UFC 3 -520-01
June 10, 2002
For facility applications, continuously energized capacitors are preferred over
switching capacitors, even though this might necessitate the addition of a smaller
amount of capacitance. If a pre -selected quantity of capacitors cannot be connected
and left connected to the line in a constantly energized state, the facility electrical
system would probably be better off without them. Each capacitor on-off cycle causes
transient voltage surges that are potentially damaging to other equipment over time.
Additional design considerations such as soft -start or pre -insertion resistors are
necessary if capacitors are routinely switched. Also, apply surge protection for switched
capacitors in accordance with the criteria of Chapter 11. Refer to IEEE 141 for
additional design considerations for switched capacitors.
Evaluate capacitors installed strictly for motor applications based on the
number of motors to have power factor correction. If only a single motor or a small
number of motors require power factor correction, the capacitor can be installed at each
motor such tha t it is switched on and off with the motor. If several motors connected to
a single bus require power factor correction, install the capacitor(s) at the bus. For new
installations, specify the MCC to contain the capacitor(s). For existing installations,
determine if spare cubicles can be refurbished to accept the capacitor(s). Refer to
NEMA MG 1, IEEE 141 and IEEE 1036, IEEE Guide for Application of Shunt Power
Capacitors, for additional information.
Do not install capacitors directly onto a moto r circuit under the following
If solid-state starters are used.
If open-transition starting is used.
If the motor is subject to repetitive switching, jogging, inching, or plugging.
If a multi -speed motor is used.
If a reversing motor is us e d .
If a high-inertia load is connected to the motor.
Size the ampacity of capacitor circuit conductors at least 135 percent of the
rated capacitor current in accordance with NEC Article 460.8 (2002 Edition) . Provide
overcurrent protection and disconnection means as specified by the NEC. IEEE 1036
provides additional guidance for sizing protective devices for the maximum possible
Consult with the manufacturer before applying a capacitor under any of the
specified abnormal service conditions of IEEE 18.