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Appendix B
Circuit Breaker Adjustments
Adjustments B-1MN894
Circuit Breaker Adjustment (Inrush current estimation)
Mechanical adjustment of the trip setting may be necessary if the circuit breaker is
replaced. The first step of this adjustment procedure is to determine the motor inrush
current (in amperes). Inrush current is also called “Locked Rotor AMPS” or “LRA”.
In soft start applications, the initial voltage applied to the motor is reduced to
approximately 40% to 70% of nominal line voltage. It is necessary to set set the trip
current for the circuit breaker to the proper inrush current value. This inrush current is
determined by motor horsepower and motor design characteristics. To define inrush
characteristics, code letters are used. This code letter defines both the low voltage and
high voltage inrush values for dual voltage motors. Table B-1 shows these code letters.
(KVA = Kilovolt-Amperes; HP = Horsepower).
KVA/HP Calculation of motor
Table B-1 Code Letter Definition
Code Letter KVA/HP
Range
KVA/HP
Mid-Range Value
A 0.00 - 3.14 1.6
B 3.15 - 3.54 3.3
C 3.55 - 3.99 3.8
D 4.00 - 4.49 4.3
E 4.50 - 4.99 4.7
F 5.00 - 5.59 5.3
G 5.60 - 6.29 5.9
H 6.30 - 7.09 6.7
J 7.10 - 7.99 7.5
K 8.00 - 8.99 8.5
L 9.00 - 9.99 9.5
M 10.00 - 11.19 10.6
N 11.20 - 12.49 11.8
P 12.50 - 13.99 13.2
R 14.00 - 15.99 15.0
3 Phase Inrush Current Calculation (Use mid range value for KVA/HP)
Inrush AMPS +
ǒ
KVA
HP
mid range value
Ǔ
xHPx577
Rated Volts