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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).

1

Summary of Contents for Standby Generators

Page 1: ...Sizing Guide standby GENERATORS ...

Page 2: ...trical 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 highe...

Page 3: ... 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 2...

Page 4: ...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 3 ...

Page 5: ...s 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...

Page 6: ...460 130 50 215 150 30 150 LP vapor LPV Pipe Size in kW 0 75 1 1 25 1 5 2 2 5 3 7 165 570 10 70 255 1000 13 45 170 690 16 30 130 540 20 15 80 340 745 25 50 235 520 35 20 125 290 1030 45 82 195 725 60 45 115 445 1095 70 20 60 260 660 80 15 50 230 590 100 30 165 430 1305 130 70 205 660 150 45 150 490 Note Table values are maximum pipe run in feet Pipe sizing is based on 5 H2O pressure drop Sizing inc...

Page 7: ... 500 300 119 37 46 40 792 540 20 528 360 0 264 180 850 510 165 41 50 40 1 217 700 20 811 800 0 405 900 1000 600 192 41 50 40 1 416 960 20 944 640 0 472 320 LP Vapor LPV Withdrawal Fuel Consumption Load kW BTU Hr Gal Hr 7 110 000 1 2 10 190 000 2 1 13 215 000 2 4 16 252 000 2 8 20 340 000 3 8 25 390 000 4 3 35 500 000 5 5 45 620 000 6 8 60 800 000 8 8 70 950 000 10 5 80 1 100 000 12 2 100 1 400 000...

Page 8: ...a minimum of 2 hertz Generator Sizing Recommendation Limit the total UPS loading to 15 20 of the generator capacity Double Conversion This technology is most common for critical load applications Double conversion UPS s constantly rectify AC to DC and then invert the DC back into AC This configuration results in an output that corrects for voltage and frequency deviations There are single and thre...

Page 9: ... with no listed HP calculate HP based on running amps in the chart on the right Recommended Generator Size ________ Refer to Generator Sizing Instructions on other side of this sheet INSTALL NOTES 1 Suggested concrete pad minimum thickness of 4 with 12 overhang on all sides Fiberglass pad included with air cooled products 2 Consult manual for installation recommendations 3 Consult local authority ...

Page 10: ...e Table 1 for typical motor sizes and electrical requirements 2 Enter all non motor loads expected to run during peak load levels into Table 7 Reference Table 2 for typical residential loads and rules of thumb 3 Sum the running motor load data but do not include the largest motor that is cycling Add to this value the non motor load data and the starting kW for the largest cycling motor Motor runni...

Page 11: ... with no listed HP calculate HP based on running amps in the chart on the right Recommended Generator Size ________ Refer to Generator Sizing Instructions on other side of this sheet INSTALL NOTES 1 Suggested concrete pad minimum thickness of 4 with 12 overhang on all sides Fiberglass pad included with air cooled products 2 Consult manual for installation recommendations 3 Consult local authority ...

Page 12: ... Table 1 for typical motor sizes and electrical requirements 2 Enter all non motor loads expected to run during peak load levels into Table 7 Reference Table 2 for typical residential loads and rules of thumb 3 Sum the running motor load data but do not include the largest motor that is cycling Add to this value the non motor load data and the starting kW for the largest cycling motor Motor runnin...

Page 13: ... with no listed HP calculate HP based on running amps in the chart on the right Recommended Generator Size ________ Refer to Generator Sizing Instructions on other side of this sheet INSTALL NOTES 1 Suggested concrete pad minimum thickness of 4 with 12 overhang on all sides Fiberglass pad included with air cooled products 2 Consult manual for installation recommendations 3 Consult local authority ...

Page 14: ... Table 1 for typical motor sizes and electrical requirements 2 Enter all non motor loads expected to run during peak load levels into Table 7 Reference Table 2 for typical residential loads and rules of thumb 3 Sum the running motor load data but do not include the largest motor that is cycling Add to this value the non motor load data and the starting kW for the largest cycling motor Motor runnin...

Page 15: ...Notes ...

Page 16: ...siemens com generators RPSA GENSZ 0506 New 3M0506B A Printed in USA 2006 Siemens Energy Automation Inc All Rights Reserved Siemens is a registered trademark of Siemens AG Product names mentioned may be trademarks or registered trademarks of their respective companies Specifications are subject to change without notice ...

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