Preliminary electrical load calculation course share

2013
Electrical Load Estimation Course

Ali Hassan
Certified Energy Manager – AEE ‐ USA

Copyrights Reserved for www.Electrical‐Knowhow.com

About Author

Hi, I'm Ali Hassan el‐Ashmawy, I began my career from 1999

as a site electrical engineer then as area manager from 2001

then as electrical designer from 2003 then as senior

electrical designer from 2006 and up to date.

In my past experience, I designed and construct about 100

projects in different countries like Egypt, Kuwait, Indonesia, KSA, Gabon and Iraq.

My designs were approved by many international authorities like USA corps of

engineers and USA ministry of exterior – OBO Office.

I'm certified energy manger CEM from AEE – USA since 2006 and I hope to become a

well‐known designer in the field of electrical design.

To contact me please email to Ali1973hassan@yahoo.com

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Course Description:

This course is intended to prepare the target persons with the ability to recognize,

understand, and perform preliminary electrical load calculation/estimation for any

building type by many calculation methods.

The target Persons:

Design engineers, new graduate engineers, under graduate engineering students.

Skills Development:

On completion of this course the target person will be able to:

• Recognize different calculation method for electrical load estimation.

• Understand the procedures and logic of each method for electrical load

estimation.

• Perform the calculations steps of each method for electrical load estimation.

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Table of Contents

Page
S/N Item
No.
1 Introduction 5
2 Importance of Electrical Load Estimation (Preliminary Load 5
Calculations)
3 Definition of Important terms in Load Estimation 5
3.1 Connected load 5
3.2 Demand load   5
3.3 Demand Interval 6
3.4 Maximum demand 6
3.5 Demand factor (in IEC, Factor of maximum utilization ku) 6
3.6 Coincidence factor (in IEC, Factor of simultaneity ks) 7
3.7 Diversity factor 10
3.7.1 Difference between demand and diversity factor 11
3.8 Load factor 18
4 Methods of Electrical load estimation 19
5 Preliminary Electrical Load estimate   19
5.1 Difference between preliminary and final load estimate 19
5.2 Preliminary load calculations sub‐methods 20
5.3 Space‐by‐Space Method (functional area method) 21
5.3.1 Usage conditions of Space‐by‐Space Method   21
5.3.2 Area Measurement in space by space method   21
5.3.3 Method of estimation by using Space‐by‐Space Method   21
First Case 21
Second case 25
5.4 The Building Method 29
5.4.1 Comparison between space‐by‐space and building methods 29
5.4.2 Usage conditions of Building Method 29
5.4.3 Area Measurement in Building Method 29
5.4.4 Method of estimation by using Building Method 29
First Case 30
Second case 31
5.5 Area method 35
5.5.1 Usage conditions of Area Method 35
5.5.2 Method of estimation by using Area Method   36
First: basic method 36
Second: Optional Method (Load centers method) 38
5.6 General notes for all methods of electrical load estimations 41

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1‐ Introduction

At the beginning of the project, in the draft design (early design) stage, the electrical
design professional should do the following:

• Make Analysis of load characteristics,
• Review The available voltage system types/classes and levels,
• Review the utility’s rate structure,
• Make roughly a key single‐line diagram and a set of subsidiary single‐line
diagrams. The key single‐line diagram should show the sources of power e.g.
generators, utility intakes, the main switchboard and the interconnections to
the subsidiary or secondary switchboards.
• Develop Demand factor relationship between connected loads and the actual
demand imposed on the system.

2‐ Importance of Electrical Load Estimation (Preliminary Load Calculations)

Electrical Load Estimation is very important in the draft design (early design) stage
because it help to:

• Plan the connection to upstream network and MV circuit configurations.
• Plan the transformers substation(s) (if any) and the main switchgear room.
• Apply to Power Company for supply.
• Calculate initial budget for the electrical works.

3‐ Definition of Important terms in Load Estimation:

There are many important terms which must be understood before performing the
load estimation, these terms are:

3.1 Connected load

It is the Sum of all the loads connected to the electrical system, usually expressed in
watts.

3.2 Demand load

It is the electric load at the receiving terminals averaged over a specified demand

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interval of time, usually 15 min., 30 min., or 1 hour based upon the particular utility’s
demand interval. Demand may be expressed in amperes, kilo‐amperes, kilo‐watts,
kilo‐vars, or kilo‐volt‐amperes.

3.3 Demand Interval

It is the period over which the load is averaged, usually 15 min., 30 min., or 1 hour.

3.4 Maximum demand

It is the greatest of all demands that have occurred during a specified period of time
such as 5 minutes, 15 minutes, 30 minutes or one hour. For utility billing purposes
the period of time is generally one month.

