Motor Cabinet Sizing –NEC Article 430
Motor control cabinets are composed of the following items
1. A housing for all the devices
2. Feeder Conduit and Conductors
3. Main Circuit Breaker – Ground Fault / Short Circuit Protection
4. Main Power distribution bus for the motor control devices
5. Individual plug in bays for motor control devices, containing:
a. Motor Branch Circuit – Ground Fault / Short Circuit Protection
b. Motor Control Devices
c. Motor Branch Circuit conductors
Figure 1 - Motor Control Cabinet
The photo to the left is a typical motor control cabinet and
can be seen in person in room B103.
The purpose of this document will be to size the items listed
above. You will be designing and selecting
a.)
b.)
c.)
d.)
e.)
f.)
g.)
h.)
i.)
j.)
k.)
Feeder Wire Size
Feeder Conduit Size (EMT)
Main Circuit Breaker Size
Branch Circuit Breaker Size
Branch Circuit Wire Size
Branch Circuit Grounding Conductor
Branch Circuit Conduit (LFMC)
Magnetic Starter Size
Overload Size
Switch Enclosure Size
Feeder Grounding Conductor Size
NOTES: For this project
Minimum conduit size = ½”
Minimum Conductor size = 12 AWG
Motor Cabinet Sizing –NEC Article 430
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Figure 2 NEC Table
NEC Article 430 - The NEC table on the left is your road
map to Article 430. This table should be used to help you
navigate around the article as needed.
Examples:
1. Feeder – 430.24, 430.25, 430.26
2. Motor Circuit Conductor – Part II
3. Motor Overload Protection – Part III
This table should be your starting point for determining all
the related code sections to the ten items listed on the
previous page.
As with other articles, references in Article 430 will take
you outside of article 430. Be sure and follow all the
references in determining proper values to utilize.
NEMA Sizes - The National Electrical Manufacturers
Association (NEMA) has set a series of standard motor
switch and starter sizes. This allows different
manufacturers parts to be easily substituted for existing
installations. That list follows below.
NEMA MAGNETIC STARTER SIZES
NEMA MAG 208V
230V
460V
575V
STARTER MAX
MAX
MAX
MAX
SIZE
HP
HP
HP
HP
0
3
3
5
5
1
7.5
7.5
10
10
2
10
15
25
25
3
25
30
50
50
4
40
50
100
100
5
75
100
200
200
6
150
200
400
400
NEMA Switch
Sizes
30
60
100
200
400
600
1000
Motor Cabinet Sizing –NEC Article 430
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Excel Motor Sizing Worksheet
Students will be provided with a motor sizing worksheet to assist in the process of designing the motor control
center. The worksheet is shown on the next page with descriptions of each item. An example motor branch
circuit is shown along with descriptions of each box.
MOTOR BLOCK
240V DC
DESCRIPTION
1.15 S.F. /THWN CU
N.E.C.
FEEDER
THWN COPPER
FEEDER
SERVICE
FULL LOAD AMPS
19
DC MOTOR
MOTOR #1 MOTOR # 2 MOTOR # 3 MOTOR # 4 MOTOR # 5 Percent
25
10
15
5
5
1
89
2
111
20
N.E.C.
BRANCH
3
COPPER WIRE SIZE
1
21
134
22
TIME DELAY
FUSE SIZE
24
MAG. STARTER
O.L. PROTECTION
GROUNDING
CONDUCTOR
Copper Size
x # wires
Copper * # wires
Ground Size
2-#12 Field Wires
Total Area needed
Conduit Area
CONDUIT SIZE
175
23
SWITCH SIZE
EMT
Table
25
0
4
5
6
400
7
3
8
111.3
9
6
10
0.1158
2
0.2316
0.0507
0.0266
0.3089
0.3490
11
12
13
14
15
16
17
From conduit table
1"
18
Liquidtite
Metal
(sum above 3)
1. Motor Horsepower is in the box above
2. Motor Full Load amps from Tables 430.247 to 430.251(B)
3. Minimum conductor amperage per Part II
4. Wire size in AWG or kcmils
5. Motor branch circuit short circuit / ground fault amperage calculated per Part IV
6. Motor branch circuit short circuit / ground fault protection using Value 5 and calculated per Part IV
7. NEMA Switch size using chart on previous page
8. NEMA Magnetic Starter size using chart on previous page
9. Motor Overload protection calculated per Part III
10. Motor Grounding conductor calculated per Article 250, and table 250.122
Motor Cabinet Sizing –NEC Article 430
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11. Area of the selected branch circuit conductor in square inches from Chapter 9, Table 5
12. Number of branch circuit conductors (typically 2)
13. Number of wires times the branch circuit area calculated in item 11 above.
14. Ground wire area, again from Chapter 9, Table 5
15. The area required for 2 each, 12 AWG conductors for the field wiring.
16. Sum of items 13, 14 & 15. This is the total area required.
17. Using Chapter 9, Table 4, the minimum conduit size is selected for both the LFMC and EMT. This
number is the square inches available for the conduit you selected. Since there are over 2 wires in the
conduit, you should use the 40% fill column.
18. Trade Size designation (in inches) for the selected conduit.
19. Motor Feeder amps as calculated per Part II
20. same value as 19
21. Feeder Conductor size selected in accordance per Part II
22. Time Delay Fuse or Circuit Breaker amperage as calculated in accordance per Part V
23. Standard Time Delay Fuse or Circuit Breaker size selected using value 22 and in accordance per Part V
24. NEMA Switch Size, same method as Item 7
25. Conduit sizing work area, the same method as with items 11 – 18 above, There are only three wires in this
conduit, the field wires are not included in the feeder conduit.
Figure 3 - Motor Controller Bus Access Points
Motor Cabinet Sizing –NEC Article 430
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Figure 4 - Motor Controller Bus Connections
Figure 5 - Motor Controller
Motor Cabinet Sizing –NEC Article 430
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Figure 6 - Motor Control Devices
In Figure 6, motor control devices are shown.
1. Branch Circuit Short Circuit / Ground Fault Protection
2. Motor Starter
3. Motor Overloads and Branch Circuit conductor connections
4. Control Transformer Primary and Secondary Over Current Protection
5. Control Transformer
Motor Cabinet Sizing –NEC Article 430
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