8/6/2008
Motor Calcs
2005 NEC Motor Circuit
Requirements and
Calculations
Copyright 1999-2007 Dan Dudley
Motor Calcs
NEC Motor Calculations
Basic Information
• 430.6(a)(1) Basic Calc information
– Name Plate Values (430.6(a)(2) are only used
to size overloads)
– Otherwise use T.430.247, 248, 249, & 250
• 430.22(a) Single motor 125% cont. duty
(3 hrs or more)
• T.430.22(e) Note Cont. Duty unless nature
of apparatus is otherwise.
Copyright 1999-2007 Dan Dudley
Copyright 2000-2007 Dan Dudley
8/6/2008
Motor Calcs
NEC Motor Calculations
Basic Information
• 430.52(c)(1) exception
– Next higher standard size overcurrent protective
device.
• 430.62 Feeder O.C. Protection
– Largest S.C. – G.F. Protective device plus the sum of
the F.L.C. of all other motors supplied by the same
feeder
• No provision for going UP to next standard size
must go down to next standard size
Copyright 1999-2007 Dan Dudley
Motor Calcs
Copyright 1999-2007 Dan Dudley
Copyright 2000-2007 Dan Dudley
8/6/2008
Motor Calcs
Hp
Phases
Voltage
FLC
Hz
Temp
Rise
Service
Factor
Insulation
Class
Locked
Rotor
Letter
Copyright 1999-2007 Dan Dudley
Motor Calcs
Line side
Nameplate Data
Actuating Coil
Overload Relays
Load Side
Overload Reset
Copyright 1999-2007 Dan Dudley
Copyright 2000-2007 Dan Dudley
8/6/2008
Motor Calcs
For all other
calculations use
Table values
Copyright 1999-2007 Dan Dudley
Motor Calcs
Copyright 1999-2007 Dan Dudley
Copyright 2000-2007 Dan Dudley
8/6/2008
Motor Calcs
Copyright 1999-2007 Dan Dudley
¾ @120 5 @208
Motor Calcs
25 HP
208 Volts
50 HP
Synchronous
200 HP
440 Volts
480 Volts
25-208 50 sync – 440 200 - 480
Copyright 2000-2007 Dan Dudley
Copyright 1999-2007 Dan Dudley
8/6/2008
Motor Calcs
C.B.
3-phase
Std. Fuse
Copyright 1999-2007 Dan Dudley
Motor Calcs
10HP 240V 1Φ
FLI 44
What minimum size THHN conductors are required
for this motor ?
Name Plate Not used
430.248:
430.248: 10hp @ 240v = 50 amps
430.22(a):
430.22(a): 50 X 125% = 62.5 amps
310.16:
310.16: 62.5 amps = #4 THHN
T.310.16
Copyright 2000-2007 Dan Dudley
T.430.248
110.14
Copyright 1999-2007 Dan Dudley
8/6/2008
Motor Calcs
25HP 480V 3Φ
FLI 32.5
What is the minimum ampacity of the branch circuit conductors
for a 25 HP , 480-volt, 3 phase, squirrel cage motor with
nameplate FLI of 32.5 amps ?
Name Plate value not used
430.250:
430.250: 25hp @480v = 34 amps
430.22(a):
430.22(a): 34 X 125% = 42.5 amps
310.16 : # 6 THHN @60º
@60ºC and 55 amps
T.310.16
T.430.250
Copyright 1999-2007 Dan Dudley
110.14
Circuits with more than one Motor
•
Step 1.
•
Decide which motor
has largest FLI rating
(This may not be the largest HP)
•
Step 2.
•
Largest FLI of one
motor X 125%
(only increase one)
•
•
Step 3.
Add FLI rating of all
other motors
(straight from tables)
Copyright 1999-2007 Dan Dudley
Copyright 2000-2007 Dan Dudley
8/6/2008
Motor Calcs
5HP 3Φ
208V
5HP 3Φ
208V
What is the minimum ampacity of the branch circuit
conductors for the two motors as shown ? 3∅, 208V
430.250:
430.250: 5 hp @ 208v = 16.7 amps
430.24:
430.24: Ampacity = FLI 1st motor X 125% + FLI 2nd motor
Ampacity = 16.7 X 125% + 16.7 = 37.575 amps
T.310.16
T.430.250
110.14
Copyright 1999-2007 Dan Dudley
Motor Calcs
5HP 1Φ
230V
2HP 1Φ
230V
What is the minimum ampacity of the branch circuit
conductors supplying the two motors as shown ? 1∅, 230V
430.248: 2hp @ 230v = 12 amps
430.248: 5hp @ 230v = 28 amps
Ampacity = Largest FLI X 125% + Second FLI
28 X 125% + 12 = 47 amps
T.310.16
Copyright 2000-2007 Dan Dudley
T.430.248
110.14
Copyright 1999-2007 Dan Dudley
8/6/2008
Motor Calcs
10HP 3Φ
480V
Squirrel Cage
25HP 3Φ
480V
Squirrel Cage
30HP 3Φ
480V
Synchronous
What is the minimum ampacity of the branch circuit
conductor serving the three motors as shown
above?
