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Move branch circuits to new sub panel, add transfer switch. Opinions requested

Move branch circuits to new sub panel, add transfer switch. Opinions requested

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Old 12-05-14, 11:05 AM
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Move branch circuits to new sub panel, add transfer switch. Opinions requested

Hello,

So, I'm planning a large off-grid solar project as mentioned in a previous thread here. All will be permitted/inspected/etc. I have a decent bit of experience with all of this, but, can never hurt to double check my plan and get other opinions. Jurisdiction is North Carolina, Catawba County. NC is under the 2011 NEC with minor amendments that don't appear to relate to my situation.

I want to do this in phases. I'm working on planning for phase one now.

Phase one will be to move all branch circuits in my existing two 200A service panels to two new 200A sub-panels located in a new "mechanical room" (essentially a 10x12 room in the finished basement earmarked for all of the off grid solar equipment). The new panels will be located on an interior non-load bearing wall. The new mechanical room does not have a ceiling (exposed floor joists with insulation between them).

I'm going to install two 200A manual transfer switches (huge Square D type) near the two new panels. The connection will end up being Service wiring -> Existing 200A main breaker -> 200A sub-panel breaker in service panel (the 4-position huge sucker for QO panels) -> one side of the transfer switch. Then the load (center) of the transfer switch will feed the 200A "main" breaker on the new sub-panel. Then repeat entire process for the second panel.

The existing service panels will remain in place and will eventually have the breakers for the grid interactive inverters (for automatic use of grid power during a low battery condition). So, I'm not removing them or converting them to gutted junction boxes.

I plan to use 3/0 copper (L1, L2, and N) for the service panel to transfer switch and transfer switch to sub panels and a 4 AWG ground conductor for the same. (Not bonding ground/neutral beyond of the service panel obviously). Plan is to running this all in 2" EMT.

Doing so will require the disconnection and removal of the existing branch circuits from the service panel first, since the top 2" knockout is used for the conduit stub-up for the branch circuits currently and there are no other accessible knock outs that would be practical to use for this 2" conduit connection.

Some existing branch circuits (Romex) run somewhat in the direction where I'm going to be putting the new panels. So, for those, the plan is to simple reroute the existing wiring as possible/legal without any splicing.

The majority, including the feed for an existing 100A sub-panel in the finished attic space, will not reach without splicing. My plan is to add one or more junction boxes above the drop ceiling above the existing service panels, and have the branch circuits that will not reach enter this box. The majority are 14/2 or 12/2 romex with a couple of 10/2 and a few larger 60A branches for HVAC. In total there are 28 circuits that will need relocating.

I plan to connect the existing branches each to new THHN wiring and run it through EMT or ENT to the new panels. I'll add a ground bus to the junction box and pull a 8 or 6 AWG ground (probably overkill, but I have extra of this wire) feed from there through each conduit used from the junction box ground bus bar to the sub panel ground bus bar. All existing grounds tie into the ground bus bar in the juction box, obviously. Line and neutral lines all 1 to 1, same size conductors and conductor count. I know that in some cases EMT can be used as a ground, but I've never actually solely used it for one.

I *believe* I'm OK with 8 or 9 line/neutral wires plus my ground in 3/4" EMT or ENT conduit from the j-box to the new sub-panels after taking into account NEC Table 310.15(B)(3)(a) and noting NEC 240.4. This should let me route all of the existing circuits using 4 or 5 runs of conduit. I'm not sure if using a larger conduit and up-sizing the conductors per 310.15(B)(3)(a) would make more sense. The run in the ceiling on the bottom of the floor joists is unobstructed with about a 5ft wide path available without needing any bends in the conduit.

OCPDs in the new sub-panels will be upgraded to AFCI as required by NEC 210.12(B)(1).

The distance between the existing service panels and the new sub-panels and transfer switches is approximately 25 ft. The transfer switches will be adjacent to the new sub-panels. The other side of the manual transfer switch will be left open for future connection to the output of the off-grid inverter bank. (Phase 2)

I believe this is a pretty sound plan for phase one, however I would appreciate any input/comments/questions/concerns that anyone may be willing to share.

Thank you in advance!

-k

Edit: Some notes I forgot to mention...

It is not practical to relocate the service entrance conductors to the new sub-panel area for several reasons. An additional reason is that I eventually require the existing service panels for connection to the grid-interactive inverters for battery charging during low battery conditions when insufficient renewable energy is available. Also, the service entrance is less than 2 ft from the existing panels and has no OCPDs on the meter side. Extended them would require these to be added I believe.

The transfer switches and new panels will be physically close to the inverters (~5 to 8 ft) in the end and the 25 ft run of cable from the grid-tied service panels and 200A sub-panel breaker will rarely be used except during any maintenance. The 200A breaker and the 4x50A inverter breakers in each service panel will never be energized simultaneously. Also, no power is ever being fed back into the utility grid.

In phase two, the inverter bank will be sufficiently sized to operate both 200A panels normally without any load shedding needed or additional power from the utility grid.
 
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  #2  
Old 12-06-14, 06:24 AM
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My that is quite a book you posted!

If I understood everything, it all looks OK. The only thing I would mention is is you can run 2/0 copper for 200 amps per 310.15(7)(B)
 
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Old 12-06-14, 08:08 PM
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Thank you for the input and review

I thought that 310.15(B)(7) only applied to the service entrance and not sub-panel wiring... I will probably use 3/0 anyway since it can't hurt, but, if I can use 2/0 I might use it for phase 2 of my project since those are shorter runs (only ~5 ft or so).
 
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Old 12-07-14, 07:54 AM
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With only reading a few paragraphs, I have a question. Why do you want to relocate the branch circuits in the two 200 amp panels? Why can't you leave them in the existing panels and refeed those panels as subpanels?
 
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Old 12-07-14, 08:10 AM
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I explained the reasoning for this.

Near the bottom of my post.
 
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Old 12-07-14, 08:15 AM
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I explained the reasoning for this.

Near the bottom of my post.
Never mind then. I wasn't up for reading a book.
 
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Old 12-07-14, 08:40 AM
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310.15(B)(7) Conductor types and sizes for 120/240 volt, 3-wire, single phase dwelling services and feeders. You are running a feeder.
 
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Old 12-07-14, 08:19 PM
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Is this for 2011? The 2011 NEC 310.15(B)(7) doesn't read exactly like that. I only have the print version, but it reads: .... 120/240-volt, 3-wire, single-phase service-entrance conductors, service-lateral conductors, and feeder conductors that serve as the main power feeder to each dwelling unit...

It seems to be specific to the service wires, and not applicable to sub-panels.

I could be wrong, but that's how I interpret it for the 2011 code (applicable to me).

Seems pretty safe to use 3/0 instead in any case.
 
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