Speedfax 2017

Molded Case Circuit Breakers
Capacitor Circuit Conductors
Application
Article 460.8 (NEC)
The ampacity of capacitor circuit
conductors shall not be less than 135%
of the rated current of the capacitor. The
ampacity of conductors that connect a
capacitor to the terminals of a motor or
to motor circuit conductors shall not be
less than one-third the ampacity of the
motor circuit conductors and in no case
less than 135% of the rated current of
the capacitor.
Application
Circuit breakers and switches for use
with capacitors must have a current
rating in excess of rated capacitor current
to provide for overcurrent from
overvoltages at fundamental frequency
and harmonic currents. Use the following
percent of capacitor current rating to size
circuit breaker or fused and non-fused
switches.
b Enclosed Circuit Breakers 150%
(Includes derating factor for enclosure)
b Fused and non-fused 165%
switches
Due to switching surges, and possible
overcurrent related to overvoltage and
harmonics, Siemens recommends using
150% of the capacitor current rating to
size a thermal-magnetic circuit breaker
for overload protection.
If the circuit breaker is to be applied in an
ambient greater than the marked rated
ambient, it may be necessary to derate
the continuous current rating. This is also
true for applications where harmonic
components are present. A basic formula
to use for calculation of the capacitor
current rating is as follows (assuming
three phase application):
Capacitor (Kvar) x 1000
(Voltage / 1.732) x 3
The interrupting rating of the circuit
breaker or fuse must be selected to
match the system fault current available
at the point of the capacitor application.
TECHNICAL
T
T-20
Siemens Industry, Inc. SPEEDFAX Product Catalog