T.310.16
T.430.250
Copyright 1999-2007 Dan Dudley
110.14
Motor Calcs
10HP 3Φ
480V
Squirrel Cage
25HP 3Φ
480V
Squirrel Cage
30HP 3Φ
480V
Synchronous
430.250: FLI’s, 3-phase, 480v
10hp Squirrel cage = 14 FLI
25hp Squirrel cage = 34 FLI
30hp Synchronous = 32 FLI
34 X 125% + 14 + 32 = 88.5 amps
T.310.16
Copyright 2000-2007 Dan Dudley
T.430.250
110.14
Copyright 1999-2007 Dan Dudley
8/6/2008
Motor Calcs
88.5A
10HP 3Φ
480V
Squirrel Cage
25HP 3Φ
480V
Squirrel Cage
30HP 3Φ
480V
Synchronous
What Size THWN conductors are required for:
1: Ungrounded Conductors
Design
A
2: Grounded Conductor
3: Grounding Conductor
T.310.16 & 110.14(c) = #2 THWN Ungrounded (60°C)
No Grounded Conductor Required
T.250.122 Grounding Conductor (EGC) = #8 Cu or #6 Al
T.310.16 T.250.122
110.14
T.430.250
Copyright 1999-2007 Dan Dudley
Motor Calcs
6000 watts
Incandescent Lighting
1HP 1Φ 120V
FLI 12.4
What is the minimum size branch circuit conductors
supplying the motor and incandescent lighting above ?
430.248: 1hp @ 120v = 16 amps
Lighting: I=W / E = 6000 / 120 = 50 amps
430.24(a): Ampacity = motor FLI X 125% + Lighting I
16 X 125% + 50 = 70 amps
#4 THHN Cu (60°C)
T.310.16
Copyright 2000-2007 Dan Dudley
T.430.248
110.14
Copyright 1999-2007 Dan Dudley
8/6/2008
Copyright 1999-2007 Dan Dudley
Motor Calcs
T.310.16
Copyright 2000-2007 Dan Dudley
T.430.250
110.14
Copyright 1999-2007 Dan Dudley
8/6/2008
Motor Calcs
81.25
26.25
123.25
T.310.16
T.430.250
110.14
Copyright 1999-2007 Dan Dudley
Motor Calcs
T.310.16
Copyright 2000-2007 Dan Dudley
T.430.250
110.14
Copyright 1999-2007 Dan Dudley
8/6/2008
Motor Calcs
T.310.16
T.430.250
110.14
Copyright 1999-2007 Dan Dudley
430.52
Motor Calcs
T.310.16
T.430.250
Copyright 2000-2007 Dan Dudley
110.14
430.52
240.6
Copyright 1999-2007 Dan Dudley
8/6/2008
Motor Calcs
Time Delay
Fuses
T.310.16
T.430.250
110.14
20HP 3Φ
480V FLI 29
430.52
240.6
Copyright 1999-2007 Dan Dudley
Motor Calcs
Time Delay
Fuses
20HP 3Φ
480V FLI 29
T.430.250: 20hp = 27 amps
T.430.52: Time Delay Fuse = 175%
Max. O.C. device = 27 X 175% = 47.25 amps
430.52(c)(1) exception #1: Next Larger size permitted
240.6(a) Next Larger Std size = 50 amps
Answer: 50 amp Time Delay Fuse
T.310.16
T.430.250
Copyright 2000-2007 Dan Dudley
110.14
430.52
240.6
Copyright 1999-2007 Dan Dudley
8/6/2008
Motor Calcs
Non-Time
Delay Fuses
20HP 3Φ
480V FLI 29
T.430.250: 20hp = 27 amps
T.430.52: Non-Time Delay Fuse = 300%
Max. O.C. device = 27 X 300% = 81 amps
430.52(c)(1) exception #1: Next Larger size permitted
240.6(a) Next Larger Std size = 90 amps
Answer: 90 amp Non-Time Delay Fuse
T.310.16
T.430.250
110.14
430.52
240.6
Copyright 1999-2007 Dan Dudley
Motor Calcs
Inverse Time
Breaker
20HP 3Φ
480V FLI 29
T.430.250: 20hp = 27 amps
T.430.52: Inverse Time Breaker = 250%
Max. O.C. device = 27 X 250% = 67.5 amps
430.52(c)(1) exception 1: Next Larger size permitted
