Today,
we will continue explaining in detail the applicable procedures for Generators
Sizing Calculations for Existing and New Installations.
Fourth: Applicable Procedures For Generators Sizing
Calculations

In the previous Article “Generators
Sizing Calculations – Part Ten”, we list the required steps for generator set sizing
calculation and we indicate that the required steps will differ according to
the type of the installation; new or existing as follows:
For
Existing Installations
the required steps for generator
set sizing calculation will be as follows:
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,
Step#5: Calculate the peak load of the
installation
Step#6: Check for transients or harmonics by using power analyzers and derate the
peak load value.
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.
For
New Constructions
the required steps for generator
set sizing calculation will be as follows:
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,
Step#5: Calculate Connected Loads to Generator
Step#6: Calculate
Effective Load to Generator,,
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.
Now, we will explain in detail the
above steps of Generators Sizing Calculations for Existing and New
Installations.

Step#5  Existing Installations: Calculate the peak load of the installation

We can calculate the peak load of an existing installation by one of the following methods:

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.
Step#1: Peak Amps = (L1 + L2)
Step#2: Peak kW = (L1 + L2)120 / 1000
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)

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

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:
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

4 NEC Load Calculation Methods
In our course “EE3: Basic Electrical design course – Level II”, we explain how to calculate the total load by using NEC Load Calculation Methods for both dwelling and nondwelling buildings as follows:
4.A For Dwelling Buildings
You can calculate the total load For Dwelling Buildings by using our electrical load calculators explained in the following articles:
4.B For NonDwelling Buildings
You can calculate the total load For NonDwelling Buildings by using our electrical load calculators explained in the following articles:

5 Load Estimation Method
In our course “EE2: Basic Electrical design course – Level I”, we explain how to estimate the total load by using (3) Electrical Load Estimation methods which are:
All these methods are explained in the following Articles:
Or , you can download our PDF Course for Preliminary Electrical Load Estimation from the following link: Electrical Load Estimation Course

Step#5 New Construction:
Calculate Connected Loads to Generator

It is important to note that if some nonlinear loads are
present in the system, generator rating is not being just sum of the loads.
The effective generator load will be the sum of the effective loads on
generator which must be calculated properly to avoid generator over sizing.
In step#3: Segregate the Loads
which explained in article “Generators
Sizing Calculations – Part Eleven” we indicate the (5) categories of the loads are as in
below table:
So, the Effective loads to generator will be calculated based on
load segregation done in step#3 as follows:
Connected Load_{ }for Category1 = Σ kW _{catagory1}
Connected Load_{ }for Category2 = Σ kW _{Starting
category2}
Connected Load_{ }for Category3 = Σ kW _{Running
category3}
Connected Load_{ }for Category4 = Σ K _{1} x kW
_{catagory4}
Connected Load_{ }for Category5 = Σ K _{2 }x kW
_{catagory5}

Step#6  Existing Installations: Check For Transients Or Harmonics By Using Power Analyzers And DeRate The Peak Load Value.

Power Quality Analyzers
Power Quality Analyzers
How to measure harmonics using a power quality analyzer
Step#1: Measure with a clamp meter that is capable of indicating total harmonic distortion (THD). THD for voltage should not exceed 5 %. THD for current will run considerably higher.
Step#2: Use a power quality analyzer to further investigate the magnitude and effects of the individual harmonics.
Notes:
Example for measuring harmonics by using power quality analyzer: See Figure2.
Fig.2: Harmonics on power quality analyzer
In this power quality analyzer screenshot, the harmonic frequencies appear on the harmonic axis. The percent to which the specific harmonic frequency is a component of the fundamental 60 Hz frequency appears on the vertical axis. The cursor has been placed over the third harmonic frequency, and this third harmonic current appears to represent approximately 25 % of the 60 Hz frequency.

Generator Sizing Rule For NonLinear Loads
In cases where nonlinear loads cause increased generator heating and Total Harmonic Distortion (THD) exceeds 15%, two techniques are typically used to compensate for the increased generator heating:
Method#1: Using Deration factors while sizing the generator.
Method#2: Using a generator with oversized kVA requirement.
The following generator
Rating vs. %Harmonics current Derate Chart can be used to determine the derating
factor for Harmonics and nonlinear loads:

Step#6  New Construction: Calculate Effective Load to Generator

In this step, we will calculate the Effective Load to Generator by calculating the effective loads
for each load category which are calculated by applying demand and diversity
factor to each category’s connected load as follows:
Where:
K _{demand} is the demand
factor & K _{diversity} is the diversity factor.

1 Demand
Factor
Demand Factor is the mathematical ratio
of the operating load divided by the connected load.
Demand Factor = (Total Operating kW x
100) / Total Connected kW
So, Demand factor is always less than one.
The below table shows
a range of common demand factors for different apparatus
2 Diversity Factor
The formula used to calculate diversity
factor is total maximum demand divided by total incoming kW times 100.
Diversity Factor = (Total Max. Demand kW
x 100) / Total Incoming kW
So, Diversity factor is usually more than one.
Difference
between demand and diversity factor:
Most of the electrical engineers confuse between the
demand and diversity factors, to solve this confusion, don't forget
that:

You can download tables for different
factors listed above by clicking the following links:

Effective Load to Generator of the
generator will be
Effective Load to
Generator = Σ KW _{effective }of all Load categories
And The Effective KVA rating of the generator will be
KVA _{effective}
= KW _{effective }/ PF _{overall}
_{} 
In the next article, we will continue
explaining the applicable procedures 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

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