What is the length twisted pair WIRE versus cable LENGTH?

[Holes Flow]

Experts,

In making calculations for lengths of twisted pair, there is the length of the cable itself, and based on the standard a tight or loose pitch, a different wire foot exists for every cable foot. See attached for differences in wire used per cable foot.

And yes, while I understand the slight differences in pitch between pairs for interference/performance reasons, I'm looking for values good enough to use in TDR calculations, so averages of all 4 pairs are fine. I'm thinking just CAT5/e and CAT6. Maybe I've lost my Google-Fu, but all searches bring up are questions about what the max cable run can be with CATx. And I'm still uncertain about 22/24/26 AWG being different within a CAT spec.

Thanks for any pointers!

[rstofer]

The manufacturer's datasheet will give you Velocity of Propagation as a percentage - typically 70% for network cable.

http://www.te.com/content/dam/te-com/documents/channel/marketing-materials/cat6a-ecard.pdf
https://catalog.belden.com/techdata/EN/1227A1_techdata.pdf

That VoP is exactly the number to dial in to the TDR (at least the one I had).

A lot of folks build a fast rise time pulse generator and do TDR measurements with a scope.  You have to lay the wire out on the ground, it can't be left spooled.  Measure 100' and see what you get - it will probably be on the order of 1.5 ns per foot.  You can search around here for such a pulse generator.  There is a project in one of these forums. 

w2aew has a project:

[Holes Flow]

@rstofer,

Thanks for writing. I was actually looking for documentation or a reference that explains the relationship between actually length of wire in the various STP/UTP specs with relationship to the length of the cable the TP is in.

For example, it if takes 13" of 24ga wire to make a single 12" of cable, that ratio. Pitch has a lot to do with it, but so the insulation thickness, most all of the specs being defined in the specs for CATx.

These values don't matter at all for small lengths, but with a line driver at, say 30mA at 5V, the difference between physical and electrical length start to add up. And different CATx specs specify different twist pitches.

[chris_leyson]

Essentially you need to calculate the length of a helix to get overall wire length. You just need to know the pitch of the helix, which is also known as cable lay length, and the diameter of the helix which is just the diameter of the wire for simple twisted pair. Good explanation on page 7 "http://www.nexans.com/France/group/doc/en/General.pdf". For example, if I have 1mm diameter wire and a pitch or lay length of 10mm then the overall wire length works out to be 10.48mm.