3.5 Demand factor (in IEC, Factor of maximum utilization ku)

In normal operating conditions the power consumption of a load is sometimes less
than that indicated as its nominal power rating.

The demand factor is the ratio of the maximum demand on a system to the total
connected load of the system.

Demand factor = Maximum demand load / Total load connected

Notes:

• This factor must be applied to each individual load, with particular attention
to electric motors, which are very rarely operated at full load.
• Demand factors for buildings typically range between 50 and 80 percent of
the connected load. For most building types, the demand factor at the service
where the maximum diversity is experienced is usually 60 to 75 percent of
the connected load. Specific portions of the system may have much higher
demand factors, even approaching 100 percent.

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3.6 Coincidence factor (in IEC, Factor of simultaneity ks)

It is a matter of common experience that the simultaneous operation of all installed
loads of a given installation never occurs in practice, i.e. there is always some degree
of diversity and this fact is taken into account for estimating purposes by the use of a
simultaneity factor (ks).

The coincidence factor is the ratio of the maximum demand of a system, or part
under consideration, to the sum of the individual maximum demands of the
subdivisions.

Coincidence factor = Maximum system demand / Sum of individual maximum
demands

Notes:

• The factor ks is applied to each group of loads (e.g. being supplied from a
distribution or sub‐distribution board).

Example#1 (see Fig.1):

5 storeys apartment building with 25 consumers, each having 6 kVA of installed
load.

Calculate the following:

1. The total installed load,
2. The apparent‐power supply,
3. The main service size,
4. The third level service size.

Solution:

1‐ Calculation of The total installed load,

From Fig.1, The total installed load for the building will be the sum of the installed
loads in the (5) storeys which will be as follows:

Ground floor:

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There are (4) consumers, the installed loads in this storey = 4consumers x 6 KVA
installed load per consumer = 24 KVA

First Floor:
There are (6) consumers, the installed loads in this storey = 6 x 6 = 36 KVA

Second Floor:

Third Floor:

Forth Floor:

So, the total installed load for the building = 24 + 36 + 30 + 24 + 36 = 150 kVA

Fig (1)

Page 8 of 41


Table#1: Factor of simultaneity (ks) for Apartments Block

From Table#1 in above, it is possible to determine the magnitude of currents in
different sections of the common main feeder supplying all floors.

For vertical rising mains fed at ground level, the cross‐sectional area of the
conductors can evidently be progressively reduced from the lower floors towards the
upper floors. These changes of conductor size are conventionally spaced by at least
3‐floor intervals.

2‐ Calculation of apparent power

From Table#1, since the number of downstream consumers = 25, the Factor of
simultaneity ks = 0.46
So, the apparent‐power supply required for the building = 150 KVA x 0.46 = 69 kVA

3‐ Calculation of The main service size

The current entering the rising main at ground level (main service size) = (150 x 0.46
x 1000) / (400 x √3) = 100 A

4‐ Calculation of The third level service size

The current entering the third floor (the third level service size) = sum of currents
delivered to third and fourth floors
The number of consumers in the third and fourth floors = 4 + 6 =10 consumers

From Table#1, for number of downstream consumers = 10, the Factor of
simultaneity ks = 0.63

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So, the current entering the third floor (the third level service size) = (36 + 24) x 0.63
x 1000 / (400 x √3) = 55 A

3.7 Diversity factor

the diversity factor is the reciprocal of the coincidence factor.

Diversity factor = Sum of individual maximum demands / Maximum system
demand

Notes:

• The Diversity Factor is applied to each group of loads (e.g. being supplied
from a distribution or sub‐distribution board).

Example#2:

Consider that a feeder supplies five users with the following load conditions:

• On Monday, user one reaches a maximum demand of 100 amps;
• On Tuesday, two reaches 95 amps;
• On Wednesday, three reaches 85 amps;
• On Thursday, four reaches 75 amps;
• On Friday, five reaches 65 amps.
• The feeder’s maximum demand is 250 amps.

Calculate the Diversity Factor for this feeder?

Solution:

The diversity factor can be determined as follows:

Sum of total demands = 100 + 95 + 85 + 75 + 65 = 420 A

Diversity factor = Sum of total demands ÷ Maximum demand on feeder = 420 A ÷
250 A = 1.68

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Example#3:

Calculate the size of a main feeder from substation switchgear that is supplying five
feeders with connected loads of 400, 350, 300, 250 and 200 kilovolt‐amperes (kVA)
with demand factors of 95, 90, 85, 80 and 75 percent respectively. Use a diversity
factor of 1.5.

