Sizing a Generator for a Specific Electric Motor Load, 

or for use with Large Extension Cords

 

Generator sizing becomes more difficult when electric motors are involved, due to their large starting demand.  Typically, there are two motor styles: CODE G requires 3 times running watts to start, and CODE L requires 5 to 6 times running watts to start.

 

It is not practical to think that all loads will start at the same time.  Nor is it practical to size a generator using all starting watts for all selected motors, as some motor loads will already be running (or OFF), when others are ready to start.  As you can see, it can be difficult to exactly size a generator for a group of specific loads, as they are constantly changing in their "ON" or "OFF" cycle.  Always try to limit your load selections to the bare essential items.

 

Following are (4) charts that will help in sizing large (over 1 hp) electric motor loads to be started by generator power.  Use the instructions and examples, immediately following these charts, for actual generator sizing.

 

CHART A
NEMA CODE LETER LOCKED ROTOR KVA/HP NEMA CODE LETER LOCKED ROTOR KVA/HP
A 0-3.14 L 9.0-9.9
B 3.15-3.54 M 10.0-11.19
C 3.55-3.99 N 11.2-12.49
D 4.0-4.49 P 12.5-13.99
E 4.5-4.99 R 14.0-15.99
F 5.0-5.59 S 16.0-17.99
G 5.6-6.29 T 18.0-19.99
H 6.3-7.09 U 20.0-22.39
J 7.1-7.99 V 22.4-UP
K 8.0-8.99      

NEMA STD. MG 1-10.36  JAN '84

 

CHART B

NEMA CODE AND HP ESTIMATES

CODE

TYPICAL HP RANGE

F

15 HP & UP

G

10-12 HP

H

5-9 HP

J

3-4 HP

K

1.5-2.5 HP

L

1 HP

M

LESS THAN 1 HP

 

CHART C

ESTIMATED CODE G ELECTRIC MOTOR
 STARTING AND RUNNING WATTS

MOTOR
HP

MOTOR
RUN
WATTS

MOTOR STARTING WATTS

UNIVERSAL
MOTORS

(SMALL APPLIANCES)

REPULSION

INDUCTION

MOTORS

CAPACITOR

START

MOTORS

SPLIT

PHASE

MOTORS

1/6

300

600

750

950

1500

1/4

400

800

1000

1300

2000

1/3

475

950

1185

1600

2400

1/2

650

1000

1600

2000

3200

3/4

900

1200

2200

2800

N/A

1

1000

N/A

2500

3200

N/A

1-1/2

1700

N/A

4200

5500

N/A

2

2000

N/A

5000

6800

N/A

3

3200

N/A

8000

10000

N/A

5

5000

N/A

12500

15000

N/A

 

CHART D

TO FIND:

SINGLE PHASE

THREE PHASE

KILOWATTS KW

VOLTS X AMPS X P.F. / 1000

VOLTS X AMPS X 1.73 X .8 

/ 1000 = KVA X .8

KILO-VOLT-AMPS KVA

VOLTS X AMP / 1000

VOLTS X AMPS X 1.73 

/ 1000 = KW / .8

KW REQUIRED FOR MOTOR

MOTOR HP X EFF.

HP X .746 / EFFICIENCY

KVA REQUIRED FOR MOTOR

MOTOR HP X EFF. X .8

HP X .746 / EFFICIENCY X .8

POWER FACTOR P.F.

KW / KVA

KW / KVA

AMPS - WHEN KW IS KNOWN

(KW X 1000) / (VOLTS X P.F.)

(KW X 1000) / (1.73 X VOLTS x P.F.)

AMPS - WHEN KVA IS KNOWN

KVA X 1000 / VOLTS

(KVA X 1000) / (1.73 X VOLTS)

REQUIRED PRIME MOVER (ENGINE) HORSE POWER

KW / (GENERATOR EFF. X .746)

FREQUENCY (HERTZ)

NUMBER OF POLES X R.P.M / 120

 

CHART E

WIRE GAUGE FOR EXTENSION CORDS

LOAD AMPS

0 - 50 FEET

51 - 100 FEET

101 - 150 FEET

1-11

18

16

14

12-18

16

12

10

19-25

12

10

8

26-35

10

8

6

36-40

8

6

4

41-50

6

4

2

51-60

4

2

2

 

EXAMPLES IN GENERATOR SIZING FOR LARGE ELECTRIC MOTORS 

All electric motors require large amounts of electric power to start up, due to it's starting winding. After approximately 3/4 - 1-1/2 seconds, the starting winding drops out, and the running winding continues, at a much lower power demand.  A generator must be sized to handle the load of the starting winding.

 

Chart A determines a motor code letter (found on all motor nameplates), which allows calculation of starting amps, or better known as locked rotor amps (LRA).  If motor code is not available, use Chart B for code estimate.  Following are formulae and examples:

 

Sizing a generator for single phase motor starting LRA : The formula is: Motor HP x KVA per HP x (1000 / Motor Voltage) = LRA. Example: A 3/4 HP, Code L motor connected to 240 Volts, requires what LRA?  Refer to Chart A, and find Code L.  Always use the largest KVA/HP number, so select 9.9.  The formula becomes: .75 x 9.9 x (1000/240) = 7.425x4.167 = 30.9 = 31 LRA. The generator must produce 31 Amps at 240 Volts, to safely start this motor.  Multiply 31 amps x 240 Volts = 7400 Watts or 7.5 KW generator size.

 

Sizing a generator for three phase motor starting LRA : The formula is: Motor HP x KVA per HP x (1000 / (Volts x 1.73)) = LRA. Example: A 20 HP Code G motor connected to 460 Volts requires what LRA?  Refer to Chart A and select 6.29 KVA/HP, from Chart A, for Code G.  The formula becomes: 20 x 6.29 x (1000 / (460 x 1.73)) = 125.8 x 1.26 = 158.5 = 159 LRA.  The generator must produce 159 Amps at 460 Volts, to safely start this motor.  Use formula: 1.73 x Volts x Amps x .8 / 1000 for generator KW size.

 

BOTH SINGLE PHASE AND THREE PHASE MOTOR SIZING EXAMPLES ARE BASED ON INFORMATION IN NATIONAL ELECTRICAL CODE HANDBOOK, ARTICLE 430.7 AND IS INTENDED FOR MAXIMUM VOLTAGE DIP OF 25% UPON INITIAL MOTOR START-UP.  IF 35% VOLTAGE DIP IS ALLOWABLE, REDUCE LRA, THEREFORE KW SIZE, BY 25% (LRA or KW X .75), WE DO NOT RECOMMEND ANY GENERATOR KW SIZING THAT YIELDS MORE THAN 35% VOLTAGE DIP.

 

Chart B can be used as a motor code estimate, when actual motor code is not available.

 

Chart C can be used as a quick reference for all Code G Motors.

 

Chart D shows various generator formulas that may be of use.

 

Chart E shows wire gauge for extension cords, used with various load amps and distances.

 

Starting more than one motor, at one time: make a list of all motors, totaling all running watts.  Calculate the starting demand when the largest motor is started while all others are running.  Add the largest motor starting watts, to the running watts of all smaller motors already running.

As shown by selection chart, air conditioning starting watts have the largest demand load.  It may be wise to dis-card air conditioner use, during blackouts, to keep your generator kw size, smaller.

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