Capacitor Discharge Time and Bleed Resistor (NEC 460.6)
Sizes the bleed resistor that keeps a switched-off power-factor or DC-bus capacitor from staying lethal.
Example
You enter
- Total capacitance (uF) 100
- Initial voltage at disconnect (V; ac bank: the PEAK, 1.414 x rms, e.g. 679 V at 480 V) 600
- Safe voltage target (V, 460.6 = 50) 50
You get
- R max (ohm) 241458
- Continuous power while energized 1.49 W
- Limit (s) 60
Checked against published worked examples from: International Capacitors S.A. (Lifasa).
Details, formula, and sources
ScopeNEC 460.6 requires the residual voltage to fall to 50 V within 1 minute at 1,000 V nominal or less (460.28: 5 minutes above 1,000 V). V(t) = V0 e^(-t/RC), so R_max = t_limit / (C ln(V0/50)) and the continuous burn is V0^2/R; on an ac bank enter the peak, 1.414 x rms. A 100 uF, 600 V cap needs a 241 kohm-or-smaller resistor, dissipating 1.49 W -- a permanently connected 220 kohm 2 W resistor does it. A 1 uF, 4160 V surge cap gets the 5-minute allowance, so 67.9 Mohm at 0.26 W suffices. The discharge means must be permanent or automatic on loss of line -- a manually switched bleed does not comply. A design aid; the equipment listing governs.
Formula or tableV(t) = V0 e^(-t/RC); t_discharge = R C ln(V0/V_safe); R_max = t_limit / (C ln(V0/V_safe)); P_continuous = V0^2/R. V0 is the voltage at disconnect, the peak (1.414 x rms) on an ac bank.
Source and editionThe capacitor stored-energy discharge requirement of NEC 2023 460.6 and 460.28 -- residual voltage to 50 V within 1 minute at 1,000 V nominal or less, 5 minutes above 1,000 V, by a permanent or automatic discharge means -- by name. A computational aid; the AHJ-adopted NEC edition and the equipment listing govern.
Free accessNEC is free to read at nfpa.org/freeaccess. The RC-discharge relation is first-principles; the 460.6 time limits and the permanent/automatic-connection requirement are in the published code.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namescapacitance_uf, initial_voltage, safe_voltage, r_max_ohm, p_continuous_w, limit_s
- Time limit residual to 50 V within 1 minute at 1,000 V nominal or less, 5 minutes above 1,000 V (600 V was the pre-2020 threshold)NEC 2023 460.6 / 460.28
- Connection the discharge means must be permanently connected or connect automatically on loss of line; a manually switched bleed does not complyNEC 460.6(B)
- Continuous power the resistor dissipates V0^2/R while the bank is energized (about half that on an ac bank with a peak V0); rate it with marginfirst-principles
- Initial voltage an ac bank can be disconnected at its peak, sqrt(2) x rms; the rms value returns a resistor too largeLifasa TS 03-010I Discharge Resistors