How to use the capacitor discharge calculator
Opening a disconnect does not remove the charge stored in a capacitor. DC bus capacitors in a VFD and power factor correction banks can hold hundreds of volts after the circuit is de-energized, and touching those terminals can cause a shock or an arc flash. Enter capacitance, bleeder resistance and initial voltage to see the time constant, the residual voltage and how long it takes to reach a safe level.
The math is the exponential RC discharge V = V₀ × e^(−t ÷ RC). The time constant τ = R × C is how long the voltage takes to fall to about 36.8 percent of its starting value, and after 5τ it is under 1 percent, which field practice treats as fully discharged. Time to a target voltage is t = τ × ln(V₀ ÷ V_target).
NEC 460.6 requires capacitors to be discharged to 50 volts or less within one minute for systems at 600 V or below, and within five minutes above 600 V. NFPA 70E treats stored energy the same way as a live source, so capacitors must be discharged and verified before a circuit is called electrically safe.
These figures assume the bleeder resistor is intact and properly connected. If that resistor is open or the dielectric has degraded, real discharge can take far longer. Always verify absence of voltage with a tested meter and apply grounding or shorting straps before touching terminals, and have a qualified person confirm the procedure.
Frequently asked questions
After 5Ï„ the voltage is about 0.67 percent of the starting value. It never reaches exactly zero, so a very high initial voltage can still leave a hazardous level. Always confirm with a meter.
Balance required discharge time against standing power loss. Solve R = t ÷ (C × ln(V₀ ÷ V_target)), then choose a resistor whose wattage rating handles the continuous heat at working voltage.