factors influencing capacitance of a cable

Question

I am out of my depth here so I am looking for a simple explanation. If you have twisted pair cables of differing makeup, varying in conductive material, diameter of the conductor, number of twists per meter and number of pairs in the cable. What if any effect the pairs capacitance.

This is in the sphere of telecoms.

EDIT:

C = (EoErA)/d

capacitance seems to be based on “Permittivity” which is a function of conductivity which would imply that it will differ by from copper to aluminium.

also the area of the conductor, so that conductors with a larger surface area have more capacitance.

and lastly the insulator and distance between the conductors will play a role.

Are there any effects that I am missing? does the number of pairs around the one tested make any difference?

EDIT: I seem to have misunderstood Permittivity it seems to be only to do with the insulator not the conductor.

Answer 1

Capacitance is a function of the spacing of the conductors and the dielectric constant (aka relative permittivity) of the insulating materials. All other things being equal, the capacitance between two wires will be proportional to the dielectric constant of the insulator.

A vacuum (and dry air) has a dielectric constant of 1. Insulating plastics have dielectric constants in around 2-4 generally.

The thicker the insulation, the wider spaced the wires will be, and thus the lower the capacitance per linear unit, again, all other things being equal.

You can approximate the capacitance of an unshielded twisted pair by:

C(pF/ft) = \$\frac{2.2\epsilon }{log_{10}(\frac{1.3D}{f d})}\$

Where

D is the diameter of of the wire including insulation (inches)

d is the conductor diameter (inches)

\$\epsilon\$ is the dielectric constant of the insulation

f is the stranding factor (about 1)

Here's another reference that gives a derivation of a similar approximation.

The resistance of the conductors does not directly affect the capacitance but it will have other effects on the performance, especially when the lengths are long. Look up the Heaviside condition for more on that.

Answer 2

When Insulated wires getting twisting, Capacitance also increase with respect to number twists even if you are increasing thickness.