Enter the relative dielectric constant, the wire center-to-center spacing, and the wire diameter into the calculator to determine the parallel wire capacitance per unit length.
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Parallel Wire Capacitance Formula
The following equation is used to calculate the parallel-wire capacitance per unit length for two round, parallel wires in a uniform dielectric.
C′ = (π·ε0·dc) / acosh(s/d)
- Where C′ is the parallel wire capacitance per unit length (F/m)
- dc is the relative dielectric constant (εr) of the material between the wires
- s is the center-to-center spacing of the wires (use the same length units as d)
- d is the diameter of the wires (same length units as s)
What is a Parallel Wire Capacitance?
Definition:
Parallel-wire capacitance is the distributed capacitance that exists between two parallel conductors (such as a twin-lead line). It is typically specified per unit length (for example, pF/m), and depends on the wire diameter, the spacing between the wire centers, and the dielectric constant of the material around/between the wires.
How to Calculate Parallel Wire Capacitance?
Parallel wire capacitance is usually calculated as a capacitance per unit length using the geometry of the two wires and the relative dielectric constant. For two round wires in a uniform dielectric, a common model is: C′ = (π·ε0·εr) / acosh(s/d).
If you need the total capacitance for a given cable length L, multiply the result by length: C = C′·L.
For example, in air (εr ≈ 1), with center-to-center spacing s = 5 mm and wire diameter d = 1 mm: acosh(s/d) = acosh(5) ≈ 2.292, so C′ ≈ (π·ε0)/2.292 ≈ 12.1 pF/m. Over L = 10 m, the total capacitance would be C ≈ 12.1 pF/m × 10 m ≈ 121 pF.
FAQ
What factors affect the capacitance of a parallel wire capacitor?
The capacitance per unit length between two parallel wires is influenced by the relative dielectric constant of the material around/between the wires, the center-to-center spacing between the wires, and the wire diameter. For the same geometry, increasing the relative dielectric constant increases capacitance approximately in direct proportion.
Can parallel wire capacitance be used in high-frequency applications?
The capacitance between two wires exists at all frequencies. At high frequencies, a pair of wires behaves as a transmission line with distributed capacitance and inductance, so the capacitance per unit length is commonly used when estimating impedance, signal integrity, and propagation behavior.
How does the relative dielectric constant affect parallel wire capacitance?
The relative dielectric constant (dc or εr) measures how much the surrounding material increases capacitance compared with vacuum. For a fixed wire diameter and spacing, a higher εr results in a higher capacitance per unit length.
