Generators Sizing Calculations – Part Fourteen


Today, we will explain Rules Of Thumb for Generators Sizing Calculations for Existing and New Installations.

 
 
Fifth: Rules Of Thumb For Generators Sizing Calculations
 

 
 
 
First: Rules Of Thumb For Existing Installations
 
 

 
In article “Generators Sizing Calculations – Part Twelve”,  We explain how to calculate the peak load of an existing installation by one of the following methods:
 
  1. Measurement Method,
  2. Billing History Method,
  3. Load Summation Method,
  4. NEC Load Calculation Methods,
  5. Estimate based on square footage Method.
 
 
Then we can size the generator(s) of an existing installation based on the calculated/estimated peak load and based on the next steps explained in the following articles:
 
 
Also, We can size the generator(s) of an existing installation for each of the above methods by using Rules Of Thumb as will be explained in below.
 
 

 
1- Measurement Method
 
Use a clamp-on Amp meter or power analyzer to measure facility load levels. Clamp each leg separately and take the measurement during peak usage levels.
 
1.A- For 240V 1ø Applications:
 
To determine peak usage in kW, add the highest Amp readings from the two legs then multiply by 120 and divide by 1,000.
 
  • Step#1: Peak Amps = (L1 + L2)
  • Step#2: Peak kW = (L1 + L2)120 / 1000
  • Step#3- Rules Of Thumb: Size the generator 10 to 20% larger than the peak measured load.
 
1.B- For 3ø Applications:
 
Add the peak Amp readings from all three legs and divide by 3 to determine peak Amps. Multiply peak Amps by volts, multiply the result by 1.732 (square root of 3), then divide by 1000 to convert Amps to kW.
 
  • Step#1: Peak Amps = (L1 + L2 + L3) / 3
  • Step#2: Peak kW = [(Peak Amps x Volts) x 1.732] / 1000    (Assumes power factor of 1.0)
  • Step#3- Rules Of Thumb: Size the generator 20 to 25% larger than the peak measured load.
 

 
 
2- 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, find the Peak Demand. Then Verify motor and UPS load compatibility.
 
Step#1: Peak Demand = largest peak demand from Billing History
Step#2- Rules Of Thumb: Size the generator 25% larger than the largest peak demand
 

 
 
3- Load Summation Method
 
Step#1: Enter running kW for all motor loads (except the largest) expected to run during peak load levels into Table-3. Refer to Table-1 for typical motor load sizes and electrical requirements.
 
Table-3
Motor Load Table (refer to Table 1)
Device
HP
RA
LRA
kW Running (= HP)
Starting kW
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Notes:
 
For HP < 7.5; starting kW = HP x 3
For HP > 7.5; starting kW = HP x 2
Starting kW for loads with no listed HP; calculate HP based on running amps in Table-4 below.
 
Table-4: How To Calculate kW for loads with no listed HP
120 V 1ø
Amps x 120/1000 = kW
240 V 1ø
Amps x 240/1000 = kW
208 V 3ø
(Amps x 208 x 1.732 x PF) /1000 = kW
240 V 3ø
(Amps x 240 x 1.732 x PF) /1000 = kW
480 V 3ø
(Amps x 480 x 1.732 x PF) /1000 = kW
PF is the load power factor - Typical application power factor is 0.95.
 
 
Step#2: Enter kW for all non-motor loads expected to run during peak load levels into Table-5. Refer to Table-2 for typical residential loads and rules of thumb.
 
Table-5
Non-Motor Load Table (refer to Table-2)
Device
Amps
Kw
 
 
 
 
 
 
 
 
 
 
 
 
 
Step#3: Add the running motor load kW, non-motor load kW, and the starting kW of the largest motor load.
 
From Table-3, the Motor load running total - minus largest motor = X kW
From Table-5, Non-motor load total = Y kW
From Table-3, Starting load from largest cycling motor = Z kW
Total electrical loads = X+Y+Z kW
 
Step#4- Rules Of Thumb: Select generator:
 
Commercial (add 20 to 25% to Total electrical loads kW)
Residential (add 10 to 20% to Total electrical loads kW)
 
Step#5- Rules Of Thumb: Select Generator Rating from Standard Sizes/Manufacturers Catalogs
 
Step#6- Rules Of Thumb: Confirm that voltage dip is within acceptable limits by comparing motor LRA to generator surge capability (see Table-3). If not, select the next higher standard generator size.
 
Step#7- Rules Of Thumb: Confirm UPS compatibility according to UPS type as in Table-7 in below.
 
