Electrical Load Estimation – Part Three


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 "


Today, I will explain other methods for electrical load estimations, which are: 

  • Building Area method. 
  • Area method. 



Second: The Building Area Method

The Building Area method is used for buildings, to some extent, bigger than that can be handled by the Space by space method, although it is easier than the Space by space method as you estimate the electrical load based on the activity of the whole building unlike the Space by space method which need dividing the building into multi-areas with different activities.



Comparison between space-by-space and building type methods

  • Power Allowances (densities) under the two methods are similar for buildings with typical space distributions, but may vary depending on the space distributions in particular buildings. 
  • In general the Building Area Method is easier to use than the Space-by-Space Method, but the Space-by-Space Method offers greater flexibility. 
  • In addition, the Space-by-Space Method allows the use of the Additional Interior Lighting Power Allowance for certain specific uses. 


Usage conditions of Building Area Method


The Building Area Method may be used only for:
  • An entire building (using the primary occupancy), or; 
  • Single, independent, and separate occupancies in a multi-occupancy building. 



Area Measurement in Building Area Method


  • Total Gross Area will be used in the Building Area Method and the square footage is measured from the outside surface of exterior walls, or from the outside surface of exterior walls to the center line of walls between building types. 

Method of estimation by using Building Area Method

In this method, we have two cases as follows: 

  • First case: availability of grouped load density (i.e. one value for the whole building) in (W/ft2) or/and (VA/ft2).
  • Second case : availability of individual load density (i.e. individual values for lighting, general power and power loads) in (W/ft2) or/and (VA/ft2) for the whole building.First case 



First Case:

availability of grouped load density (i.e. one value for the whole building) in (W/ft2) or/and (VA/ft2). Hence, Method of estimation by using building area Method will be as follows:


1. Calculate the total Gross Area for the building;

2. Determine the building type/usage (use a reasonable equivalent if the specific building type is not listed);

3. Determine the load density in (W/ft2) or/and (VA/ft2) from the Tables for that building type,

4. Calculate the preliminary electrical load by multiplying the total gross area of the building by its load density.

5- Multiply the preliminary electrical load for the building by the power factor value (= 0.8) and a suitable load factor (for future extensions and losses compensations) to get the main service size.



Example #1:



If the educational buildings have power densities as follows:




Load density (watt/m2) for
non air conditioned zone
Load density (watt/m2)
for air conditioned zone
15
90


Calculate the total preliminary electrical load for transformer(s) feeding an academic building which has (4) floors with a floor area 3250 m2, and calculations to be done for the following cases: 
  • Building with air conditioning, 
  • Building without air conditioning. 


The solution: 


  • The total gross area of the building = 3250 m2 x 4 = 13000 m2 
  • The building type/usage is educational 
  • The load densities is indicated in the above table 
  • The total estimated electrical load :
  1. The total estimated electrical load for the Building with air conditioning = 13000 x 90 = 1,170,000 watt = 1,170 KW 
  2. And, the total estimated electrical load for the Building without air conditioning = 13000 x 15 =195,000 watt = 195 KW 
  • The preliminary electrical load is based on a cos phi = 0.85 and a transformer load level of 70%. 
  • Transformer output rating for the Building:
  1. Transformer output rating for the Building with air conditioning = 1,170 kW / (0.7 x 0.85) = 1,967 kVA 
  2. Transformer output rating for the Building without air conditioning = 195 kW / (0.7 x 0.85) = 328 kVA (in some regulations, this load can be feed from utility low voltage network without the need for a transformer substation inside the user’s premises) 



Second case:


availability of individual load density (i.e. individual values for lighting, general power and power loads) in (W/ft2) or/and (VA/ft2) for the whole building. Hence, Method of estimation by using building area Method will be as follows:
1- Calculate the total Gross Area for the building; 

2- Determine the building type/usage (use a reasonable equivalent if the specific building type is not listed);

3- Determine the load density in (W/ft2) or/and (VA/ft2) from the Tables for that building type for each load type i.e. for lighting, small appliances, power loads.

4- Multiply the total gross area of the building by the assigned Lighting / small appliances /power load densities to get the estimated lighting / small appliances /power electrical loads for this building.

5- Take into your consideration the following points:

  • Consider requirements for supplementary lighting (for example, floodlighting, security lighting, and special task lighting). 
  • Consider requirements for the extent of using small appliances by the occupants of this space (for example, Mechanical spaces in building storage areas and similar spaces in which outlets are provided but infrequently used are usually neglected in computing loads, except for special cases). 
  • Electric power loads shall include all loads other than lighting loads and those served by general purpose receptacles and comprise the environmental system electric power requirements and the facility occupancy equipment electric power requirements 

6- Sum the estimated lighting load plus the estimated small appliances load plus the estimated power loads to get the total estimated electrical loads.

7- Multiply the total estimated electrical load for the whole building by the power factor value (= 0.8) and a suitable load factor (for future extensions and losses compensations) to get the main service size. 



Example#2:


A (4) floors office building with a floor area 2000 SF, calculate the total preliminary electrical load for the transformer(s) feeding this building By using The following tables from ASHRAE and IEEE.






The solution: 


1- The total gross area of the building = 2000 x 4 = 8000 SF

2- The building type/usage is Office

3- From above ASHRAE and IEEE tables, the load densities will be as follows:

  • Lighting = 1.81 W/SF 
  • Small appliances = 1 VA/SF 
  • Air conditioning = 6 VA/SF 

4- The subtotals estimated electrical loads will be as follows:

  • Lighting = 1.81 W/SF x 8000 = 14,480 Watt 
  • Small appliances = 1 VA/SF x 8000 = 8,000 Watt 
  • Air conditioning = 6 VA/SF x 8000 = 48,000 Watt 

5- The total estimated electrical load for the Building = 14,480 + 8,000 + 48,000 = 70,480 Watt = 70.5 KW

6- The preliminary electrical load is based on a cos phi = 0.85 and a transformer load level of 70%.
7- Transformer output rating for the Building = 70.5 kW / (0.7 x 0.85) = 118.5 kVA (in some regulations, this load can be feed from utility low voltage network without the need for a transformer substation inside the user’s premises) 



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In the next topic, I will explain the Third method for preliminary electrical load calculations; Area method. So, please keep following.




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