The Myth That Two Conductors Feeding a Load Would Not Have Capacitance

The One-Sentence Reality: Parasitic Capacitance Exists Everywhere

The idea that two conductors feeding a load would not have capacitance is a convenient fiction used in basic DC circuit theory; in physical reality, any two wires separated by an insulating dielectric form a capacitor, creating parasitic capacitance that stores energy and alters high-frequency or long-run AC behavior. People commonly confuse this parasitic (or stray) capacitance with intentional capacitance—assuming that if they didn't solder a discrete ceramic or electrolytic capacitor into the circuit, the capacitance is strictly zero. What this parasitic property changes in a real installation is the high-frequency impedance, signal rise times, and displacement leakage currents, which can trigger nuisance GFCI trips, destroy motor bearings, or cause ghost voltages on LED fixtures. In standard 60Hz residential wiring over short distances, this effect is negligible, but as cable runs exceed 50 meters or switching frequencies climb into the kilohertz range, ignoring conductor capacitance leads to catastrophic field failures.

Cable Capacitance Reference Data

To understand why the "zero capacitance" myth fails in the field, you need to look at the actual dielectric properties of the insulation materials surrounding your conductors. Polyvinyl chloride (PVC), cross-linked polyethylene (XLPE), and Teflon all have different dielectric constants, which directly dictate how much charge the cable geometry can store per meter. Below is a reference table of typical capacitance values for common electrical and signal cables. For a deeper look at the physics of how dielectrics store energy, review All About Circuits' chapter on capacitance and electric fields.

Notice how shielded and high-voltage cables (like VFD and 4-20mA pairs) exhibit significantly higher capacitance per meter. The proximity of the conductors to the grounded shield, combined with the dielectric constant of the thick insulation required for voltage isolation, creates a highly efficient parasitic capacitor. To calculate the total parasitic capacitance for any run, simply multiply the length of the cable in meters by the pF/m value in this table.

Worked Example: The Hidden VFD Capacitive Load

Let’s prove mathematically why assuming two conductors feeding a load have no capacitance will destroy your equipment in a high-frequency application. We will calculate the displacement current flowing through the parasitic capacitance of a long motor feeder. If you need to verify these measurements in the field, refer to Fluke's guide to measuring capacitance with a dedicated LCR meter.

The Scenario: You are wiring a 480V AC motor using a Variable Frequency Drive (VFD). The run is 150 meters of 10 AWG shielded VFD cable, which has a manufacturer-rated capacitance of 130 pF/m between the phase conductors and the grounded shield. The VFD outputs a PWM waveform with a carrier frequency of 4 kHz and a peak DC bus voltage of roughly 680V peak.

Step 1: Calculate Total Parasitic Capacitance
Total Capacitance (C) = Length × Capacitance per meter
C = 150 m × 130 pF/m = 19,500 pF = 0.0195 µF (or 19.5 nF).

Step 2: Calculate Capacitive Reactance (Xc) at the Switching Frequency
While the fundamental motor frequency might be 60Hz, the PWM switching edges hit the cable at 4,000 Hz. The parasitic capacitance reacts to this high-frequency carrier.
Xc = 1 / (2 × π × f × C)
Xc = 1 / (2 × 3.14159 × 4000 Hz × 0.0000000195 F)
Xc = 1 / 0.000489 = 2,045 Ω

Step 3: Calculate the High-Frequency Displacement Current
Using Ohm’s Law for the peak voltage of the PWM pulses:
I = V_peak / Xc
I = 680V / 2,045 Ω = 0.332 Amps (332 mA) of high-frequency leakage current.

Where You Meet This in Practice

Understanding conductor capacitance moves you from a theoretical hobbyist to a competent field technician. Here is where ignoring the "zero capacitance" myth causes real-world headaches:

• Ghost Voltages in Long 3-Way Switch Runs: When you run a 3-way or 4-way switch circuit, the traveler wires run parallel in the same NM-B cable for dozens of feet. The parasitic capacitance between the energized traveler and the switched-off traveler allows a tiny AC displacement current to couple across. If the load is a modern LED bulb with high-impedance driver circuitry, this coupled current charges the LED's internal capacitor until it flashes or glows dimly, even when the switch is off.
• 4-20mA Analog Sensor Signal Degradation: In industrial PLC setups, a 4-20mA loop acts as a current source, but the physical cable acts as a low-pass filter due to its parasitic capacitance and the loop's termination resistance (usually 250Ω). A 500-meter run of instrument cable (~55nF) combined with a 250Ω resistor creates an RC low-pass filter. The cutoff frequency (-3dB point) is calculated as f = 1 / (2 × π × R × C). For our 500m run, the cutoff frequency drops to roughly 11.5 Hz. While this passes slow temperature readings, it will completely filter out a fast-acting pressure transmitter's signal, causing the PLC to miss rapid process spikes.
• Nuisance GFCI Trips on Long Extension Cords: A 100-foot heavy-duty extension cord has measurable capacitance between the hot and neutral/ground conductors. When you plug a highly capacitive load (like a switching power supply) into the end of a highly capacitive cord, the inrush current waveform can become distorted enough to create a transient ground-fault imbalance, tripping a 5mA GFCI receptacle even though there is no actual fault to ground.

FAQ: Common Confusions About Conductor Capacitance