UPS Type
Generator Sizing Recommendation
Passive (also referenced as standby or off-line) and Line-Interactive
 
Limit the total UPS loading to 15% - 20% of the generator capacity.
 
Double-Conversion
 
Single phase models
Limit the total UPS loading to 25% of the generator capacity.
 
Single phase Minuteman UPS models
Limit the total UPS loading to 50% 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.
 
 
Table-7: UPS compatibility according to UPS type
 
If UPS compatibility not verified, select the next higher standard generator size.
 
You can know the UPS type, if you know the supplier name from the below table:
 
Supplier(s)
Passive (Standby)
Line-Interactive
Double-Conversion
Minuteman UPS
Enspire
Enterprise Plus
Endeavor
APC
Back-UPS Series
Smart-UPS Series
Symmetra Series
Liebert
PowerSure PST & PSP
PowerSure PSA & PSI
UPStation & Nfinity
Powerware
3000 series
5000 series
9000 series
Note: Ferrups and Delta-Conversion UPS technologies not included in discussion
 

 
 
4- NEC Load Calculation Methods
 
No Rules Of Thumbs will be used just follow the NEC rules explained in article Generators Sizing Calculations – Part Twelve”.
 

 
 
5- Load Estimation Method
 
Step#1- Rules of Thumbs: Calculate the square footage of the building. (For typical floors multiply the square footage of one floor by number of typical floors).
 
Step#2- Rules of Thumbs: Calculate the estimated KW by multiply the total square footage of the building by the application factor indicated in below table.
 
Applications
Estimated kW
Fast food, convenience stores, restaurants, grocery stores applications
50 kW + 10 watts/sq. ft.
 
Other commercial applications
30 kW + 5 watts/sq. ft.


Facility
Power Usage
Watts/sq ft
kWh/ft2/yr
Schools
 
Class Rooms
Locker Rooms, Auditoriums,
Halls and Corridors
4-5 average (43.06-53.82 W/m2 (Total)
5-6 (53.87-64.58)
2-3 (21.53-32.29)
20 Watts per running foot (65.62 W/m)
11 to 17
(1.02 to 1.58
kWh/m2/yr)
Shopping Centers
Stores, Large Department and Specialty Stores
Show Windows
5 average (Total)
 
5-6 (53.87-64.58)
500 Watts per running foot (1640.5 W/m)
28 to 34 (2.6-3.2)
Office Buildings
Private and General Offices
Professional Offices
Dentist, Drafting Rooms, etc.
5-6 average
4 (43.06)
6-7 (64.58-75.35)
7 (75.35)
28 to 34 (2.6-3.2)
Hotels and Motels
 
Lounge
Rooms
Dining Rooms
Exhibition Halls, Shops, Lobby, Kitchen
3-4 average (32.29-43.06) (Total)
2 (21.53)
3 (32.29)
4 (43.06)
 
3 (32.29)
12 to 17 (1.1-1.58)
Hospitals
 
Lobby, Wards, Cafeterias
 
Private Rooms, Operating Rooms
 
Operating Tables:
Major Surgeries
Minor Surgeries
1.5 to 2.5 kW per bed average
3 Watts/sq ft (32.29 W/m2)
5 Watts/sq ft (53.82 W/m2)
 
3000 Watts each
1500 Watts each
8500 to 11400 kWh
per bed per year
Apartment Houses
Lobby
 
Apartments
 
Small Appliances
2-3 kW per unit (Total)
2 Watts/sq ft (21.53 W/m2)
3 Watts/sq ft (32.29 W/m2)
1.5 kW/unit
11 to 17 (1.20 to
1.58)

 
 
Step#3- Rules Of Thumb: Select Generator Rating from Standard Sizes/Manufacturers Catalogs.
 

 

 
Second: Rules Of Thumb For New Constructions
 

 
 
We have two methods as Rules Of Thumb for generator sizing in New Constructions as follows:
 
  1. By using conservative rules of thumb,
  2. By using Load Form,
 

 
 
1- By Using Conservative Rules Of Thumb
 
Here are some conservative rules of thumb for generator sizing include:
 
  1. Oversize generator 20–25% for reserve capacity and for across the line motor starting.
  2. Oversize generator for unbalanced loading or low power factor running loads.
  3. Use 1/2 hp per kW for motor loads.
  4. For variable frequency drives, – 100% oversize unless pulse–width–modulated oversize the generator by at least 40%.
  5. For UPS systems, oversize the generator by 40% for 6 pulse and 15% for 6 pulse with input filters or 12 pulse.
  6. Always start the largest motor first when stepping loads. For basic sizing of a generator system,
 
The steps of generator sizing by this method will include the following steps:
 
  • Step#1: Calculate Running Amperes,
  • Step#2: Calculating Starting Amperes Using 1.25 Multiplier,
  • Step#3: Selecting kVA of Generator.
 