240.6(a) Next Larger Std size = 70 amps
Answer: 70 amp Inverse Time Circuit Breaker
T.310.16
T.430.250
Copyright 2000-2007 Dan Dudley
110.14
430.52
240.6
Copyright 1999-2007 Dan Dudley
8/6/2008
Selecting Minimum Size Overloads 430.32(A)(1)
„
Service Factor
„
„
„
„
Less than 1.15
use 115%
Temperature Rise
„
1.15 or greater
use 125%
„
No Service Factor
Listed use 115%
„
40°C or less use
125%
Over 40°C use
115%
No Temp Listed
use 115%
Copyright 1999-2007 Dan Dudley
Selecting Maximum Size Overloads 430.32(C)
„
Service Factor
„
„
„
Less than 1.15
use 130%
1.15 or greater
use 140%
No Service Factor
Listed use 130%
„
Temperature Rise
„
„
„
40°C or less use
140%
Over 40°C use
130%
No Temp Listed
use 130%
Copyright 1999-2007 Dan Dudley
Copyright 2000-2007 Dan Dudley
8/6/2008
Motors with a Duty Cycle - Welders
Solution:
Welder No.1
630.11(a) Nonmotor Generator
Duty cycle 30 Multiplier = 0.55
I=RPCXM
I = 28 amps x 0.55 = 15.4 amps
Welder No.2
630.11(a) Nonmotor Generator
Duty cycle 80 Multiplier = 0.89
I=RPC xM
I = 18 amps x 0.89 = 16.02 amps
630.11(b)
Total = No.1+ No.2
RPC 28A
Duty Cycle
30%
RPC 18A
Duty Cycle
80%
Total = 15.4 + 16.02 = 31.42 amps
Ans. 31.42 amps
T.310.16
T.630.11
110.14
430.52
240.6
Copyright 1999-2007 Dan Dudley
Motor Calcs
5HP 230V 1Φ
FLI 24.5
Periodic Duty
15 Min
Solution:
Nameplate current rating is used.
Table 430.22(e)
FLI 15 min. periodic duty = 90%
Ampacity -Nameplate FLI x 90%
= 24.5 x 90% = 22.05 amps.
Answer: 22.05 amperes
T.310.16 T.430.22(e)
Copyright 2000-2007 Dan Dudley
110.14
430.52
240.6
Copyright 1999-2007 Dan Dudley
8/6/2008
Motor Calcs
15HP 230V 3Φ
FLI 16.5
Varying Duty
Solution:
Nameplate FLI is used.
Table 430.22(e)
Continuous use, varying duty= 200%
Ampacity = nameplate FLI x 200 %
Ampacity= 16.5 x 200 = 33 amps
Answer: 33 amperes
T.310.16 T.430.22(e)
110.14
430.52
240.6
Copyright 1999-2007 Dan Dudley
Motor Calcs
Motor Efficiency
No motor is 100% efficient
•
Efficiency = Output / Input
Efficiency = Output / Input
•
5HP, 240V, 1∅ = 28A F.L.C.
T.430.248
•
W = E X I = 240V X 28A =
6720 VA
•
6720 VA input and 5712 VA
output
•
5712/6720 = .85
•
6720 VA X .85 = 5712 VA
output
•
85% efficiency
Copyright 1999-2007 Dan Dudley
Copyright 2000-2007 Dan Dudley
8/6/2008
Motor Calcs
Motor Duty Rating
„
„
„
„
„
„
Intermittent Duty = T.430.22(e)
Nameplate F.L.C. used
50HP,480V,3∅, 15 minute rated elevator
motor, Name Plate FLC =61.2A
T.430.22(e) = 85%
Name Plate FLC = 61.2 X 85% = 52.02A
Branch Circuit Ampacity
Conductor Required = #6 (60°C column)
Copyright 1999-2007 Dan Dudley
Motor Calcs
Motor Calculations
Practice Questions
10 Questions
30 Minutes
Copyright 1999-2007 Dan Dudley
Copyright 2000-2007 Dan Dudley
8/6/2008
Motor Calcs
Copyright 1999-2007 Dan Dudley
Motor Calcs
Motor Calculations Answers
1.
(a)
33A
13.2 X 250% = 33A
KW
2.
(a)
38.5A
30.8 X 125% = 38.5A
KW
3.
