What I'd do to get you out of the corner you painted yourself into
Your original plan to flushmount the panel painted you into a bit of a corner as you'd have to rip out significant chunks of the wall sheathing whenever you wanted to rewire due to the fact that in SDC D2, you need 12.5' of intact shearwall sheathing at maximum (50') shearwall spacing. Instead, I'd bend the service conduit and adjust the panel location so that the panel can be surface-mounted to the wall atop the garage-side sheathing, 1" out from the plane of the studs, with some temporary 5/4 shims holding it in place until the wall can be sheathed. With that out of the way, you can then install the sheathing, although it'll take some tricky piece-jointery and fire-taping to work around the existing conduit when doing this. You'll need ⅜" structural panels (plywood or OSB) along with ⅝" Type X drywall atop the structural panels for this; the ⅜" thickness comes from IRC Table R602.10.4 for CS-WSP bracing, while the ⅝" Type X drywall is needed to match the test conditions for the rest of the parts we're using here.
Once that's done, you can then use some STI SpecSeal LCI (about $10 or so per tube from building supply houses or industrial MRO suppliers) to seal around the conduit penetration (max ⅞" annular space, minimum ⅝" bead depth, finished flush with the wall) in accordance with XHEZ.W-L-1527; this is permitted for an angled pipe by the following text from XHEZ.GuideInfo:
Where the uninsulated penetrating item in the individual design is indicated as a metallic pipe, conduit, tube, duct or cable, and the firestop system consists at minimum of a fill material (such as sealants, putty or mortar), the penetrant may pass through the opening in the wall or floor assembly at any angle, provided the annular space is maintained on both sides of the wall or floor assembly and all other specifications in the design are satisfied. In all other cases, except where otherwise indicated in the system, the penetrating item should penetrate the wall or floor assembly at a 90° angle.
Now that we have the wall sheathed and the service conduit penetration firestopped, we can then install an STI EZ-Path 22 (or two of them, one each side of the service conduit, if you need more room) mounted above the panel so that they discharge into the attic space over the dining room, with 5/8" Type X on the house side of the wall in this area. These devices provide UL tested and listed firestopping for as many or few NM cables (up to 8/3) as you can fit into them (visual 100% max fill), and can be re-entered at a later date without having to take anything apart, for around $100 a piece through the same suppliers that you can find the SpecSeal at.
Why all this work?
There is a more important reason than "shearwall sheathing replacement annoyance" that I'm doing it this way, though. As Square-D explains in data bulletin 0100DB0704, loadcenters are not intended to go into any sort of fire-rated assembly, and the intent of IRC 302.6 is to provide a degree of fire separation between the house and the garage:
Square D™ panelboards and load centers by Schneider Electric™ are not
third-party certified for firewall installations. As of the publication of this article, we are not aware of any other manufacturer of panels that have
successfully evaluated the use of panelboards within a fire wall.
Square D has investigated fire ratings for panels in the past with
unsuccessful results. The temperature on the “unexposed” side of the wall
must remain below a certain value over a specified period of time. The
problem with panelboards (or any other device with a metal enclosure) is
that the heat transfer is very rapid and the temperature on the unexposed
sides quickly exceeds the allowable limits.
If a wall is required to have a particular hourly fire rating, then the entire wall assembly, which includes all its individual components, is expected to maintain that rating. When you place a panelboard or load center in the wall, you introduce a foreign component that has not been evaluated to insure the integrity of the wall is maintained. The same is true for doors. For example, UL tests door assemblies for fire ratings to make sure the entire wall will be resistant to fire for the required duration of time.
Square D manufactures equipment that may be used through fire walls.
Take for instance busway passing through many floors in a high rise
building. The UL ® Fire Directories (see reference below) provide materials
and assemblies that ensure the fire would not spread up a high rise through
those penetrations. Although there are methods to “Fire Stop” busway for
two hours, there have not been any panelboards successfully evaluated for
such purpose. Keep in mind that even if you were able to get the panelboard
to pass, you then also add other materials (wire conduit, penetrations to exit the panel and fire rated wall) that would have to be considered in order to maintain the integrity of the fire rated wall assembly.
So, in order to maintain the integrity of the IRC R302.6-required fire separation, we use a surface-mounted panel instead. The penetration firestopping and ⅝" Type X drywall exceed the IRC's requirements for this wall, but penetration firestop testing assumes that the wall in question is constructed as a fully rated wall, not the "cross fingers and hope we get the 30 minutes predicted by IBC 722 calculations" basis R302.6 works from.
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