Cable Capacitance Calculator
Calculate cable capacitance using conductor diameter, insulation diameter, dielectric constant and cable length for power transmission, underground cable and electrical design applications.
🌞 Free Tool · No Signup · Instant Results
Cable Capacitance & Charging Current Calculator
How to Use the Cable Capacitance Calculator
Computing standard electrical parameters for shielding and insulation design takes less than a minute. Follow these simple instructions to analyze your cable system:
- 1Enter Conductor Diameter (d). Measure or look up the bare conductor core diameter and enter it into the first input. Select the appropriate unit (mm, cm, or m).
- 2Enter Insulation Outer Diameter (D). Input the diameter across the insulation layer (inner diameter of the metallic shield/sheath). Ensure it uses the correct matching units.
- 3Select Insulation Material Dielectric Constant. Choose a standard material (XLPE, PVC, PE, or EPR) from the dropdown to automatically load its relative permittivity (εr), or choose "Other" to enter a custom value.
- 4Enter Cable Length. Input the physical run length of the cable in meters or kilometers.
- 5Add Operating Frequency. Enter the AC system operating frequency (default is 50 Hz, but 60 Hz is common in North America).
- 6Press Calculate. Click the Calculate Capacitance button to run the engineering math models.
- 7Review Capacitance and Charging Current Outputs. Analyze the total capacitance, the capacitance per km, and the safe charging current values.
How to Calculate Cable Capacitance
Because insulated cables consist of a conductor separated from a grounded metallic shield by a solid dielectric sheath, they physically function as coaxial electrostatic capacitors. Finding these values is vital because cable capacitance affects voltage regulation, insulation stress, and grid reactive power.
Cable Capacitance Formula
The standard radial electrostatics formula to determine the physical capacitance of a single-core armored or shielded cable is:
Where:
- C: Total cable capacitance in Farads (F)
- ε0: Permittivity of free space constant ≈ 8.854 × 10⁻¹² F/m
- εr: Relative permittivity (dielectric constant) of the insulating compound
- L: Total physical length of the cable run in meters (m)
- D: Outer diameter of the solid insulation (or inner shield diameter)
- d: Outside diameter of the copper or aluminum conductor core
- ln: Natural logarithm base-e operator
Cable Charging Current Formula
Alternating AC voltage continuously charges and discharges the cable dielectric layer. This produces an active charging current flowing from the system line to ground, calculated using:
Where:
- Ic: Cable capacitive charging current in Amperes (A)
- f: System operating frequency in Hertz (Hz)
- C: Total capacitance in Farads (F)
- V: Operating phase-to-ground voltage in Volts (V). In standard 3-phase networks, phase-to-ground voltage equals the Line-to-Line voltage divided by √3 (1.732).
Step-by-Step Engineering Walkthrough
Real-Life Scenario: An 11 kV underground 3-phase power line uses a 1000-meter XLPE insulated cable ($d = 18\text{ mm}$, $D = 40\text{ mm}$, $\varepsilon_r = 2.3$, $f = 50\text{ Hz}$).
Step 1: Compute the Diameter Ratio Natural Logarithm:
Compute the logarithmic ratio of physical spacing:
Step 2: Calculate the Cable Capacitance:
Multiply electrostatic constants against the log spacing ratio:
C ≈ 1.27953 × 10⁻⁷ / 0.79851 ≈ 1.6024 × 10⁻⁷ F ≈ 0.1602 μF
This gives a per-kilometer capacitance of 0.1602 μF/km.
Step 3: Calculate the Operating Phase Voltage:
Convert the 11 kV Line-to-Line voltage into Line-to-Ground phase voltage:
Step 4: Calculate AC Charging Current:
Multiply charging speed elements together:
Ic ≈ 314.159 × (1.6024 × 10⁻⁷) × 6,350.85 ≈ 0.3197 Amperes
- Total Cable Capacitance: 0.1602 μF
- Capacitance per km: 0.1602 μF/km
- AC Charging Current: 0.320 A (320 mA)
Cable Capacitance Comparison Chart
Use this reference table to evaluate typical capacitance per kilometer indices across standard high-voltage (HV), medium-voltage (MV), and low-voltage (LV) underground insulation configurations. Actual capacitance varies depending on exact shielding clearances, structural spacing, and custom dielectric properties.
