Sizing Guide
standby
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).
1
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
2
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
3
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.
Generator Sizing Guide
Table 4 – Fuel Pipe Sizing
Natural Gas
Pipe Size (in.)
kW 0.75" 1" 1.25" 1.5" 2" 2.5" 3"
7 55 200 820
10 20 85 370 800
13 10 50 245 545
16 40 190 425
20 20 115 265 950
25 10 75 180 660
35 35 95 370 915
45 15 60 260 650
60 25 145 390 1185
70 5 75 225 710
80 65 195 630
100 40 140 460
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 includes a nominal number of elbows and tees.
– Please verify adequate service and meter sizing.
5
Generator Sizing Guide
Table 5 – LP Vapor (LPV) Tank Sizing
Vapor Withdrawal
Tank Capacity Tank Capacity Length Diameter Overall Ht. Minimum Temp Tank Capacity
Total (Gal.) Useable (Gal.) (Inches) (Inches) (Inches) (°F) (btu/hr.)
120 72 57 24 33 40 246,240
20 164,160
0 82,080
150 90 68 24 33 40 293,760
20 195,840
0 97,920
250 150 94 30 39 40 507,600
20 338,400
0 169,200
325 195 119 30 39 40 642,600
20 428,400
0 214,200
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 15.5
130 1,800,000 19.9
150 2,050,000 22.7
Note: Tank BTU capacity and generator run times based upon maintaining a minimum tank fuel level of 20%.
Note: Fuel consumption based on a generator 80% loaded.
6
Generator Sizing Guide
UPS Generator Compatibility
Passive (also referenced as standby or off-line) and
Line-Interactive
These technologies are most common for personal workstations
and point of sale applications. They are typically single phase
equipment with size ranges of 350 VA – 2000 VA for passive
and 500 VA to 5000 VA for line-interactive.
Passive UPS’s are the simplest type. Under normal conditions
AC power passes straight through to the UPS load. When the
input power supply goes outside of specifications, the UPS
transfers the load from input power to the internal DC to AC
power inverter. Passive UPS’s do not correct for voltage or
frequency deviations under “normal” operation.
Line-interactive is similar to the passive technology except
it has circuitry that attempts to correct for standard voltage
deviations. Frequency deviations under “normal” power
operation are not corrected.
Equipment Notes:
These devices tend to be electrically / harmonically very noisy.
A single small UPS is not a significant concern, but applications
with multiple UPS’s can be problematic.
Passive UPS technology typically has normal tolerances of
10 – 25% on voltage and 3 hertz on frequency. If the input
source goes outside of these tolerances, the UPS will switch
onto the UPS battery source. Some line-interactive units may
have frequency tolerances factory set to .5 hertz. These units
will need to have their frequency tolerance increased to 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 three phase models covering small
through large applications. Most UPS applications larger than
5000 VA use double conversion technology. This approach is
also the preferred technology for generator applications.
Equipment Notes:
Double-conversion UPS’s that are single phase or unfiltered
three phase models tend to create a significant level of
electrical/ harmonic noise. This is illustrated by harmonic
current distortions that are greater than 35%. When three
phase models are supplied with harmonic filters (current
distortion less than 10%), this concern is no longer an issue.
Generator Sizing Recommendation:
Single phase models: limit the total UPS loading to 25%
of the generator capacity.
Three phase models without filters (current distortion > 30%):
limit the UPS loading to 35% of the generator capacity.
Three phase models with filters (current distortion < 10%):
limit the UPS loading to 80% of the generator capacity.
Supplier(s) Passive (Standby) Line-Interactive Double-Conversion
APC Back-UPS Series Smart-UPS Series Symmetra Series
Liebert PowerSure PST and PSP PowerSure PSA and PSI UPStation and Nfinity Series
Powerware 3000 Series 5000 Series 9000 Series
Note: Ferrups and Delta-Conversion UPS technologies not included in discussion.
