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  03.14  L  9.09.9 
B  3.153.54  M  10.011.19 
C  3.553.99  N  11.212.49 
D  4.04.49  P  12.513.99 
E  4.54.99  R  14.015.99 
F  5.05.59  S  16.017.99 
G  5.66.29  T  18.019.99 
H  6.37.09  U  20.022.39 
J  7.17.99  V  22.4UP 
K  8.08.99  
NEMA STD. MG 110.36 JAN '84 
NEMA CODE AND HP ESTIMATES 

CODE 
TYPICAL HP RANGE 
F 
15 HP & UP 
G 
1012 HP 
H 
59 HP 
J 
34 HP 
K 
1.52.5 HP 
L 
1 HP 
M 
LESS THAN 1 HP 
ESTIMATED CODE G ELECTRIC MOTOR STARTING AND RUNNING WATTS 

MOTOR 
MOTOR 
MOTOR STARTING WATTS 

UNIVERSAL (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 
11/2 
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 
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 
KILOVOLTAMPS 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 
WIRE GAUGE FOR EXTENSION CORDS 

LOAD AMPS 
0  50 FEET 
51  100 FEET 
101  150 FEET 
111 
18 
16 
14 
1218 
16 
12 
10 
1925 
12 
10 
8 
2635 
10 
8 
6 
3640 
8 
6 
4 
4150 
6 
4 
2 
5160 
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  11/2 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 STARTUP. 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 discard air conditioner use, during blackouts, to keep your generator kw size, smaller. Gillette Generators, Inc.
