This document provides sizing calculations for conductors, controllers, overcurrent protection devices (OCPDs), and disconnects for three 3-phase 480V induction motors rated at 50HP, 60HP, and 75HP that are powered from a single feeder. It calculates the maximum demands for each motor including full load current, locked rotor current, branch circuit conductors, overload protection, controllers, OCPDs, disconnects, feeder conductors, feeder OCPD and disconnect to ensure adequate protection and compliance with National Electrical Code standards.

1. WS003 Motors Calculations-3Phase 480V Induction
BKV Group-Chad Kurdi-2015 Electrical Training Seminar
Size the following:
# Motor#1 Motor#2 Motor#3 NEC
1-FLC 65 77 96 T430-250
1.1-LRC (3.54*50*1000)/(1.73
*480)=213A
(3.99*60*1000)/(1.73*
480)=288A
(4.49*75*1000)/(1.73
*480)=406A
T430.7B
2-BC
Conductors
1.25*65=81.3
(3)#4AWG
1.25*77=96.3
(3)#3AWG
1.25*96=120
(3)#1AWG
430.22
2.1-BC EGC (1)#6AWG (1)#6AWG (1)#6AWG T250.122
2.2-BC EMT 3*0.0824+1*0.0507=
0.2979 (1)1.0”
3*0.0973+1*0.0507=
0.3426 (1)1.0”
3*0.0973+1*0.0507=
0.3426 (1)1.0”
Ch#9
T4&T5
2.3-BC FMC 3*0.0824+1*0.0507=
0.2979 (1)1.0”
3*0.0973+1*0.0507=
0.3426 (1)1.0”
3*0.0973+1*0.0507=
0.3426 (1)1.0”
Ch#9
T4&T5
3-Overload 1.15*62=71.3A 1.25*74=92.5A 1.25*93=116.3A 430.32A
4-Controller 50HP@480V
NEMA#3
60HP@480V
NEMA#4
75HP@480V
NEMA#4
430.83
5-OCPD
DETD Fuse
1.75*65=114
125A
1.75*77=134.8
150A
1.75*96=168
175A
T430.52
240.6
6-Disconnect 1.15*65=75.8 100A 1.15*77=88.6 100A 1.15*96=110.4 200A 430.110A
7-Feeder
Conductors
(1.25*96)+77+65=262A (3)#300KCM 430.24
7.1-Feeder EGC (1)#3AWG T250.122
7.2-Feeder
EMT
3*0.4608+1*0.0973=1.4797 (1)2.5” Ch#9
T4&T5
8-Feeder
OCPD
(175)+77+65=317A 300A 430.62
240.6
9-Feeder (AMP)
Disconnect
1.15*(96+77+65)=274A 400A 430.110C
9.1-Feeder (HP)
Disconnect
96+77+65=238----------- 250HP, 313+288+405=1006-----150HP T430.250
T430.251B
Three Induction motors are powered from one feeder. Power system is
480V, 3Ø, 60HZ. Terminals are rated 75 D. C. Conductors are CU, THHN.
Motors are continuous duty. Name Plates have the following:
Volts HP FLA SF Temp Rise Code Letter
Motor#1 460V 50 62 1.00 50 B
Motor#2 460V 60 74 1.10 40 C
Motor#3 460V 75 93 1.15 30 D