Example#1:
 
Size the generator to supply the following loads:
  • 200 hp motor,
  • 100 hp motor,
  • 60 hp motor,
  • 68A Lighting load,
  • 95A Miscellaneous loads.
The system voltage is 480 V and PF is 0.77.
 
 
Solution:
 
Step#1: Calculate Running Amperes
 
Motor loads
Load in KW (from rule#3 above)
Running Amperes
200 hp motor
100 KW
= (100x1000)/(1.732x480x0.77)= 156 A
100 hp motor
50 KW
= (50x1000)/(1.732x480x0.77)= 78 A
60 hp motor
30 KW
= (30x1000)/(1.732x480x0.77)= 48 A
Normal Loads
Lighting load
= 68 A
Miscellaneous loads
= 95 A
Total Running Amperes
= 445 A
 
 
Step#2: Calculating Starting Amperes Using 1.25 Multiplier
 
Motor loads
Running Amperes
Starting Amperes
200 hp motor
156 A
= 156 x 1.25 = 195 A
100 hp motor
78 A
= 78x 1.25 = 98 A
60 hp motor
48 A
= 48 x 1.25 = 60 A
Normal Loads
Lighting load
= 68 A
Miscellaneous loads
= 95 A
Total Starting Amperes
= 516 A
 
Step 3: Selecting kVA of Generator
 
Running kVA
= (445 A x 480 V x 1.732)/1000 = 370 kVA
Starting kVA
= (516 A x 480 V x 1.732)/1000 = 428 KVA
Generator must have a minimum starting capability of 428 kVA and minimum running capability of 370 kVA.
 

 

2- By Using the Load Form

 
The load form is prepared by the generator manufactures to help and facilitate the process of generator sizing. A copy of the load form is in below image.
 
Load Form
 
The steps of generator sizing by using the load form will include the following steps:
 

Step#1: Write down all resistive loads on column 12 and 13, KW = KVA so entries are the same on the rows of resistive loads.

 
Note: Resistive loads consist of incandescent lights, water heaters, electric heaters, stoves, and electric furnaces. Power factor = 1, KW = KVA. Running and starting KW are the same.
 
Step#2: List all motor loads with the worst case (largest load applied last) in order on the load form.
 
Note: Motors are inductive loads with KVA always larger than KW. The power factor running usually is between 0.6 and 0.85.
 
Step#3: List the following items for each motor load (If the motor is existing, use nameplate data. If this is a new installation, use Table-2):
 
  • Column-1: HP =
  • Column-2: Starting Code Letter =
  • Column-3: nos. of phases =
  • Column-4: Volts =
  • Column-5: reduced voltage motor starting =
  • Column-6: Start KVA =
  • Starting power factor =
  • Column-7: Start KW = start KVA x start P.F. =
  • Column-8: Run KVA =
  • Column-9: Run KW =
 Table-2
 
Special cases for step#3:
 
A- Only locked rotor amps are known:
 
To find starting KVA if only locked rotor amps are known;
  • For 1-phase: KVA start = (locked rotor amps) (voltage)
  • For 3-phase: KVA start = √3 (locked rotor amps) (voltage)
 
B- For submersible water pumps only:
 
Submersible water pump motors have a higher normal locked rotor code letter and running current than Onan tables shown for average motors. The running current is higher than NEC 430-148 when run at their service factor amps. Use the following two tables:
 
 
 
 
Step#4: calculate the values of columns 10, 11, 12 & 13 as follows:
 
  • Max. KVA - Column 10: Add column 6 to previous line column 12.
  • Max. KW - Column 11: Add column 7 to previous line column 13.
  • Continuous KVA - Column 12: Add column 8 to previous line column 12.
  • Continuous KW - Column 13: Add column 9 to previous line column 13.
 
 
Step#5: Look at the largest numbers in columns 10, 11, 12 and 13. Generator output must be greater than these numbers.
 
Note:
These calculations do not take into account voltage drop on generator. Consult manufacturer at this point.
 
 
Example#2:
 
For the following Loads, calculate the suitable generator size.
 
  • Motor: 2HP, 230V, 1-phase, code J
  • Resistance Loads: 4.5 KW, 230V, 1-phase
Assume resistance loads are balanced.
 
