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: NEC 460.6 requires the residual voltage to fall to 50 V within 1 minute at or below 600 V (5 minutes above 600 V). V(t) = V0 e^(-t/RC), so R_max = t_limit / (C ln(V0/50)) and the continuous burn is V0^2/R. 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 and source
V(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.
The capacitor stored-energy discharge requirement of NEC 2023 460.6 -- residual voltage to 50 V within 1 minute at or below 600 V, 5 minutes above 600 V, by a permanent or automatic discharge means -- by name. A computational aid; the AHJ-adopted NEC edition and the equipment listing govern.
Audience
This tile is built for electricians and the adjacent professions in the Electrical group. The interactive calculator runs entirely in your browser. No account, no fee, no advertising, no tracking.
Related tools
Posture
Rough Logic answers the math question the working professional asks on the job. The site is a calm, fast, ad-free, account-free, ever-free reference. It does not interpret code. It does not replace the licensed professional. It does not store your inputs. The Authority Having Jurisdiction governs all installations and inspections.