Solution:

1‐ Calculate demand for each feeder:

Feeder#1 demand = 400 kVA × 95% = 380 kVA

2‐ Sum all of the individual demands = 380 + 315 + 255 + 200 + 150 = 1,300 kVA

3‐ If the feeder were sized at unity diversity, then the size of the main feeder = 1,300
kVA ÷ 1.00 = 1,300 kVA

However, using the diversity factor of 1.5, the size of the main feeder = 1,300 kVA ÷
1.5 = 866 kVA.

3.7.1 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:

• The Demand factor must be applied to each individual load, with particular
attention to electric motors, which are very rarely operated at full load.
• The Diversity Factor is applied to each group of loads (e.g. being supplied
from a distribution or sub‐distribution board).

Example #4:

An industrial building consists of (3) nos. workshops A, B & C, each workshop will
include the following loads:

Page 11 of 41


Workshop A:

• 4 nos. lathe with 5 KVA each,
• 2 nos. pedestal drill with 2 KVA each,
• 5 nos. sockets outlets 10/16 A on one circuit with 18 KVA total,
• 30 nos. fluorescent lamps on one circuit with 3 KVA total.

Workshop B:

• One nos. Compressor with 15 KVA,
• 3 nos. sockets outlets 10/16 A on one circuit with 10.6 KVA total,

Workshop C:
• 2 nos. ventilation fans with 2.5 KVA each,
• 2 nos. Oven with 15 KVA each,
• 5 nos. sockets outlets 10/16 A on one circuit with 18 KVA total,

• Draw a key single line diagram for this building?
• Determine both the demand (utilization) factor and simultaneity factor with
the help of tables # 2 & 3 in below?
• Calculate the demand load for each level in the key single line diagram?

Table#2: Factor of simultaneity for distribution boards (IEC 60439)

Page 12 of 41


table#3: Factor of simultaneity according to circuit function

solution:

Follow the solution steps in below and in fig.2.

fig.2

Page 13 of 41


Step#1: List all the loads in each workshop and write the apparent power of each
load in KVA beside it.

Step#2: write the utilization factor for each load, IEC gives Ku estimation values for
these loads as follows:

• For motor Ku = 0.8
• For socket outlets Ku = 1 (depend on the type of appliances being supplied
from the sockets concerned)
• For light circuits Ku= 1

The Table of Calculation for Steps# 1&2 will be as follows:

Apparent
Apparent Utilization
Workshop Power
Load Type Load No. Power Factor
Name Demand
(KVA) Max.
Max.KVA

lathe No.1 5 0.8 4
No.2 5 0.8 4
No.3 5 0.8 4
Workshop No.4 5 0.8 4
A: pedestal drill No.1 2 0.8 1.6
No.2 2 0.8 1.6
5 nos. sockets outlets 10/16 A 18 1 18
30 nos. fluorescent lamps 3 1 3
Compressor 15 0.8 12
3 nos. sockets
Workshop
outlets 10/16 A
10.6 1 10.6
B:
10 nos. fluorescent
lamps
1 1 1

ventilation fan No.1 2.5 1 2.5
No.2 2.5 1 2.5
Workshop Oven No.1 15 1 15
C: No.2 15 1 15
5 nos. sockets outlets 10/16 A 18 1 18
20 nos. fluorescent lamps 2 1 2

Step#3: calculate the Max. Demand apparent power in KVA for each load = apparent
power X Ku for each load.

Step# 4: group same type of loads on one distribution panel/box and this will be the
first Level of distribution (LEVEL 1).

Page 14 of 41


Step# 5: in level 1 and from table #2, write the simultaneity factor for each
distribution panel/box and from table # 3 write the simultaneity factor for each for
each separate load.

Step# 6: calculate the Max. Demand apparent power in KVA for each distribution
panel/box = sum of all branch loads’ Max. Demand apparent power in KVA X
simultaneity factor for each distribution panel/box.

The Table of Calculation will be as follows:

Level‐1
Appar
ent Appar
Appar Utilizat
Power ent
Worksho Load ent ion
Load Type Dema simultan Power
p Name No. Power Factor
nd eity Dema
(KVA) Max.
Max.K factor nd
VA Max.K
VA
lathe No.1 5 0.8 4
No.2 5 0.8 4
No.3 5 0.8 4
0.75 14.4
Worksho No.4 5 0.8 4
p A: pedestal drill No.1 2 0.8 1.6
No.2 2 0.8 1.6
5 nos. sockets outlets 10/16 A 18 1 18 0.2 3.6
30 nos. fluorescent lamps 3 1 3 1 3
Compressor 15 0.8 12 1 12
3 nos. sockets
Worksho
outlets 10/16 A
10.6 1 10.6 0.4 4.3
p B:
10 nos. fluorescent
lamps
1 1 1 1 1

ventilation fan No.1 2.5 1 2.5
No.2 2.5 1 2.5
1 35
Worksho Oven No.1 15 1 15
p C: No.2 15 1 15
5 nos. sockets outlets 10/16 A 18 1 18 0.28 5
20 nos. fluorescent lamps 2 1 2 1 2

Step# 7: group the distribution panel/box in each workshop in one main distribution
panel/box. So, we will have (3) main distribution panel/box for the (3) workshops
and this will be the second level of distribution (LEVEL 2).

