Sizing of MCC equipments like Fuses, Circuit Breakers, Overload Relays and Contactors for Induction Motors.

1. MOTOR CONTROL CENTRE (MCC)

2. NEMA CLASS B
Power : 10 kW
Rated Voltage : 400 V
Rated Current : 19.7 A
Frequency: 50 Hz
Connection: Y
Power Factor : 0.86
RPM: 990
Efficiency: 85.2 %
Insulation Class: F
IP: 55
Motor runs in Industrial application having part load of 70 to 87 % (14 to 17.1 A)

3. NEMA (National Electrical Manufacturers Association) Designs
NEMA Design A:
Maximum 5% slip, high to medium starting current,
normal locked rotor torque, normal breakdown torque, and suited for a broad
variety of applications - as fans and pumps.
NEMA Design B:
Maximum 5% slip, low starting current, high locked
rotor torque, normal breakdown torque, suited for a broad variety of
applications, normal starting torque - common in HVAC application with fans,
blowers and pumps.
NEMA Design C: Has maximum 5% slip, low starting current, high locked
rotor torque, normal breakdown torque, and suited for equipment with high
inertia starts - as positive displacement pumps.
NEMA Design D:
Maximum 5% slip, low starting current, very high
locked rotor torque, and suited for equipment with very high inertia starts - as
cranes, hoists etc.

4. NEMA Code letter designation to classify motors by the ratio of locked rotor KVA per
horsepower.
NEMA
Code Letter
KVA/HP
With Locked rotor
Approximate MidRange Value
A
0 - 3.14
1.6
B
3.15 - 3.55
3.3
C
3.55 - 3.99
3.8
D
4.0 - 4.49
4.3
E
4.5 - 4.99
4.7
F
5.0 - 5.59
5.3
G
5.6 - 6.29
5.9
H
6.3 - 7.09
6.7
J
7.1 - 7.99
7.5
K
8.0 - 8.99
8.5

5. Locked Rotor Current:
It is current drawn by the
motor when the motor rotor
is locked (stopped) but full
voltage is applied to the
motor terminal.
Example of Locked rotor current is
Motor Stall.
Starting Current:
It is the current drawn by the motor
at the time of motor starting period.
It is 5 to 7 times the Full Load
Current.
Induction Motor Takes
approximately 10 second to reach
its rated speed depending upon the
size.
When motor reaches to its
rated speed, starting current
comes down to the normal
valu.
In some starting methods, lower
voltage is applied to the motor
terminal and hence the starting
current is reduced

6. Power Rating in HP and Locked Roror Codes
Less Than 1 HP
L
1 to 2 HP
L or M
3 HP
K
Code Letter : G ( for 10 kW Motor )
Minimum Locked Rortor Current : 5.6 x FLC = 110.3 Amp
Maximum Locked Rotor Current : 6.3 x FLC = 124.1 Amp
5 HP
J
7.5 to 10 HP
H
More than 15
HP
G

8. MOTOR CONTROL CENTRE MAIN EQUIPMENTS
FUSE
CIRCUIT BREAKER
CONTACTOR
OVERLOAD RELAY

9. Motor Control Centre Schematics

10. What is the difference between overcurrent protection and overload protection?
Overcurrent protection
is protection against excessive
currents or current beyond the
acceptable
current
rating
of
equipment. It generally operates
instantly. Short circuit is a type of
overcurrent.
Magnetic
circuit
breakers, fuses and overcurrent
relays are commonly used to provide
overcurrent protection.
Overload
protection
is a
protection against a running overcurrent
that would cause overheating of the
protected equipment. Hence, an overload is
also type of overcurrent. Overload
protection typically operates on an inverse
time curve where the tripping time
becomes less as the current increases.
Overload relays as well as “slow blow”
fuses are commonly used to provide
overload protection.
Some devices provide both overcurrent and overload protection. A thermal-magnetic circuit
breaker has both thermal (overload) and magnetic (overcurrent) elements. Both elements
operate as described above. Likewise the dual element fuse has both instantaneous and
inverse time characteristics in the same fuse providing both overcurrent and overload
protection

11. FUSE RATING Based on NEC 430-152
No
TYPE OF MOTOR
Time Delay Fuse
1
Single Phase
300 %
175 %
2
3 Phase
300 %
175 %
3
Synchronous
300 %
175 %
4
Wound Rotor
150 %
150 %
5
Direct Current
150 %
150 %
Maximum Size of Time Delay Fuse : 300 % x FLC
: 3 x 19.7 = 59 Amp
Maximum size of Non Time Delay Fuse: 175 % x FLC
: 1.75 x 19.7 = 34.5 Amp
Non Time Delay Fuse

12. Dual Element Fuses (Time Delay
Fuses)with both thermal and
instantaneous trip features that allow the
motor starting current to flow for a short
time without blowing the fuse.
Time delay fuses can also be used to
provide some degree of overload
protection which standard fuses cannot.
The NEC allows time delay fuses to be
sized up to a maximum of 175% of a
motor’s FLA for overcurrent protection
Time-delay fuses will hold 500% of
their amp rating for 10 seconds which
will allow most motors to start without
opening the circuit.
Under normal conditions, a 100-amp
time-delay fuse will start any motor
with a locked-rotor current rating of
500 amps or less.
Time delay (Dual
Element) LV HRC
Fuse Links
3 NA 3014
35 A, 500 VAC
Make: SIEMENS

