Formula for CAPACITANCE in 2 Conductor (Shielded & Non-Shielded) Wires & Coax
Balanced Pair
Cable Type
CAPACITANCE (pF/ft)
IMPEDANCE (Ω)
Unshielded
Twisted Pair
Shielded
Twisted Pair
Overall
Shielded &
Cabled
where:
C = Mutual capacitance (pF/ft.) d = Diameter of the conductor (in.) D = Diameter over the insulation (in.) ε = Insulation dielectric constant (see Table II) f = Stranding factor (see Table I)
ZO = Characteristic impediance
No. of
Strands
Stranding
Factor
f
1
1.000
7
0.939
19
0.970
37
0.980
61
0.985
91
0.988
TABLE I
Material
Dielectric
Constant ε (pF/ft)
Velocity of
Propagation
VP (%)
ECTFE (Halar®)
2.60
63
PFA (Teflon®)
2.15
68
PVC
5.00
45
PVC (Semi-rigid)
3.60
53
PVDF (Kynar®, SOLEF®)
7.70
36
Polyethylene
2.29
66
Polypropylene
2.25
67
Polyurethane
6.50
39
Rubber – butyl
4.0
50
Rubber – natural
5.0
45
Rubber – SBR
4.0
50
Rubber – silicone
3.1
57
TFE Teflon®
2.1
69
TPE
5.0
45
Teflon®
2.10
69
Tefzel®
2.6
62
TABLE II
Coaxial Cable
Measure
Equation
UNITS
CAPACITANCE
pF/ft
IMPEDANCE
Ω
INDUCTANCE
µH/ft
VELOCITY OF
PROPAGATION
%
TIME DELAY
nsec/ft
CUTOFF
FREQUENCY
GHz
where:
C = Capacitance (pF/ft.) d = Diameter of the center conductor (in.) D = Diameter under the shield (in.) ε = Insulation dielectric constant (see Table III) f = Stranding factor (see Table I on previous page) L = Inductance