(b)
#3/0 THHN
2 – 7.5 hp 1Ø = 44 A each
2 – 5 hp 1Ø = 30.8 A
1 – 2 hp 1Ø = 13.2 A
1 - 1 HP 1Ø = 8.8 A
56 X 125% + 44 + 30.8 + 24 + 13.2 + 11
70 + 44 + 30.8 + 13.2 + 13.2 + 8.8
Answer: 193 amps
*Note #2
T. 430.248: 5hp, 208v, 1∅ = 30.8 amps
30.8 X 125% = 38.5 amps
T.310.16
T.430.248
Copyright 2000-2007 Dan Dudley
T.430.250
T.430.250
1999-2007 Dan
Dudley
110.14Copyright
430.52
240.6
8/6/2008
Motor Calcs
Motor Calculations Answers
4.
(c)
#3 TW
65 X 125% = 81.25 amps
required. T. 310.16
5.
(b)
9152 va
Power(va) = E X I
W= 208v X 44 = 9152 va
6.
(d)
15.18 A
13.2 X 115% = 15.8 amps
7.
(b)
80 amp
44 A FLC X 175% = 77 A
8.
(a)
250 amp
175 A + 44 A + 30.8 A
+24 A + 13.2A + 11 A = 298 amps
240.6 = 250 amp
Can’t go up on FEEDER
T.310.16
T.430.248
T.430.250
T.430.250
1999-2007 Dan
Dudley
110.14Copyright
430.52
240.6
Motor Calcs
Motor Calculations Answers
9.
10.
(d)
200 amps
1.
175 + 21 + 21 = 217
2.
Can’t go up on feeder - drop down to 200 amp
(240.6)
(d)
#4 41 amps X 1.25 = 51.25a
Be careful of TYPE of MOTOR, in this case
SYNCHRONOUS Also be careful of type of
CONDUCTOR
Look in Right side – aluminum conductors in the
middle column since design “D” is stated
** For motors marked with design letters B, C, or D conductors having an
insulation rating of 75°C (167°F) or higher shall be permitted to be used
provided the ampacity of such conductors does not exceed the 75°C
(167°F) ampacity.
T.310.16
T.430.248
Copyright 2000-2007 Dan Dudley
T.430.250
T.430.250
1999-2007 Dan
Dudley
110.14Copyright
430.52
240.6
8/6/2008
Motor Calcs
Copyright 1999-2007 Dan Dudley
Motor Calcs
25-208 50 sync – 440 200 - 480
Copyright 1999-2007 Dan Dudley
Copyright 2000-2007 Dan Dudley
8/6/2008
Motor Calcs
Copyright 1999-2007 Dan Dudley
Motor Calcs
Copyright 1999-2007 Dan Dudley
Copyright 2000-2007 Dan Dudley
8/6/2008
Motor Calcs
Copyright 1999-2007 Dan Dudley
Motor Calcs
Copyright 1999-2007 Dan Dudley
Copyright 2000-2007 Dan Dudley
8/6/2008
Motor Calcs
Copyright 1999-2007 Dan Dudley
Motor Calcs
Copyright 1999-2007 Dan Dudley
Copyright 2000-2007 Dan Dudley
8/6/2008
Motor Calcs
Copyright 1999-2007 Dan Dudley
Motor Calcs
Copyright 1999-2007 Dan Dudley
Copyright 2000-2007 Dan Dudley
8/6/2008
Motor Calcs
Copyright 1999-2007 Dan Dudley
Motor Calcs
Question 1 Keywords
What size short circuit/ground fault protection is
required for the branch circuit serving motor #1 in
motor diagram #1?
What size short circuit/ground fault protection is
required for the branch circuit serving motor #1 in
motor diagram #1?
We are looking for the smallest fuse or circuit breaker
required to protect motor #1 on Motor Diagram #1.
Copyright 1999-2007 Dan Dudley
Copyright 2000-2007 Dan Dudley
8/6/2008
Motor Calcs
Question 2 Keywords
What is the minimum size overloads required for
motor #2 on motor diagram #1 ?
What is the minimum size overloads required for
motor #2 on motor diagram #1 ?
We are looking for the smallest heater/overload size
allowed for motor #2 on motor diagram #1.
Overloads = name plate full load current times either
115% or 125% depending on S.F. and/or Temp Rise.
Copyright 1999-2007 Dan Dudley
Motor Calcs
Question 3 Keywords
What is the minimum size THHN feeder required for
the six motors as shown on Motor Diagram #1 ?
What is the minimum size THHN feeder required for
the six motors as shown on Motor Diagram #1 ?
We are looking for the smallest legally allowed THHN
copper conductor to feed these 6 motors. Feeder
conductor = three steps:
1 find largest FLA motor and multiply by 1.25 (125%)
2. Add FLA of all other motors on same feeder
3. Can’t go up in size = drop down to standard at 240.6
Copyright 1999-2007 Dan Dudley
Copyright 2000-2007 Dan Dudley