 
Solution:

Step#1: Write down all resistive loads on column 12 and 13, KW = KVA so entries are the same on the rows of resistive loads.

 
Step#2: List all motor loads with the worst case (largest load applied last) in order on the load form,
 
 

 
Step#3: List the following items for each motor load (use Table-2):
 
  • Column-1: HP = 2
  • Column-2: Starting Code Letter = code J
  • Column-3: nos. of phases = 1
  • Column-4: Volts = 230V
  • Column-5: reduced voltage motor starting = Not Applicable
  • Column-6: Start KVA = 16.4 KVA
  • Starting power factor = 0.8
  • Column-7: Start KW = start KVA x start P.F. = 16.4 x 0.8 = 13.12 KW
  • Column-8: Run KVA = 2.47 KVA
  • Column-9: Run KW = 2 KW
 
 
Step#4: calculate the values of columns 10, 11, 12 & 13
 
  • Max. KVA - Column 10: Add column 6 to previous line column 12 = 20.9 KVA
  • Max. KW - Column 11: Add column 7 to previous line column 13 = 17.62 KW
  • Continuous KVA - Column 12: Add column 8 to previous line column 12 = 6.97 KVA
  • Continuous KW - Column 13: Add column 9 to previous line column 13 = 6.5 KW
  
Step#5: Look at the largest numbers in columns 10, 11, 12 and 13. Generator output must be greater than these numbers.
 
 
 

Column 10

Column 11

Column 12

Column 13
Max. KVA
Max. KW
Cont. KVA
Cont. KW
20.9
17.62
6.97
6.5
 
 
In this case, a 20 KW generator would handle the continuous KVA and KW, and maximum KVA and KW.
 
We can use for example, Onan generator 20ES which has the following ratings:
 

Max. KVA

Max. KW

Cont. KVA

Cont. KW
42
22
25
20
 

 

In the next article, we will explain the special cases for Generators Sizing Calculations. So, please keep following.

The previous and related articles are listed in the below table:

Subject of Previous Article
Article
Glossary of Generators – Part One
Glossary of Generators – Part Two
First: Reasons for having on-site generators

Second: Applicable performance standards for generator sets

Third: Selection Factors Used For Generators Sizing Calculations
  1. Generator Power Ratings
  2. Application type
Third: Selection Factors Used For Generators Sizing Calculations
3- Location Considerations,
4- Fuel Selection Considerations,
5- Site Considerations,
 
Third: Selection Factors Used For Generators Sizing Calculations
6- Environmental Considerations,
7- System Voltage and Phase,
 
 
Third: Selection Factors Used For Generators Sizing Calculations
8- Acceptable percent of voltage & frequency dip,
9- Acceptable duration of the voltage & frequency dip,

Third: Selection Factors Used For Generators Sizing Calculations
10- Percent And Type Of Loads To Be Connected – Part One
 
10- Percent And Type Of Loads To Be Connected – Part Two
 
 
Third: Selection Factors Used For Generators Sizing Calculations
11- Load step sequencing
12- Future needs
 
Fourth: Applicable Procedures For Generators Sizing Calculations
1.1- Generator Load Factor
1.2- Load Demand Factor
1.3- Load Diversity Factor
 
Fourth: Applicable Procedures For Generators Sizing Calculations
Step#1: Determine the Required Generator(S) Set Rating,
Step#2: Assign the System Voltage and Phase,
Step#3: Segregate the Loads
Step#4: Match the System to the Load Profile, Calculate the Required Number of Generator Sets and Paralleling Requirement,
 
Fourth: Applicable Procedures For Generators Sizing Calculations
Step#5-Existing Installations: Calculate the peak load of the installation
Step#5-New Constructions: Calculate Connected Loads to Generator
Step#6- Existing Installations: Check for transients or harmonics by using power analyzers and de-rate the peak load value.
Step#6-New Constructions: Calculate Effective Load to Generator,
 
Fourth: Applicable Procedures For Generators Sizing Calculations
Step#7: Adjust the Generator Rating According To Transient Voltage Dip,
Step#8: Adjust the Generator Rating According To Site Conditions,
Step#9: Adjust the Generator Rating According To Fuel Type,
Step#10: Adjust the Generator Rating According To Future Needs,
Step#11: Adjust the Generator Rating According To Power Factor,
Step#12: Calculate the Adjusted Generator Rating,
Step#13: Select Generator Rating from Standard Sizes/Manufacturers Catalogs,
Step#14: Assign Required Number Of Steps/Starting Sequence.
 

 


 

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