Sizing Guide
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GENERATORS
Generator Sizing Guide
General Information
Important Notice
This booklet is designed to familiarize estimators and installers
with proper sizing guidelines for residential and commercial
generators. The information is not comprehensive, nor does
it replace or supercede any material contained in any of
the written documents shipped with the equipment. This
booklet should only be used in conjunction with the Owner’s
Manual, Installation Manual and other technical documents
shipped with each product. Always read all accompanying
documentation carefully before attempting to install any
generator, transfer switch or related equipment.
How to Use this Booklet
Within this booklet, you will find electrical load information,
plus an outline of generator surge capability, fuel pipe sizing,
liquid propane tank sizing, and UPS / generator compatibility.
The final pages are perforated for easy removal and can be
photocopied to create additional Onsite Estimating Sheets
for use with individual jobs.
Safety Information
Proper sizing of the generator is crucial to the success of
any installation and requires a good working knowledge
of electricity and its characteristics, as well as the varying
requirements of the electrical equipment comprising the load.
When analyzing the electrical load, consult the manufacturer’s
nameplate on each major appliance or piece of equipment
to determine its starting and running requirements in terms
of watts, amps and voltage. When choosing the generator
output for commercial or industrial applications, select a
rating that is approximately 25% higher than the peak load
(for example, if the load is about 40 kilowatts, select a 50 kW
genset). A higher rated generator will operate comfortably
at approximately 80% of its full capacity and will provide a
margin of flexibility if the load increases in the future.
For safety reasons, Siemens recommends that the backup
power system be installed, serviced and repaired by a
Generac Authorized Service Dealer or a competent,
qualified electrician or installation technician who is
familiar with applicable codes, standards and regulations.
It is essential to comply with all regulations established by
the Occupational Safety and Health Administration (OSHA)
and strict adherence to all local, state and national codes
is mandatory. Before selecting a generator, check for
municipal ordinances that may dictate requirements
regarding placement of the unit (setback from building
and/or lot line), electrical wiring, gas piping, fuel storage (for
liquid propane or diesel tanks), sound and exhaust emissions.
If you have a technical question regarding sizing or
installation, contact Siemens Technical Service Center
toll free at
800-844-0029 during normal business hours
(8 a.m. to 5 p.m. CST).
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Generator Sizing Guide
Table 1 – Motor Load Reference
AC and Heat Pumps
Running Load Starting Load
Running Amps at Amps at Amps at Starting LR Amps LR Amps LR Amps
Description Hp kW 240V 1ø 208V 3ø 480V 3ø kW at 240V 1ø at 208V 3ø at 480V 3ø
1 Ton (12,000 BTU) 1 1 5 3 1 3 25 17 7
2 Ton (24,000 BTU) 2 2 10 7 3 6 50 33 14
3 Ton (36,000 BTU) 3 3 15 10 4 9 75 50 22
4 Ton (48,000 BTU) 4 4 20 13 6 12 100 67 29
5 Ton (60,000 BTU) 5 5 25 16 7 15 125 83 36
7.5 Ton (85,000 BTU) 7.5 7.5 37 24 11 17 188 125 54
10 Ton (120,000 BTU) 5 Hp (x2) 10 49 33 14 15 125 83 36
10 Ton (120,000 BTU) 10 Hp 10 49 33 14 20 250 167 72
15 Ton (180,000 BTU) 7.5 Hp (x2) 15 74 49 21 17 188 125 54
15 Ton (180,000 BTU) 15 Hp 15 74 49 21 30 375 250 108
20 Ton (240,000 BTU) 10 Hp (x2) 20 98 65 28 20 250 167 72
20 Ton (240,000 BTU) 20 Hp 20 n/a 65 28 40 500 333 144
25 Ton (300,000 BTU) 25 25 n/a 82 35 50 625 416 180
30 Ton (360,000 BTU) 15 Hp (x2) 30 n/a 98 42 30 375 250 108
30 Ton (360,000 BTU) 30 Hp 30 n/a 98 42 60 750 500 217
40 Ton (480,000 BTU) 20 Hp (x2) 40 n/a 131 57 40 500 333 144
40 Ton (480,000 BTU) 40 Hp 40 n/a 131 57 80 1000 666 289
50 Ton (480,000 BTU) 25 Hp (x2) 50 n/a 163 71 50 625 416 180
50 Ton (480,000 BTU) 50 Hp 50 n/a 163 71 100 1250 833 361
General Residential
Running Load Starting Load
Running Amps at Amps at Starting LR Amps LR Amps
Description Hp kW 120V 1ø 240V 1ø kW 120V 1ø 240V 1ø
Refrigerator, Sump Pump, 0.5 0.5 4.9 2.5 1.5 25 13
Furnace, Garage Opener
Freezer, Washer, Septic Grinder 0.75 0.75 7.4 3.7 2.3 38 19
General 1 Hp 1 1 9.8 4.9 3 50 25
Well and Septic Lift Pump 2 2 19.6 9.8 6 100 50
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Generator Sizing Guide
Table 2 – Non-Motor Load Reference
General Residential
Running Load
Amps at Amps at
Description kW 120V 1ø 240V 1ø
Electric heat per 1000 ft.
2
12 n/a 50
Heat pump elements per 1000 ft.
2
7 n/a 29
Dryer 5.5 n/a 23
Hot tub 5 n/a 21
Range oven 5 n/a 21
Hot water 4.5 n/a 19
Stove top per burner 1.5 n/a 6
General receptacles per 1000 ft.
2
1 8.3 n/a
Lighting per 1000 ft.
2
0.75 6.3 n/a
Blow dryer 1.25 10.4 n/a
Dishwasher 1.5 12.5 n/a
Microwave 1 8.3 n/a
Toasters 1 8.3 n/a
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Generator Sizing Guide
Table 3 – Surge Capability
Siemens Liquid Cooled Generators Operating at <3600 RPM
Rated Output Surge Capability Surge Capability
(Running Amps) LR Amps at 15% Voltage Dip LR Amps at 30% Voltage Dip
Size (kW) 240V 1ø 208V 3ø 480V 3ø 240V 1ø 208V 3ø 480V 3ø 240V 1ø 208V 3ø 480V 3ø
25 104 87 38 71 47 26 133 89 52
70 292 243 105 275 183 106 550 366 212
80 333 278 120 275 183 106 550 366 212
100 417 347 150 371 247 142 738 491 284
130 542 451 195 546 364 209 1088 724 419
Siemens Generators Operating at 3600 RPM
Rated Output Surge Capability Surge Capability
(Running Amps) LR Amps at 15% Voltage Dip LR Amps at 30% Voltage Dip
Size (kW) 240V 1ø 208V 3ø 480V 3ø 240V 1ø 208V 3ø 480V 3ø 240V 1ø 208V 3ø 480V 3ø
7 29 24 11 23 n/a n/a 46 n/a n/a
10 42 35 15 31 n/a n/a 63 n/a n/a
13 54 45 20 38 n/a n/a 75 n/a n/a
16 67 56 24 46 n/a n/a 92 n/a n/a
20 83 69 30 63 42 24 121 80 47
25 104 87 38 71 47 26 138 92 53
35 146 121 53 104 69 40 204 136 78
45 188 156 68 146 97 57 292 194 112
60 250 208 90 179 119 69 354 236 136
70 292 243 105 246 164 95 496 330 190
100 417 347 150 333 222 128 663 441 255
150 625 520 226 558 372 215 1121 747 431
4
Note: All kW models listed above are based on nominal LP rating.