Calculating stranded and solid cable length (for 22AWG)

Question

Reading up on how to calculate cable lengths as a follow-up to my previous question, I understand that you have to use (102-H)/(1+D) to calculated the stranded length (C) and H+C to calculate the channel length. And that the channel's max length is 100m (in my example for 1000BASE-T).

Using the standard for D which is 0.2 (for 24AWG UTP) and 90m horizontal get me C=10. Does this mean that 22AWG cables (D=0.2) will always be out of specs (since the channel lengh will be 102 regardless of the size of the horizontal length)?

Where I'm also uncertain, but I can post that into a separate questions if needed is this example:

For example, if using 60 meters of horizontal solid category 6A cable and 40 meters of stranded 24 AWG category 6A patch cable with a 0.2 de-rating factor, the total length of the channel must be reduced to 97.5 meters

How exactly would I reduce the cable? If horizontal link is permanent an can not be change I have to reduce to 37,5m stranded in order to get to 97.5?

And the last question is, what's physically behind the math/specs? Why can't I use more then 90m in my first example? With 91m horizontal I get 9.2 stranded and 100.2 for the channel length. Even if reduce the stranded to to almost zero I'm out of specs... I just can't warp my head around this. I just want to select "the right patch cable" in an existing setup (in an office building)... and thought it would be a good idea to know a bit about the limits. But this whole approach seems to be a bit complicated or did I just start out to naive?!

Answer 1

It may be that you can exceed the length specifications and get it to work, sometimes. To guarantee trouble-free networking, you need to closely adhere to all the specifications. Sometimes, the cabling seems to work, but randomly fails for unknown reasons. You may be getting a lot of frame errors and retransmissions and not even notice it until the link is used for a particular application. It can cost a lot of time and money trying to track down problems that you may not even realize are cabling problems.

Network cabling is expensive and permanent, and there are ordinances and laws about it that could cause problems if there is an inspection by a building inspector, fire marshal, etc., who can close down the building until the problems are corrected. For such an investment that should last many years, you really need to do it correctly the first time.

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First, to properly install Category-6a cabling and have it actually pass the category test suite and work for an average distance, you really need a professional installer. Next, trying to install so close to the distance limits will make it difficult, even for an experienced installer to get it to pass the category test suite and consistently work correctly. Something as simple as having the blue wire on top of the blue-white wire at the termination punchdown can cause a test failure, and the proper test equipment is expensive and requires experience to use. The installer will use the equipment to test each cable run an present you with a report for the tests on each cable run.

A properly designed and implemented cable plant should not need patch cables over five meters, and if the horizontal cable is so close to the 90 meter limit, then you should really add another data closet. All the data closets should be tied back to the main data closet with fiber optic cable.

To accommodate modular furniture there is an exception called a MUTOA (Multi-User Telecommunications Outlet Assembly), but you really need professional cable designer and installers for something like that.

Modern network cabling for modern network speeds requires close adherence to the specifications. You can join BICSI or consult with BICSI RCDDs (Registered Communications Distribution Designers) and installers, but it is no longer a DIY project if you want things to work correctly and consistently.