7
Generator Sizing Guide
Onsite Estimating Sheet
Contractor _____________________________________________
Email _________________________________________________
Phone _________________________ Fax ___________________
Job Name _____________________________________________
Date___________ Location _______________________________
VOLTAGE
120/240 1Ø 120/208 3Ø 277/480 3Ø
TYPE
Natural Gas LP Vapor LPV)
ELEC. SERVICE
100 Amp 200 Amp 400 Amp
600 Amp Other________
Before installation contact local jurisdiction to confirm all
requirements are met. Jurisdictions may vary. Siemens
recommends contacting local authorities prior to installation.
Loads: Look for heavy building loads such as refrigeration,
air conditioning, pumps or UPS systems.
Use the following for sizing and determining generator kW.
QT Upgrade Required
These applications require an upgrade from the QT Series:
NEC 695 Fire Pumps
NEC 700 Emergency Systems
NFPA 20 Fire Pumps
NFPA 99 Healthcare
NFPA 110 Emergency Systems
Reference Codes
Related Codes and Standards:
NEC 225 Branch Circuits and Feeders
NEC 240 Overcurrent Protection
NEC 250 Grounding
NEC 445 Generators
NEC 701 Legally Required Standby
NEC 702 Optional Standby
NFPA 37 Installation and Use of Stationary Engines
NFPA 54 National Fuel Gas Code
NFPA 58 LP Gas Code
Motor Load Table (refer to Table 1)
Device HP RA LRA kW Running Starting
(= HP) kW
Table 6
Non-Motor Load Table (refer to Table 2)
Device Amps kW
Table 7
Starting kW for HP < 7.5 starting kW = HP x 3
Starting kW for HP > 7.5 starting kW = HP x 2
Starting kW for loading 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 having jurisdiction for local requirements.
To Calculate kW (refer to page 2 for shortcut)
120 V 1Ø Amps x 120/1000 = kW
240 V 1Ø Amps x 240/1000 = kW
208 V 3Ø Amps x 208 x √3 x PF/1000 = kW
240 V 3Ø Amps x 240 x √3 x PF/1000 = kW
480 V 3Ø Amps x 480 x √3 x PF/1000 = kW
PF is application power factor (worst case 1.0)
Typical application power factor is 0.95.
Resources in Sizing Guide
Surge Capability Chart – References running amps of units and LRA
Propane Tank Sizing Chart –
Measures fuel consumption of generator units
Motor Load Reference Guide – Reference guide for basic motor loads
Non-Motor Load Reference Guide – Reference guide for non-motor loads
Fuel Piping Sizing Chart – Assists in calculating adequate pipe size for
natural gas and propane
UPS – Generator Compatibility
UPS Information
1.5 x kVA rating for a filtered system
3 – 5 x kVA rating for an unfiltered system
Siemens recommends you refer to the Siemens UPS Generator
Compatibility sheet and notify the manufacturer of the UPS
system to assist in your installation.
Transfer Switch Availability
SR6ST
– 100 and 200 Amp service entrance rated
RTS – 100, 200, 400 Amp
SR6SI switch only works with R100 controller.
XT 6XR – 100, 150, 200, 300, 400, 600, 800 Amp
HTS switch only works with H100 controller. Avail. in
NEMA 1, NEMA 3R and NEMA 12. Refer to Siemens
product catalog for the appropriate transfer switch.
8
Generator Sizing Guide
Onsite Estimating Sheet
Generator Sizing Instructions:
There is not a single correct sizing solution. The instructions
below identify multiple methods that, when mixed with good
judgment, should result in a moderately sized generator.
Remember to consider load growth, seasonality, and effects
of starting motors.
When motors start, they create a current surge that step loads
the generator. As a result of this step loading, the generator
will experience a voltage dip. After selecting a generator,
reference the generator's surge capability using Table 3. Verify
that the generator voltage dip is adequate for the application.
Most commercial applications should be limited to 15%
voltage dip and residential applications should be limited to
30% voltage dip.