| Cable Configuration Type | Insulation Type | Typical Capacitance (μF/km) | Voltage Class | Typical Application & Notes |
|---|---|---|---|---|
| Single Core XLPE | XLPE | 0.15 – 0.25 μF/km | MV (Medium Voltage) | Standard municipal and industrial electrical distribution. |
| Single Core PVC | PVC | 0.20 – 0.35 μF/km | LV (Low Voltage) | Commercial building risers, low load general grid use. |
| 3 Core XLPE (Shielded) | XLPE | 0.25 – 0.45 μF/km | MV (Medium Voltage) | Medium-voltage armored power transmission networks. |
| EPR Cable (Rubber) | EPR | 0.18 – 0.32 μF/km | MV (Medium Voltage) | Heavy flexible industrial wiring and mining supply runs. |
| HV Underground Power Line | XLPE | 0.30 – 0.60 μF/km | HV (High Voltage) | Bulk power grid cross-country underground lines. |
Note: Standard engineering reference estimates based on BS 6622, IEC 60502, and standard manufacturing tolerances. Grounding spacing and ambient soil moisture ratios can cause small shifts in actual dielectric capacitance.
Copper vs. Aluminum Conductor Sizing for Cable Capacitance (Per km, Underground & Charging Current)
Choosing the correct conductor material directly affects sizing, weight, and installation cost. Copper has a higher electrical conductivity, while Aluminum is lighter and less expensive. However, aluminum has only 61% of copper's conductivity, requiring larger physical sizes:
| Material Property | Copper (Cu) | Aluminum (Al) | Sizing Impact |
|---|---|---|---|
| Resistivity (Ω·m) | 1.72 × 10⁻⁸ | 2.82 × 10⁻⁸ | Aluminum requires 1-2 sizes larger |
| Density (g/cm³) | 8.89 | 2.70 | Aluminum is ~70% lighter |
| Thermal Expansion | 16.5 × 10⁻⁶ | 23.1 × 10⁻⁶ | Aluminum requires special compression lugs |
Aluminum is widely used for major service feeders, while copper is the standard for branch circuits in Cable Capacitance (Per km, Underground & Charging Current) systems due to terminal connection reliability.
Short-Circuit Thermal Capacity of Cable Capacitance (Per km, Underground & Charging Current) Conductors
Under short-circuit conditions, cables experience high currents for a fraction of a second. The conductor must have sufficient thermal mass to absorb this fault energy without letting its insulation melt (160°C for PVC, 250°C for XLPE). The minimum cross-sectional area required is calculated as:
Where t is the breaker trip time in seconds and k is a material constant (115 for copper with PVC). If the ground fault currents in your Cable Capacitance (Per km, Underground & Charging Current) setup are high, you may need to increase the cable or ground wire sizing to handle short-circuit stresses.
Cable Capacitance Frequently Asked Questions
Cable capacitance is the electrical capacitance that exists between the conductor of a cable and its outer metallic shield, sheath, or ground. Because the conductor and the grounded sheath are separated by an insulating dielectric material, they form a coaxial capacitor. This capacitance is a key parameter in high-voltage and underground cables.
Cable capacitance is calculated using the formula C = 2πε0εr L / ln(D/d), where ε0 is the permittivity of free space, εr is the relative permittivity of the insulation material, L is the cable length, d is the conductor diameter, and D is the outer insulation diameter.
Underground cables have significantly higher capacitance than overhead lines because their conductors are placed much closer to the grounded earth (separated by only a few millimeters of insulation) and they use dielectric materials (like XLPE or PVC) with much higher permittivity than the air surrounding overhead conductors.
Yes, the insulation material directly affects capacitance through its relative permittivity (εr), also known as the dielectric constant. Materials with higher permittivity, such as PVC (εr ≈ 3.0), store more electrostatic charge and result in higher cable capacitance than materials like XLPE (εr ≈ 2.3) or PE (εr ≈ 2.25).
Cable charging current is the continuous AC current that flows into a cable's capacitance even when no load is connected. It is driven by the alternating voltage applied across the insulation dielectric between the conductor and the grounded shield, calculated as Ic = 2πfCV_phase.
Typical underground medium-voltage and high-voltage power cables have a capacitance of approximately 0.1 to 0.6 μF/km depending on their geometry and insulation materials. XLPE cables usually range between 0.15 and 0.35 μF/km, while PVC cables may exhibit higher capacitance.