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.
Today, I will explain these methods for electrical load estimations.
Note:
I will explain the (3) preliminary load estimation methods only in this course, and the two other methods, NEC load calculations and Final load calculations will be explained later in course " EE3: Basic Electrical design course – Level II " , because these two methods are applied in the final stages of design.
Preliminary Electrical Load estimate
Difference between preliminary and final load estimate
S/N

Preliminary
load calculations

Final
load calculations

1

Units
of Loads will be in (W/ft2) watts per square foot or/and (VA/ft2)
voltamperes per square foot

Units
of Loads will be in KW (kilowatt), or/and KVA (kilovoltampere),
or/and HP (horse power)

2

units are
used interchangeably because unity power factor is assumed

Units can’t
used interchangeably. So, Hp will be converted to kVA; and kVA may be
multiplied by the estimated power factor to obtain kW if required

3

Unity power factor is assumed

Different values of power factors according to load
types.

4

Demand and load factors values will be selected from tables based on
the designer estimation and they will be Used to calculate the transformer
and service size.

Demand and load
factors values are Real values that will document and reflect the number, the
type, the duty
rating
(continuous, intermittent, periodic, short time,
and varying), and the wattage or voltampere rating
of equipment supplied by a common source of
power, and the diversity of operation of equipment served by the common
source.

5

The
connected load will be estimated based on area or population

Actual demand load will be
calculated based on summation of individual building connected loads modified
by suitable demand and diversity factors

6

Easy and Fast
calculations

economical, cost effective calculations
insuring that items of
equipment and
materials are adequate to serve
existing, new,
and future load demands

Preliminary load calculations will be divided to:
 Space by space (functional area method),
 Building Area method,
 Area method.
Note:
A particular design may use one Preliminary load estimate method or a combination from two or even the three methods.
First: SpacebySpace Method (functional area method)
In the SpacebySpace Method, the building will be divided into different space based on its function like offices, conference halls, corridors and lobbies, shops, parking areas, workshops and etc. The Load density in (W/ft2) or/and (VA/ft2) is prescribed for these different spaces, these load densities in addition to spaces area will be used to estimate the preliminary electrical load of this building as described in below.
Usage conditions of SpacebySpace Method
 The SpacebySpace Method is used only for individual spaces in the building.
 The SpacebySpace Method may be used for any building or portion of a building.
Area Measurement in space by space method
The square footage is measured from the outside surface of exterior walls to the centerline of walls between interior partitions of the spaces. The sum of the Gross Interior Area equals the total Gross Area of the building.
Method of estimation by using SpacebySpace Method
In this method, we have two cases as follows:
 First case: availability of grouped load density (i.e. one value covering all lighting, general power and power loads) in (W/ft2) or/and (VA/ft2) for each space.
 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 each space.
First case
1 Divide the building into different space based on its function (for example, office, storage, mechanical, and corridor).
2 Calculate the gross interior area of each space.
3 Determine the grouped load density for each space (from tables).
4 Multiply each space gross interior area by its grouped load density to get the estimated electrical load for this space.
5 Sum all the estimated electrical loads for all spaces to get the total preliminary electrical load for the whole building.
6 Multiply the total preliminary 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# 1:
For a highrise office building with shopping arcade, the power supply has to be planned for a 10story building (12 floors) with a floor area of approx. 25 m x 60 m.
There is a car park for customers in front of the building, the access way to the parking garage (levels 1 to 3 for employees) and a fountain (80 m x 20 m). (Fig. 1)
Real floor area approx. 1350 m2 (14 levels + technical equipment on roof level).
Floor heights of levels 00 to +10: 4 m, levels 1 to 3: 3 m
Calculate main electrical service size for this building.
Fig (1) 
The solution:
Consider first case: availability of grouped load density for each space. Hence, calculations will be as follows:
1 The building will be divided to the following spaces:
 Parking garage / utilities areas (incl. roof area)
 Shopping center / bank
 Offices
2 Calculate The gross areas of each space will be as follows:
Parking garage / utilities areas (incl. roof area) Basement levels 1 to 3 with 1,350 m2 each + utilities areas ca. 210 m2 (areas between utilities hubs), Area = (3 x 1,350 m2 + 210 m2) = 4260 m2
Shopping center / bank Ground level 00 with 1,350 m2 Area = 1 x 1,350 m2 = 1350 m2
Offices Levels +1 to +10 with 1,350 m2 each Area = 10 x 1,350 m2 = 13500 m2
Table# 1 
3 Determine the grouped load density for each space (from table#1) as follows:
Parking garage / utilities areas (incl. roof area)
assumed average power demand: 10 W/m2
Shopping center / bank
assumed average power demand: 60 W/m2
Offices
assumed average power demand: 50 W/m2
4 Multiply each space gross interior area by its grouped load density to get the estimated electrical load for this space as follows:
Parking garage / utilities areas (incl. roof area) Estimated electrical load = (3 x 1,350 m2 + 210 m2) x 10 W/m2 = 42,600 W
Shopping center / bank
Estimated electrical load = 1 x 1,350 m2 x 120 W/m2 = 162,000 W
Offices Estimated electrical load = 10 x 1,350 m2 x 110 W/m2 = 1,485,000 W
5 Sum all the estimated electrical loads for all spaces to get the total preliminary electrical load for the whole building.
Total preliminary electrical load for the whole building = 42,600 + 162,000 + 1,485,000 = 1,690 kW
6 Required transformer output
The established total power demand determines the required transformer output. The determination is based on a cos phi = 0.85 and a transformer load level of 70%.
Transformer output rating = 1,690 kW / (0.7 x 0.85) = 2,840 kVA
Second Case
Method of estimation by using SpacebySpace Method will be as follows:
1 Divide the building into different space based on its function (for example, office, storage, mechanical, and corridor).
2 Calculate the gross interior area of each space.
3 Calculate the estimated lighting, small appliance and power loads for each space as follows:
A Estimated Lighting Load
 Determine the type of light source for each space.
 Determine the Lighting load density in (W/ft2) or/and (VA/ft2) for this type of light source.
 Consider requirements for supplementary lighting (for example, floodlighting, security lighting, and special task lighting).
 Multiply the gross interior area of the space by the assigned Lighting load density to get the estimated lighting electrical load for this space.
B Estimated Small Appliance Loads (general Power Load)
 Small appliance loads shall include those served by general purpose receptacles. In general, the dividing of areas by function for estimating lighting loads will serve for estimating small appliance loads.
 Determine the Small Appliance load density in (W/ft2) or/and (VA/ft2) for this space.
 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).
 Multiply the gross interior area of the space by the assigned Small Appliance load density to get the estimated Small Appliance electrical load for this space.
 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.
HVAC Equipment
 Use the same method for small appliance loads
 These loads are related to the whole building and not related to the divided spaces. So, The SpacebySpace Method cann’t estimate the required ratings for these loads and we must use the building method for this purpose.
Example# 2 :
Consider the same building in example# 1 to be solved if individual load densities are known for each load type as follows:
Space