Some commercial applications utilize one or multiple
uninterruptible power supplies (UPS) to backup critical loads.
Please read sizing guidelines for this load type.
Measurement Method
Use a clamp-on amp meter or power analyzer to measure facility load levels. The measurement should be made at peak load
levels. Size the generator 25% larger than the peak measured
load. Verify motor and UPS load compatibility.
Measured Amps = _______
Billing History Method
Many commercial customers have a utility rate structure that
has a peak demand charge. Using a year's worth of electric
bills, size the generator 25% larger than the largest peak
demand. Verify motor and UPS load compatibility.
Peak Demand = _______
Load Summation Method
1) Enter all motors loads expected to run during peak load
levels into Table 6. Reference 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 running load total
(minus largest cycling motor): ______ kW (Ref. Table 6)
Motor starting load from
largest cycling motor: + ______ kW (Ref. Table 6)
Non-motor load total: + ______ kW (Ref. Table 7)
Total (above items): = ______ kW
Select generator (Total x 1.25) ______ kW
4) Verify voltage dip compatibility using generator Surge
Capability Table 3. Verify UPS compatibility using sizing
guidelines provided.
Initial Estimate and Cross Check Methods
These methods are for initial estimates and cross checks only.
Size the generator using one of the above methods.
Estimate based on 60% service size:
240 Volts, 1
Ø: ________ amps x .15 = ________ kW
208 Volts, 3
Ø: ________ amps x .22 = ________ kW
480 Volts, 3
Ø: ________ amps x .50 = ________ kW
Estimate Based on Square Footage
Fast food, convenience stores, restaurants, grocery stores
= 50 kW + 10 watts / sq. ft.
Other commercial = 30 kW + 5 watts / sq. ft.
Square footage = ____________
Estimated kW = ____________
LP
LPG: 8.55 ft.3/lb., 4.24 lbs./gal.,
2500 btu/ft.
3
LPG: 36 ft.3= 1 gal.
Natural Gas
1 cubic foot = 1,000 BTUs
1 therm = 100,000 BTUs
Gas consumption = 13,000-16,000 btu
per kW/hr.
Pressure
1 inch mercury = 13.61 inches
Water Column
1 inch Water Column = 0.036 psi
5-14 inches water column = 0.18 psi
to 0.50 psi
Air Conditioning
1 hp per 1 ton
1 ton = 12,000 btu
Rule of Thumb
For 480 volt systems kW x 1.5 = Amps
For 208 volt systems kW x 3.5 = Amps
For 240 volt single phase systems
kW x 4 = Amps
9
Generator Sizing Guide
Onsite Estimating Sheet
Contractor _____________________________________________
Email _________________________________________________
Phone _________________________ Fax ___________________
Job Name _____________________________________________
Date___________ Location _______________________________
VOLTAGE
120/240 1Ø 120/208 3Ø 277/480 3Ø
TYPE
Natural Gas LP Vapor LPV)
ELEC. SERVICE
100 Amp 200 Amp 400 Amp
600 Amp Other________
Before installation contact local jurisdiction to confirm all
requirements are met. Jurisdictions may vary. Siemens
recommends contacting local authorities prior to installation.
Loads: Look for heavy building loads such as refrigeration,
air conditioning, pumps or UPS systems.
Use the following for sizing and determining generator kW.
QT Upgrade Required
These applications require an upgrade from the QT Series:
NEC 695 Fire Pumps
NEC 700 Emergency Systems
NFPA 20 Fire Pumps
NFPA 99 Healthcare
NFPA 110 Emergency Systems
Reference Codes
Related Codes and Standards:
NEC 225 Branch Circuits and Feeders
NEC 240 Overcurrent Protection
NEC 250 Grounding
NEC 445 Generators
NEC 701 Legally Required Standby
NEC 702 Optional Standby
NFPA 37 Installation and Use of Stationary Engines
NFPA 54 National Fuel Gas Code
NFPA 58 LP Gas Code
Motor Load Table (refer to Table 1)
Device HP RA LRA kW Running Starting
(= HP) kW
Table 6
Non-Motor Load Table (refer to Table 2)
Device Amps kW
Table 7
Starting kW for HP < 7.5 starting kW = HP x 3
Starting kW for HP > 7.5 starting kW = HP x 2
Starting kW for loading 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 having jurisdiction for local requirements.
