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How can you pass cables through a fire-rated wall and keep "free air" ampacities?

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    How can you pass cables through a fire-rated wall and keep "free air" ampacities?

    I have a set of cables in free air that I need to pass through an 8-inch cinder block fire-rated wall and then in free air back to the transformer. I would like to use the "free air" ampacity ratings from Table 310.15(B)(17). Is there anyway to pass through a free-rated wall and keep the "free air" ampacity ratings?

    1. One approach is to transfer the cables to a cable tray, in a single layer with over a diameter space between per [COLOR=#333333]392.80(A)(2)(c). Then use a cable tray wall penetration sleeve with fire stop blocks. However, the cables will no longer be in free air for the 8 inches it is encapsulated by the fire stop material. So I can increase the size of conductors to get a necessary ampacity. Are there any exceptions for small lengths of "free air" cables not being in free air for small lengths (such as 8" out of a 50 ft run)?[/COLOR]

    [COLOR=#333333]2. Another option is to mount a fire-proof splice box of some sort at the wall. Attach cables to terminals on either side of the wall. Are there products that are designed for passing through fire-rated walls? Possibly use a wall penetration bus duct with cable terminals on each side.

    I am curious if anyone has any creative ideas.[/COLOR]

    See 310.15(A)(2) exception.


      Conductors sleeved in a raceway 24" or less need not be derated.


      (2) Adjustment factors shall not apply to conductors
      in raceways having a length not exceeding
      600 mm (24 in.).
      [COLOR="blue"]"Electricity is really just organized lightning." George Carlin