Electrical Load Estimation – Part Four


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:

  1. Space by space (functional area method), 
  2. Building Area method. 
  3. 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: 
  1. Basic method,
  2. 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 
Total building demand (for identical buildings) = building demand (step# 7) X number of identical buildings 
  • 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: 
Total Area demand = ∑ (All total buildings demands from step# 8)+ general load (step# 9) 

  • 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 sub-main 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.



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