Motor Feeder Calculations
By John C. Pfeiffer, P.E.
Pfeiffer Engineering Co., Inc.
 

First, we have taken a chart out of the National Electrical Code, Section 430, Motors and expanded the chart to better define where the various aspects of motors and motor control are located.  Next we have developed a chart that defines the rules for sizing wire, overloads, etc. for motors.  We follow this with an example of the calculations. 

 

 

Example Calculations:

Refer to the above chart as the connection diagram in this example>

Motor 1 -  100 hp 480 VAC 3ph  Nameplate Full Load Amps 128 amps, Design B, fused

Motor 2 10 hp 480 VAC 3ph Nameplate full Load Amps 19 amps, Design B, MCP,

Heater 50 KVA 480 VAC 3ph

Maximum ambient temperature 104 Degrees F

Terminal rating 75 Degree C

 

 

Motor 1

Motor 2

Heater

Size

1

100 hp

10 hp

50 kva

NEC FLA

1

124 amps

14 amps

60 amps

Disconnect Size *FLA

2&7

115%

115%

125%

Disconnect Size

2&7

142.6 Amps

16.1 amps

75 amps

Nominal size

2&7

200 amp

30 amp

100 amp

Branch Circuit Size

3

 

 

 

125%

3

155 amps

17.5 amps

75

Wire Size

3

2/0 (175 amps)

#14 (20 amps)

#4 (85amps)

Wire Derating

3

.88

.88

.88

Derated Size

3

154 amps

17.6 amps

74.8 amps

 

3

Violation

OK

Violation

Revised Wire size

3

3/0 (200 amps)

 

#3 (100amps)

Derated Size

3

176 amps

 

88 amps

Nameplate Amps

4

128 amps

19 amps

 

Overload sizing

4

1.15%

1.15%

NA

Overload Size

4

147.2 amps

21.85 amps

NA

Starter Size

5

Size 4

Size 1

100 amp

Branch Circuit Protection

6

175% (max)

250% (max)

125% (max)

Protection Size

6

217 amps

35 amps

75 amps

Next Largest Size

6

225 amps (max)

35 amps (max)

75 amps (max)

Equipment Ground

9

#4

#10

#8

Feeder Conductor

10

 

 

 

FLA

10

124 Amps

14 amps

60 amps

25% of Largest Motor

10

31 amps

 

 

Total Amps

10

(124+14+60+31)

229 amps

 

 

Wire Size

10

4/0 (230 amps)

 

 

Wire Derating

10

.88

 

 

Derated Size

10

202 amps

 

 

 

10

Violation

 

 

Revised Wire size

10

300 kcmil (285 amps)

 

 

Derated Size

10

250 amps

 

 

Feeder Protection

11

 

 

 

Largest Motor BCP

11

225 amps

 

 

FLA other loads

11

 

14 amps

60 amps

Total Load

11

(225+14+60)

299 amps

 

 

Feeder Protection

11

250 Amps

 

 

Feeder Disconnect

12

 

 

 

Full Load Amps

12

124 amps

14 amps

60 amps

Total FLA

12

198 amps

 

 

115% of FLA

12

227.7 amps

 

 

Lock Rotor Amps

12

725 amps

81 amps

60 amps

Total LRA.

12

866 amps

 

 

Equivalent Hp

12

125 hp

 

 

Disconnect Size

12

200hp (240amps)

 

 

 

Copyright 2011 Pfeiffer Engineering Company, Inc. All rights reserved