Fire Stop devices, for walls having cables that run through a wall opening, are provided on exposed ends of a conduit placed in the wall opening. Each fire stop device has a hollow housing fitted to the exposed end of the conduit, and a front wall of the housing has moveable cable clamping jaws to minimize the size of the slot through which the cables are provided. Intumescent material is provided inside the hollow housing. The rear wall of the housing has a separable back plate that contacts the conduit at only limited abutment surface to minimize heat transfer from the conduit to the housing. In a retro-fit version of the device the housing has a separable top and bottom housing portions.