To Calculate kW (refer to page 2 for shortcut)
120 V 1Ø Amps x 120/1000 = kW
240 V 1Ø Amps x 240/1000 = kW
208 V 3Ø Amps x 208 x √3 x PF/1000 = kW
240 V 3Ø Amps x 240 x √3 x PF/1000 = kW
480 V 3Ø Amps x 480 x √3 x PF/1000 = kW
PF is application power factor (worst case 1.0)
Typical application power factor is 0.95.
Resources in Sizing Guide
Surge Capability Chart – References running amps of units and LRA
Propane Tank Sizing Chart –
Measures fuel consumption of generator units
Motor Load Reference Guide – Reference guide for basic motor loads
Non-Motor Load Reference Guide – Reference guide for non-motor loads
Fuel Piping Sizing Chart – Assists in calculating adequate pipe size for
natural gas and propane
UPS – Generator Compatibility
UPS Information
1.5 x kVA rating for a filtered system
3 – 5 x kVA rating for an unfiltered system
Siemens recommends you refer to the Siemens UPS Generator
Compatibility sheet and notify the manufacturer of the UPS
system to assist in your installation.
Transfer Switch Availability
SR6ST
– 100 and 200 Amp service entrance rated
RTS – 100, 200, 400 Amp
SR6SI switch only works with R100 controller.
XT 6XR – 100, 150, 200, 300, 400, 600, 800 Amp
HTS switch only works with H100 controller. Avail. in
NEMA 1, NEMA 3R and NEMA 12. Refer to Siemens
product catalog for the appropriate transfer switch.
10
Generator Sizing Guide
Onsite Estimating Sheet
Generator Sizing Instructions:
There is not a single correct sizing solution. The instructions
below identify multiple methods that, when mixed with good
judgment, should result in a moderately sized generator.
Remember to consider load growth, seasonality, and effects
of starting motors.
When motors start, they create a current surge that step loads
the generator. As a result of this step loading, the generator
will experience a voltage dip. After selecting a generator,
reference the generator's surge capability using Table 3. Verify
that the generator voltage dip is adequate for the application.
Most commercial applications should be limited to 15%
voltage dip and residential applications should be limited to
30% voltage dip.
Some commercial applications utilize one or multiple
uninterruptible power supplies (UPS) to backup critical loads.
Please read sizing guidelines for this load type.
Measurement Method
Use a clamp-on amp meter or power analyzer to measure facility load levels. The measurement should be made at peak load
levels. Size the generator 25% larger than the peak measured
load. Verify motor and UPS load compatibility.
Measured Amps = _______
Billing History Method
Many commercial customers have a utility rate structure that
has a peak demand charge. Using a year's worth of electric
bills, size the generator 25% larger than the largest peak
demand. Verify motor and UPS load compatibility.
Peak Demand = _______
Load Summation Method
1) Enter all motors loads expected to run during peak load
levels into Table 6. Reference 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 running load total
(minus largest cycling motor): ______ kW (Ref. Table 6)
Motor starting load from
largest cycling motor: + ______ kW (Ref. Table 6)
Non-motor load total: + ______ kW (Ref. Table 7)
Total (above items): = ______ kW
Select generator (Total x 1.25) ______ kW
4) Verify voltage dip compatibility using generator Surge
Capability Table 3. Verify UPS compatibility using sizing
guidelines provided.
Initial Estimate and Cross Check Methods
These methods are for initial estimates and cross checks only.
Size the generator using one of the above methods.