Load

Load Density (W/M^{2})

Parking Garage / Utilities Areas (Incl. Roof Area)

Lighting

7

Small Appliance

3


Power Load

Inc. in building power


Shopping Center / Bank

Lighting

28

Small Appliance

20


Power Load

Inc. in building power


Offices

Lighting

28

Small Appliance

20


Power Load

Inc. in building power


Building Power Load

HVAC  Pumps  Elevators
 Others

60

Calculate main electrical service size for this building.
The solution:
1 The following table summarizes the calculations steps.
Space

Area

Load

Load
Density

Estimated
Load

Estimated
Electrical
Load For
Space


Parking Garage /
Utilities Areas
(Incl. Roof Area)

4,260

Lighting

7

29,820

42,600

VA

4,260

Small
Appliance

3

12,780


4,260

Power Load

0

0


Shopping Center /
Bank

1,350

Lighting

28

37,800

64,800

VA

1,350

Small
Appliance

20

27,000


1,350

Power Load

0


Offices

13,500

Lighting

28

378,000

648,000

VA

13,500

Small
Appliance

20

270,000


13,500

Power Load

0

0


Building Power
Load

14,850

HVAC –
Pumps –
Elevators –
Others

60

891,000

891,000

VA

Total Estimated Electrical Load

1,646,400

VA


Application Of Cos Phi = 0.85

1,936,941

VA


Application Of Load Level = 70%

2,767,059

VA


Transformer Output Rating

2,767

KVA

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In the next topic, I will explain Other methods for preliminary electrical load calculations. So, please keep following.
In the next topic, I will explain Other methods for preliminary electrical load calculations. So, please keep following.
the best thing in your lessons is the examples , keep going and we with you .
ReplyDeletethanks, medo88
Deleteشكر الله لك مازلت أستفيد منك دائما وأكمل مشروعك التعليمي
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ReplyDeleteBuilding Power
ReplyDeleteLoad
Area14,850m2how did we arrive at this?
Area14,850m2 = shooping Center + officies = 1350 + 13500 = 14850 m2
Deleteparking garage and utilites area are not served for building power load (HVAC –Pumps –Elevators – Others)
thanks,good work that you are doing.
DeleteExample #1
ReplyDeleteShopping center / bank
assumed average power demand: 60 W/m2
Offices
assumed average power demand: 50 W/m2
but in calculation you used 110w/m2 and 120 w/m2 whats the logic behind this
correct me if am wrong, but I think in the first example when calculating for transformer output, we should multiply the total power 1,690KW by 0.7 instead of dividing by the same.
ReplyDeleteLoad level 70% means that the load of the transformer must be equal or less than 70% of the transformer rating . so, 1,690 KW must equal 70% of the transformer rating. how much is the transformer rating? of course must be more than the load 1,690 KW and therefore, the load is divided by( the 70% multiplied by the P.F=0.8.)
Deletehi Ali,
ReplyDeleteCould you inform us fr average demand we can refer to IEC standard number ?
Keep up with good work.
ReplyDelete