I was planning to get 3/4" conduit (Carlon ENT/Smurftube) to run network cables through the house.
Just how many CAT6 cables can safely fit into 3/4" conduit?
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This depends on whether you are using pre-assembled cables or not. The following images were made using The Engineering Toolbox's Smaller Circle in Larger Circle Tool to find the optimal packing.
If using bare cable (No Connectors)
However, this assumes the cables are perfectly circular and non-deformable. If you assume you can deform the cables, or you use a slightly smaller cable, you can fit up to 7 without destroying the cables.
If using pre-assembled cable
If running connectorized cables, you can probably get away with 2, max. However, if using bare cable, you can comfortably fit 5, but can potentially fit up to 7 without completely squishing the cables. Using the ratio of areas, we could theoretically fit ((.75^2)/(.25^2)) = 9 cables, but if we abide by the National Electrical Code's 40% rule, this translates to only 3.6 cables (more accurately, ((0.203)/(pi*.125^2))=4.1). Therefore, if you go with bare cable, by code, you can only legally run 4 cables through the conduit. (I'm not sure how the NEC calculates this 0.203 although their math allows up to 4 cables - my math would limit us to only 3 cables). Although I do not endorse illegal activity, I would feel comfortable running up to 5 cables in that size conduit.
When it comes to data (Cat6) cable it in not so much about the number of wires you can fit in to the conduit, because the more you get in the conduit the more the speed of the information could degrade. I have always put no more then 4 Cat6e in a 3/4in Conduit. So pick the best cable for your job and then the correct size of conduit or run more then one 3/4in.
The following is from, http://www.datcominc.com/edit/files/catalogues/Mohawk%20Conduit%20Fill%20Guide.pdf
If you're following National Electrical Code, you'll need to know the actual size of the cable. In the Notes to tables section of chapter 9, there are two important notes.
(5) For conductors not included in Chapter 9, such as multiconductor cables,the actual dimensions shall be used.
(9) A multiconductor cable, optical fiber cable, or flexible cord of two or more conductors shall be treated as a single conductor for calculating percentage conduit fill area...
So assuming you'll be pulling more than two cables, according to Table 1 of chapter 9 you'll have to use the 40% fill column from the conduit fill tables.
You'll need to know the cross sectional area of the cable you're installing, and the type and size of conduit you want to use. Then you'll look up the allowable fill for that size conduit, using the 40% fill column of the applicable table.
3/4" Electrical Nonmetallic Tubing (ENT), has a 40% fill value of 0.203 in.².
Then you'll divide the value from the table, by the area of the cable (and drop the remainder). This will tell you how many cables you can pull though that conduit.
According to this document, the outside diameter of UTP Category 6 cable is 0.25 in.
0.25 in. / 2 = 0.125 in.
A = pi * r² = pi * 0.125² = 0.0490873852 in.²
0.203 in.² / 0.0490873852 in.² = 4.1354820423
So if you're pulling similar sized cable through 3/4" ENT, you'd be able to pull 4 cables through the conduit and be code compliant. Though in reality, without lube and special cable pulling tools. You'd probably only be able to pull 2 or 3, unless it's a short straight-ish pull.
For what it's worth, I recently ran 5 in 3/4" conduit and it was a tough pull. However, it had a couple bends to work through. I used foam lube and fish tape to help ease the friction. I do worry about degrading the signal and may remove one. I'd say 4 should be more than doable without concern of damaging wire.