13. Standard (Non-Time Delay) Fuses:
Standard fuses protect against short circuits and
ground faults using thermal features to sense a
heat buildup in the circuit.
The NEC allows standard fuses as overcurrent
protection devices sized up to a maximum of
300% of the motor’s FLA to allow the motor
to start.
Standard Fuse Response hold 500% of their
current rating for approximately one-fourth
of a second.
In order for a standard fuse to used as
motor overload protection, the motor would
have to start and reach its running speed in
one-fourth of a second or less.
Standard fuses will not generally provide any
overload protection for hard starting
installations because they must be sized well
above 125% of a motor’s FLA to allow the motor
to start.

14. CIRCUIT BREAKER RATING based on NEC 430-52
No
Type Of Motor
Instantaneous Trip
Breaker
Inverse Time Circuit
Breaker
1
Single Phase
800 %
250 %
2
3 Phase
800 %
250 %
3
Synchronous
800 %
250 %
4
Wound Rotor
800 %
150 %
5
Direct Current
200 %
150 %
Max Size of Instantaneous Trip Circuit Breaker : 800 % x Full Load Current
: 8 x 19.7 = 158 A
Max Size of Inverse Trip Circuit Breaker
: 250 % x Full Load Current
: 2.5 x 19.7 = 49 A

15. Inverse Time Circuit Breakers:
Inverse time circuit breakers have both thermal and instantaneous trip features and are
preset to trip at standardized levels. This is the most common type of circuit breaker
used.
The thermal action of this circuit breaker responds to heat.
If a short should occur, the magnetic action of the circuit breaker will detect the
instantaneous values of current and trip the circuit breaker.
The National Electrical Code requires inverse time circuit breakers to be sized to a
maximum of 250% of the motor FLA.
The rating of an inverse time circuit breaker can be multiplied by 3 and this total
amperage will start any motor with less locked-rotor amperage.
The time it takes to reach the 300% level varies with the amperage and voltage ratings
of the breaker.

16. Instantaneous Trip Circuit Breakers:
Instantaneous trip circuit breakers respond to immediate (almost instantaneous) values
of current from a short circuit, ground fault, or locked rotor current.
The National Electrical Code allows instantaneous trip circuit breakers to be sized to a
maximum of 800% of a motors FLA value.
They are used where time-delay fuses set at five times their ratings or circuit breakers
at three times their rating will not hold the starting current of a motor.
Some instantaneous trip circuit breakers have adjustable trip settings. The
instantaneous trip ratings of an instantaneous trip circuit breaker can be adjusted
above the locked-rotor current of a motor to allow the motor to start and come up to
its running speed.

17. Make: SIEMENS
Model: 3RV10414JA10
CIRCUIT-BREAKER SIZE :
S3
FOR MOTOR PROTECTION,
CLASS 10
A-REL. 45...63A,
Current range: 45-63 A

18. Thermal Overload Relay
Min Thermal Overload Relay setting:
(According to part load conditions)
Full Load Current : 19.7 A
Running Load Ampere : 14-17.1 Amp
Relay Setting : 17.5 A ( 88 % of FLC)
Max Thermal Overload relay setting :
120 % of FLC
Full Load Current : 19.7 A
Relay Setting : 23.6 A ( 120 % of FLC)
Siemens
3RU1136-4BB0
Thermal
Overload Relay, for induction motors
with a rating of 7.5 kW with a set
current value of the inverse-time
delayed overload release of 14 .. 20 A..

19. CONTACTOR SIZE & TYPE
No
APPLICATION
CONTACTOR
MAKING CAPACITY
1
Non Inductive, Slightly
Inductive, Resistive Loads
AC 1
1.5
2
Slip ring Motor
AC 2
4
3
Squirrel Cage Motor
AC 3
10
4
Rapid Start/Stop
AC 4
12
5
Switching of Transformers
AC 6a
12
6
Switching of Capacitor Banks
AC 6b
12
7
Motor Load in House hold
application
AC 7b
8
Type of Contactor: AC 3
Size of Contactor : 100 % x Full Load Current = 1 x 19.7 = 19.7 A
Making / Breaking Capacity of contactor : Full Load Current x Valu in Chart
: 19.7 x 10 = 197 A

20. Contactor:
3RT10-26-1 XB40-OLA2
Make: SIEMENS
Rated Operational current at
400 VAC :
25 Amp
Rating of Induction Motor at
50 Hz: 11 kW
Rated Control supply voltage:
24 VDC

21. THANKS

22. Problem: Vibrating Screen having two unbalanced motor (each 10 kW), one motor
rotate CW and other CCW.
Issue: Out of two, one particular motor burnt out three times since
commissioned (five months before).
Report: While Overload Relay is tripped, electrician went to the equipment for
investigation and found motor is already burnt. He came back to switchboard and
checked fuse and power contactor is intact.
Problem: As with same arrangement other equipment is running trouble free, all the running
data also normal so why particular one motor is burning frequently ?
What could be the preventive measures ?