Estimate based on 60% service size:
240 Volts, 1
Ø: ________ amps x .15 = ________ kW
208 Volts, 3
Ø: ________ amps x .22 = ________ kW
480 Volts, 3
Ø: ________ amps x .50 = ________ kW
Estimate Based on Square Footage
Fast food, convenience stores, restaurants, grocery stores
= 50 kW + 10 watts / sq. ft.
Other commercial = 30 kW + 5 watts / sq. ft.
Square footage = ____________
Estimated kW = ____________
LP
LPG: 8.55 ft.3/lb., 4.24 lbs./gal.,
2500 btu/ft.
3
LPG: 36 ft.3= 1 gal.
Natural Gas
1 cubic foot = 1,000 BTUs
1 therm = 100,000 BTUs
Gas consumption = 13,000-16,000 btu
per kW/hr.
Pressure
1 inch mercury = 13.61 inches
Water Column
1 inch Water Column = 0.036 psi
5-14 inches water column = 0.18 psi
to 0.50 psi
Air Conditioning
1 hp per 1 ton
1 ton = 12,000 btu
Rule of Thumb
For 480 volt systems kW x 1.5 = Amps
For 208 volt systems kW x 3.5 = Amps
For 240 volt single phase systems
kW x 4 = Amps
11
Generator Sizing Guide
Onsite Estimating Sheet
Contractor _____________________________________________
Email _________________________________________________
Phone _________________________ Fax ___________________
Job Name _____________________________________________
Date___________ Location _______________________________
VOLTAGE
120/240 1Ø 120/208 3Ø 277/480 3Ø
TYPE
Natural Gas LP Vapor LPV)
ELEC. SERVICE
100 Amp 200 Amp 400 Amp
600 Amp Other________
Before installation contact local jurisdiction to confirm all
requirements are met. Jurisdictions may vary. Siemens
recommends contacting local authorities prior to installation.
Loads: Look for heavy building loads such as refrigeration,
air conditioning, pumps or UPS systems.
Use the following for sizing and determining generator kW.
QT Upgrade Required
These applications require an upgrade from the QT Series:
NEC 695 Fire Pumps
NEC 700 Emergency Systems
NFPA 20 Fire Pumps
NFPA 99 Healthcare
NFPA 110 Emergency Systems
Reference Codes
Related Codes and Standards:
NEC 225 Branch Circuits and Feeders
NEC 240 Overcurrent Protection
NEC 250 Grounding
NEC 445 Generators
NEC 701 Legally Required Standby
NEC 702 Optional Standby
NFPA 37 Installation and Use of Stationary Engines
NFPA 54 National Fuel Gas Code
NFPA 58 LP Gas Code
Motor Load Table (refer to Table 1)
Device HP RA LRA kW Running Starting
(= HP) kW
Table 6
Non-Motor Load Table (refer to Table 2)
Device Amps kW
Table 7
Starting kW for HP < 7.5 starting kW = HP x 3
Starting kW for HP > 7.5 starting kW = HP x 2
Starting kW for loading 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 having jurisdiction for local requirements.
To Calculate kW (refer to page 2 for shortcut)
120 V 1Ø Amps x 120/1000 = kW
240 V 1Ø Amps x 240/1000 = kW
208 V 3Ø Amps x 208 x √3 x PF/1000 = kW
240 V 3Ø Amps x 240 x √3 x PF/1000 = kW
480 V 3Ø Amps x 480 x √3 x PF/1000 = kW
PF is application power factor (worst case 1.0)
Typical application power factor is 0.95.
Resources in Sizing Guide
Surge Capability Chart – References running amps of units and LRA
Propane Tank Sizing Chart –
Measures fuel consumption of generator units
Motor Load Reference Guide – Reference guide for basic motor loads
Non-Motor Load Reference Guide – Reference guide for non-motor loads
Fuel Piping Sizing Chart – Assists in calculating adequate pipe size for
natural gas and propane
UPS – Generator Compatibility
UPS Information
1.5 x kVA rating for a filtered system
3 – 5 x kVA rating for an unfiltered system
Siemens recommends you refer to the Siemens UPS Generator
Compatibility sheet and notify the manufacturer of the UPS
system to assist in your installation.
