The photo shows a disconnected generator transfer switch and a sub-panel in an interior room on a wall that is shared with an attached garage.

Transfer switch and sub-panel

The transfer switch is pre-wired with some sort of flexible metal conduit. (Greenfield? AC / armored cable? BX?) The conduit is about 1" interior diameter and 1.25" exterior.

As part of a remodel, I want to mount the transfer switch in the attached garage, so the metal conduit comes through the wall and into the subpanel. I want to mount the switch roughly opposite the subpanel.

The wall is plywood with drywall on the garage side as a fire-barrier.

May I just drill a big hole through the wall into the garage and run the metal conduit through? I would seal the edges of the hole with some fireproof caulk. Or is some sort of connector fitting required?

1 Answer 1


I'm no code expert but as I understand it, yes if you use fire caulk as per exception #2 in 713.3.1 and you are going to surface mount the switch on the drywall in the garage.

713.3 Fire-resistance-rated walls. Penetrations into or through fire walls, fire barriers, smoke barrier walls and fire partitions shall comply with Sections 713.3.1 through 713.3.3. Penetrations in smoke barrier walls shall also comply with Section 713.5.

713.3.1 Through penetrations. Through penetrations of fire-resistance-rated walls shall comply with Section 713.3.1.1 or 713.3.1.2.

Exception: Where the penetrating items are steel, ferrous or copper pipes, tubes or conduits, the annular space between the penetrating item and the fire-resistance-rated wall is permitted to be protected as follows:

  1. In concrete or masonry walls where the penetrating item is a maximum 6-inch (152 mm) nominal diameter and the area of the opening through the wall does not exceed 144 square inches (0.0929 m2), concrete, grout or mortar is permitted where it is installed the full thickness of the wall or the thickness required to maintain the fire-resistance rating; or

  2. The material used to fill the annular space shall prevent the passage of flame and hot gases sufficient to ignite cotton waste when subjected to ASTM E 119 or UL 263 time-temperature fire conditions under a minimum positive pressure differential of 0.01 inch (2.49 Pa) of water at the location of the penetration for the time period equivalent to the fire-resistance rating of the construction penetrated.

    713.3.1.1 Fire-resistance-rated assemblies. Penetrations shall be installed as tested in an approved fire-resistance-rated assembly.

    713.3.1.2 Through-penetration firestop system. Through penetrations shall be protected by an approved penetration firestop system installed as tested in accordance with ASTM E 814 or UL 1479, with a minimum positive pressure differential of 0.01 inch (2.49 Pa) of water and shall have an F rating of not less than the required fire-resistance rating of the wall penetrated.


Side note: whats wrong with it where it is in the picture? Don't want it inside your house looking ugly? Also, some of those wires look a little short. You may want to cut as much of the BX as you can leaving just enough to connect it.

  • Thanks! Why not mount the transfer switch inside? If I mount it inside, I still have to plug the generator into it. I think that means either leaving the connecting door to the garage ajar and running the generator cable through it (noise, maybe some fumes even though I'll wheel the generator out of the garage to run it), or installing some sort of through-wall connectors to get power from the generator to the transfer switch (involving more work than mounting the transfer switch in the garage).
    – Michael H
    Commented Jul 7, 2014 at 14:09

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