In places where the block had to be up first and then channeled, I have been using emt, and then they plaster over it. 2nd floor and up though the masons stop at the second course, which they have made a 3" x 3" channel in the top of it. The channels have been cut extra smooth, unlike what you get trying to make knockout block using regular hollow tile. The dimensions of the wall block is 23-1/2" long, x 10" wide, x 8" high each. There is plenty of space from any outside edge of the blocks to the channels in the middle. I route out for boxes where I want them, and drill a 1-1/8" hole from the channel at an angle down thru the second block to my boxes. A regular router with a wood bit works really good routing out the boxes, takes about 1 minute per box to route. The compressive strength of the block is only about 700 lbs per square inch, so I don't really think this stuff is all that great like they try to hype it. During a hurricaine when the coconuts start flying will be the real test.
