Wednesday, March 4, 2015

Residential Cable Pathways and Conduit

Hello world. Sorry it's been a while. I got a job at Google and moved to California therefore my posting frequency had gone down for a bit. It's a mix of being very busy and general NDAness of working at Google. 

Riser between garage and attic
I'm adding a home security system as well as adding wired network to several locations in my new home in San Jose. After having some very weird and un-reusable pathways in my last house, I decided to implement some best practices for wiring by installing actual conduit to my wiring closet (office closet). I found a great space between my garage and the attic that allows the venting of the water heater and furnace as well as the main ductwork for the upper floor. It also shares a wall with the office closet which is an ideal location for network switches and the alarm system. My hope is that this will allow me to pull future cable with a pull-string through the conduit instead of drilling more holes or ripping apart the drywall for another pathway.

Conduit and mounting materials
Typical low voltage conduit is metal EMT due to the low cost, rigid/protective nature and plethora of mounting hardware available. Unfortunately, my local Home Depot didn't have EMT in the 3" size I wanted so I decided on schedule 40 PVC. I verified that Art. 352 of the NEC allows PVC conduit to be concealed within walls, floors, or ceilings; directly buried; or embedded in concrete in buildings of any height. This will protect the cabling and simplify future cable pulls. I purchased a piece of unistrut, 4 3" PVC strut clamps, a 90° elbow and a 10' section of bell-ended schedule 40 conduit.

NEC article 352.30 requires supports within 3' of each termination and least every 6' so I went with a bottom, middle and top support after cutting down the conduit to 90" (7'6").

Access hole in closet
The first step was to measure the width of the riser space and cut the unistrut with a metal blade on my saber saw. 3 cuts burned up one of my cheap thick metal blades that came from Harbor Freight in a 20-pack. I then cut an access hole in the office closet to allow installation of the unistrut and strut clamp.

Mounted unistrut
Next I fed the assembled conduit into the hole and did a test-fit for location. I then mounted the 3 pieces of unistrut at the base of the riser, in the middle where I cut the access hole and finally at the belled transition to the 90° elbow. I used wood screws and washers to fasten the unistrut to 2 points on the studs in all 3 locations.

Holding the elbow base against the top strut, I located and cut a 3" diameter hole in the drywall using the elbow as a template. I then ran the elbow through the hole from my access hole in the side of the closet for final test fit.

Conduit hole in drywall
From the garage, I fed the straight piece of PVC into the hole and had my daughter attach the elbow at the access hole in the closet and then guide the assembled conduit into place on the wall. I slid the first strut clamp into place while she supported it and tightened it down until she could release it, fully supported.

I reached through the access hole and attempted to install the other two clamps and found that the offset from the wall caused the clamp to not quite close. Longer bolts (2 1/2") fixed the problem allowing me to support the conduit in the upper 2 locations.
Midpoint support strap with 2 1/2" bolts
Elbow support at the top of the conduit
The final step is to cut a passthrough in the metal plate at the bottom and repair the drywall. Then it's time to drop a weighted pull string through and start pulling cable.

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