In the previous topic " Electrical Load Estimation – Part One ", I explain the definitions of the most important terms in electrical load estimation and provides download links for factors used in the electrical load estimation.
Also, I listed the (5) methods of electrical load estimations which were:
A Preliminary load calculation which divide to:
 Space by space (functional area method),
 Building Area method.
 Area method.
B NEC load calculations.
C Final load calculations.
I explained the first method of preliminary load calculations; Space by space (functional area method) in the previous topic " Electrical Load Estimation – Part Two ". Also, I explained the second method; Building Area method in the previous topic " Electrical Load Estimation – Part Three “.
Today, I will explain the third method for electrical load estimations which is Area method.
Third: Area method
Area method is used for Area loads which consist of groups of individual building loads that be constructed on a large area and will be served by a same subdivision of the electric distribution system.
Examples for cases where area loads method will be applied are industrial, residential, commercial and military areas. The area method will be used for the design of substations serving these areas.
Industrial Area 
Usage conditions of Area Method
The Area Method may be used only for:
 Groups of individual building loads within an area.
Method of estimation by using Area Method
We have two methods as follows:
 Basic method,
 Optional method (Load center method).
First: basic method
This method is used for one level of electrical distribution inside the area (i.e. the feeders for individual buildings originated from the main substation switchgear) and the steps of calculations will be as follows:
 Step #1: list the buildings names included in the area and determine the numbers of identical buildings.
 Step#2: calculate the total connected load for each building by using the one or both of the previous methods; Space by Space and building area.
 Step#3: determine the classification of each building from table #1.
 Step#4: determine the demand factor for each building from table #1.
 Step# 5: determine the load factor for each building from table #1.
 Step# 6: find the coincidence factor for each building from table #1.
 Step# 7: calculate the building demand load, where:
Building Demand load = building connected load (step# 2) x demand factor (step# 4) x coincidence factor (step# 6)
 Step# 8: calculate the total building demand for the identical buildings, where
 Step# 9: calculate the general loads for this area which include roadway lighting, area lighting, obstruction lighting, and other loads not included in individual building loads.
 Step# 10: calculate the total Area demand, where:
 Step# 11: apply a load growth factor of 25% and calculate the final total Area demand, where:
Final total Area demand= total area demand (step# 9) X 1.25
Table#1
you can download table#1 by clicking Here.
Second: Optional Method (Load centers method)
This method is used for multi levels of electrical distributions inside the area (i.e. each group of individual buildings can be connected to submain switchgear far from the main substation switchgear. The calculation steps will be as follows:
 Steps from #1 to #7 are the same as in basic method.
 Step# 8: an additional coincidence factors – differs from that given in Table# 1  will be applied to each load center which can be called as “load center coincidence factor”.
After determination of load center coincidence factor for each load center, the total building demand = ∑ (All load centers demands).
 Steps from #9 to # 11 are the same as in basic method.
Load center coincidence factor values
 For loads of a similar nature, it should be in the range 0.93 to 1.00.
 For loads of a varying nature (Evening loads and daytime loads) are combined; it should be in the range of 0.70 to 1.00.
 The lower values will occur when the magnitudes of the loads are nearly balanced, and the higher ones when the combined load is predominantly one type.
Example:
An airforce base has a maintenance area including the following buildings:
Description

Number
of Buildings

Total connected
load, kw

Fuel oil pump house

1

0.3

Filling station

1

3

Filling station building

1

0.3

Receiver building

1

2.1

Transmitter building

1

37.2

Tacan building

1

0.7

Radar building

1

1.2

Aircraft fire and rescue
station

1

8

Aircraft operations
building

1

80.2

Photographic building

1

10.5

Academic instruction
building

1

47

Operational trainer
facility

1

0.1

Aircraft overhaul and
repair shop

1

7600

Paint/finishing hangar

1

127

Engine preparation and
storage shop

1

405

Engine maintenance shop


Engine test cell

1

360

Missile equipment
maintenance facility

1

3

Auto vehicle maintenance
facility

1

370

Fire station

1

14.6

Calculate the main electrical service size for this area using the building connected loads indicated above.
The solution:
By using table #1 and the basic method described above for estimating the electrical load of an area, the following table summarizes the solution steps:
Description

Total
Connected
load, kw

Demand
factor,
%

Maximum
demand,
%

Load
factor,
%

Coinciden
ce factor, %

Coincide
nce peak, kw

Fuel oil
pump house

0.3

100

0.3

52

52(1)

0.2

Filling
station

3

60

1.8

18

57(1)

1

Filling station
building

0.3

80

0.2

20

61(1)

0.1

Receiver
building

2.1

65

1.4

72

79

1.1

Transmitter
building

37.2

65

1.4

72

79

19.1

Tacan building

0.7

65

0.5

72

79

0.4

Radar building

1.2

70

0.8

72

79

0.6

Aircraft fire
and rescue
station

8

30

2.4

15

52(1)

1.2

Aircraft
Operations
building

80.2

80

64.2

28

68(1)

43.6

Photographic
building

10.5

70

7.4

18

57(1)

4.2

Academic
Instruction
building

47

60

28.2

22

62(1)

17.5

Operational
trainer facility

0.1

80

0.1

15

52(1)

0.1

Aircraft
overhaul
and repair
shop

7600

38

2890

25

95(2)

2745

Paint/
Finishing
hangar

127

70

89

26

66(1)

58.3

Engine
Preparation
and storage
shop

405

40

162

15

52(1)

84.2

Engine
Maintenance
shop


Engine test
cell

360

45

162

28

68(1)

110

Missile
equipment
maintenance
facility

3

40

1.2

22

62(1)

0.7

Auto
vehicle
Maintenance
facility

370

60

222

25

65(1)

145

Fire station

14.6

30

4.4

15

52(1)

2.3

Total

3235


System loss (6%)

194


Grand total (Main
electrical service size)

3429

Free download
You can download an excel worksheet for preliminary load calculation as per area method b y clicking Here.
General notes for all methods of electrical load estimations
1 Method combination
A particular design may use one Preliminary load estimate method or a combination from two or even the three methods.
2 System loss
A system loss of approximately 6 %, based on calculated maximum demand, should be added to the preliminary building load.
3 Load growth
Determine the requirements for load growth for anticipated usage and life expectancy with particular attention to the possibility of adding heavy loads in the form of air conditioning, electric heating, electric data processing, and electronic communication equipment. No more than 10% spare capacity will be considered during design unless otherwise noted.
Free download
In the next topics, I will explain how to design and draw an optimal single line diagram. So, please keep following.
You can download an excel worksheet for preliminary load calculation as per area method b y clicking Here.
General notes for all methods of electrical load estimations
1 Method combination
A particular design may use one Preliminary load estimate method or a combination from two or even the three methods.
2 System loss
A system loss of approximately 6 %, based on calculated maximum demand, should be added to the preliminary building load.
3 Load growth
Determine the requirements for load growth for anticipated usage and life expectancy with particular attention to the possibility of adding heavy loads in the form of air conditioning, electric heating, electric data processing, and electronic communication equipment. No more than 10% spare capacity will be considered during design unless otherwise noted.
Free download
In the next topics, I will explain how to design and draw an optimal single line diagram. So, please keep following.
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