the building with a single feeder, then split that out to the other panels.

  • Can you quote the relevant section, because I seem to have missed something major... As it stands, there will always be a single main disconnect for each building. My concern is that 225.30 seems to indicate that I can't feed two subpanels on a remote building with two separate multiwire branch circuits. – Hari Ganti Mar 17 '17 at 21:07
  • @HariGanti sorry, I guess I misunderstood your question. In that case the code is still clear, you cannot supply a building with more than one feeder. You'll have to supply the building with a single feeder, then split that out to the other panels. – Tester101 Mar 17 '17 at 21:12
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    @HariGanti If there's a reason to (fire, earthquake, etc), emergency services should only have to access a single point to shut off power. – Tester101 Mar 17 '17 at 21:18
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    Da rules is da rules. You could ask the inspector for an exception. – Tester101 Mar 18 '17 at 2:04
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    A building can have multiple occupancy's and a service for each with a maximum of 6 disconnects for the service. Also some buildings have multiple services with different voltages. The NEC handbook from 2008 shows examples starting at exhibit 230.1 through 230.13 there are also exemptions if the serving utility normally would not supply a large enough service a second service is allowed. I know this information is still in the 2014 code because I needed to add a new service to a separate section of a building. The few circuits that were in that section from the original service were removed. – Ed Beal Mar 18 '17 at 2:53

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