Transfer Switch Availability
SR6ST
– 100 and 200 Amp service entrance rated
RTS – 100, 200, 400 Amp
SR6SI switch only works with R100 controller.
XT 6XR – 100, 150, 200, 300, 400, 600, 800 Amp
HTS switch only works with H100 controller. Avail. in
NEMA 1, NEMA 3R and NEMA 12. Refer to Seimens
product catalog for the appropriate transfer switch.
12
Generator Sizing Guide
Onsite Estimating Sheet
Generator Sizing Instructions:
There is not a single correct sizing solution. The instructions
below identify multiple methods that, when mixed with good
judgment, should result in a moderately sized generator.
Remember to consider load growth, seasonality, and effects
of starting motors.
When motors start, they create a current surge that step loads
the generator. As a result of this step loading, the generator
will experience a voltage dip. After selecting a generator,
reference the generator's surge capability using Table 3. Verify
that the generator voltage dip is adequate for the application.
Most commercial applications should be limited to 15%
voltage dip and residential applications should be limited to
30% voltage dip.
Some commercial applications utilize one or multiple
uninterruptible power supplies (UPS) to backup critical loads.
Please read sizing guidelines for this load type.
Measurement Method
Use a clamp-on amp meter or power analyzer to measure facility load levels. The measurement should be made at peak load
levels. Size the generator 25% larger than the peak measured
load. Verify motor and UPS load compatibility.
Measured Amps = _______
Billing History Method
Many commercial customers have a utility rate structure that
has a peak demand charge. Using a year's worth of electric
bills, size the generator 25% larger than the largest peak
demand. Verify motor and UPS load compatibility.
Peak Demand = _______
Load Summation Method
1) Enter all motors loads expected to run during peak load
levels into Table 6. Reference 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 running load total
(minus largest cycling motor): ______ kW (Ref. Table 6)
Motor starting load from
largest cycling motor: + ______ kW (Ref. Table 6)
Non-motor load total: + ______ kW (Ref. Table 7)
Total (above items): = ______ kW
Select generator (Total x 1.25) ______ kW
4) Verify voltage dip compatibility using generator Surge
Capability Table 3. Verify UPS compatibility using sizing
guidelines provided.
Initial Estimate and Cross Check Methods
These methods are for initial estimates and cross checks only.
Size the generator using one of the above methods.
Estimate based on 60% service size:
240 Volts, 1
Ø: ________ amps x .15 = ________ kW
208 Volts, 3
Ø: ________ amps x .22 = ________ kW
480 Volts, 3
Ø: ________ amps x .50 = ________ kW
Estimate Based on Square Footage
Fast food, convenience stores, restaurants, grocery stores
= 50 kW + 10 watts / sq. ft.
Other commercial = 30 kW + 5 watts / sq. ft.
Square footage = ____________
Estimated kW = ____________
LP
LPG: 8.55 ft.3/lb., 4.24 lbs./gal.,
2500 btu/ft.
3
LPG: 36 ft.3= 1 gal.
Natural Gas
1 cubic foot = 1,000 BTUs
1 therm = 100,000 BTUs
Gas consumption = 13,000-16,000 btu
per kW/hr.
Pressure
1 inch mercury = 13.61 inches
Water Column
1 inch Water Column = 0.036 psi
5-14 inches water column = 0.18 psi
to 0.50 psi
Air Conditioning
1 hp per 1 ton
1 ton = 12,000 btu
Rule of Thumb
For 480 volt systems kW x 1.5 = Amps
For 208 volt systems kW x 3.5 = Amps
For 240 volt single phase systems
kW x 4 = Amps
13
Notes
Siemens Energy & Automation, Inc.
3333 Old Milton Parkway
Alpharetta, GA 30005
1-800-964-4114
infosea@siemens.com
www.sea.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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