How to Use the Cable Charging Current Calculator
Shielded underground cable behaves as a capacitor between conductor and shield, so current flows even with nothing connected at the far end. That is charging current, and on a long medium voltage feeder it grows large enough to matter. Enter system voltage, cable capacitance per 1,000 ft and circuit length to get the per-phase charging current and three-phase charging kVAR.
The expression is Ic = 2πf · C · V ÷ √3. Here C is the catalog capacitance multiplied by the installed length and the number of parallel circuits, and the phase to ground voltage is line voltage divided by the square root of three. Three-phase charging kVAR is three times phase voltage times charging current. For a 13.8 kV feeder running 6,000 ft of cable at 0.08 µF per 1,000 ft, the result is about 1.4 A per phase and roughly 34 kVAR.
Charging current occupies transformer and breaker capacity with no load present and drives the Ferranti effect, where receiving end voltage exceeds sending end voltage. On an ungrounded system a single line to ground fault returns the capacitive current of all three phases through the fault point, so roughly three times the per-phase value appears there. Ground overcurrent relay pickup has to sit above that figure or the relay will trip on healthy system charging current. Always use manufacturer capacitance data, and have relay settings reviewed by a qualified protection engineer.
Frequently Asked Questions
From the electrical characteristics table in the manufacturer catalog, usually given in microfarads per 1,000 ft. It changes with conductor size, insulation thickness and shield construction, so two cables of the same voltage class differ. Shielded 15 kV EPR or XLPE typically falls between 0.05 and 0.10 microfarads per 1,000 ft.
It flows with no load connected, so it occupies transformer and breaker capacity and raises the receiving end voltage, known as the Ferranti effect. It also feeds ground-fault relays, which can misread healthy phase charging current as a fault and trip without cause.
On an ungrounded system a single phase contacting earth raises the other two phases to full line voltage above ground, and the current through all three phase-to-ground capacitances returns through the fault. The result is roughly three times the per-phase charging current.