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:
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 clampon 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.
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.

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 Table3. Refer to Table1 for typical motor load sizes and electrical requirements.
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 Table4
below.
Step#2:
Enter
kW for all nonmotor loads expected to run during peak load levels into Table5.
Refer to Table2 for typical residential loads and rules of thumb.
Step#3:
Add
the running motor load kW, nonmotor load kW, and the starting kW of the largest
motor load.
From
Table3, the Motor load running total  minus largest motor = X kW
From
Table5, Nonmotor load total = Y kW
From
Table3, 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 Table3). If
not, select the next higher standard generator size.
Step#7
Rules Of Thumb: Confirm UPS compatibility according to UPS type as in Table7
in below.
Table7: 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:
Note:
Ferrups and DeltaConversion 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.
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
Here are some
conservative rules of thumb for generator sizing include:
The steps of
generator sizing by this method will include the following steps:
Example#1:
Size the
generator to supply the following loads:
The system
voltage is 480 V and PF is 0.77.
Solution:
Step#1:
Calculate Running Amperes
Step#2:
Calculating Starting Amperes Using 1.25 Multiplier
Step
3: Selecting kVA of Generator

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.
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 Table2):
Special cases for step#3:
A Only locked rotor amps are
known:
To find starting KVA if only locked rotor amps are known;
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 430148 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:
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.
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 Table2):
Step#4: calculate the values of columns 10, 11, 12 & 13
Step#5: Look at the largest numbers in columns
10, 11, 12 and 13. Generator output must be greater than these numbers.
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:

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 onsite generators
Second: Applicable performance standards for generator sets Third: Selection Factors Used For Generators Sizing Calculations


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#5Existing Installations: Calculate
the peak load of the installation
Step#5New Constructions: Calculate Connected Loads to Generator
Step#6 Existing Installations: Check for
transients or harmonics by using power analyzers and derate the
peak load value.
Step#6New 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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