Page 15 of 41


Step# 8: in level 2 and from table #2, write the simultaneity factor for each main
distribution panel/box.

Step# 9: calculate the Max. Demand apparent power in KVA for each main
distribution panel/box = sum of all branch distribution boxes’ Max. Demand
apparent power in KVA X simultaneity factor for each main distribution panel/box.


Worksh Load Type Load App Utiliz App Level‐1 Level‐2
op No. aren ation aren
Name t Facto t
simult App simult App
Pow r Pow
er Max. er
aneity aren aneity aren
factor t factor   t
(KVA Dem Pow Pow
) and er er
Max. Dem Dem
KVA and and
Max. Max.
KVA KVA
Works lathe No.1 5 0.8 4 0.75 14.4 0.9 18.9
hop A:    No.2 5 0.8 4
   No.3 5 0.8 4
   No.4 5 0.8 4
pedestal drill No.1 2 0.8 1.6
   No.2 2 0.8 1.6
5 nos. sockets outlets 18 1 18 0.2 3.6
10/16 A
30 nos. fluorescent 3 1 3 1 3
lamps
Works Compressor    15 0.8 12 1 12 0.9 15.6
hop B:   3 nos. sockets    10.6 1 10.6 0.4 4.3
outlets 10/16 A
10 nos.    1 1 1 1 1
fluorescent
lamps
Works ventilation fan No.1 2.5 1 2.5 1 35 0.9 37.8
hop C:      No.2 2.5 1 2.5
Oven No.1 15 1 15
   No.2 15 1 15
5 nos. sockets outlets 18 1 18 0.28 5
10/16 A
lamps

Page 16 of 41


Step# 10: group the (3) main distribution panel/box in one main general distribution
board MGDB and this will be the third level of distribution (LEVEL 3).

Step# 11: in level 3 and from table #2, write the main general distribution board
MGDB.

Step# 12: calculate the Max. Demand apparent power in KVA for main general
distribution board MGDB = sum of the (3) workshop main distribution boxes’ Max.
Demand apparent power in KVA X simultaneity factor for main general distribution
board MGDB.


Load Type Load App Utiliz App Level‐1 Level‐2 Level‐3
Workshop No. aren ation aren
Name t Facto t
Pow r Pow simult App simult App simult App
er Max. er aneity aren aneity aren aneity aren
(KVA Dem factor   t factor   t factor   t
) and Pow Pow Pow
Max. er er er
KVA Dem Dem Dem
and and and
Max. Max. Max.
KVA KVA KVA
Works lathe No. 5 0.8 4 0.75 14. 0.9 18. 0.9 65
hop A: 1 4 9
   No. 5 0.8 4
2
   No. 5 0.8 4
3
   No. 5 0.8 4
4
pedestal drill No. 2 0.8 1.6
1
   No. 2 0.8 1.6
2
5 nos. sockets outlets 18 1 18 0.2 3.6
10/16 A
lamps

Works Compressor    15 0.8 12 1 12 0.9 15.
hop B:   3 nos. sockets    10. 1 10. 0.4 4.3 6
outlets 10/16 A 6 6
10 nos.    1 1 1 1 1
fluorescent
lamps

Works ventilation fan No. 2.5 1 2.5 1 35 0.9 37.

Page 17 of 41


hop C:   1 8
   No. 2.5 1 2.5
2
Oven No. 15 1 15
1
   No. 15 1 15
2
5 nos. sockets outlets 18 1 18 0.28 5
10/16 A
lamps

3.8 Load factor

The load factor is the ratio of the average load over a designated period of time,
usually 1 year, to the maximum load occurring in that period.

Load factor = Average load / Maximum load

Free download

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

• IEEE Demand Factor Values
• Unified Facilities Criteria ‐UFC‐ Demand Factor Values
• NEC Demand Factor Values
• Demand Factor Values From Other Regulations
• Diversity Factor Values
• Unified Facilities Criteria ‐UFC‐Load Factor Values
• IEC Factor of Simultaneity Values

Page 18 of 41


4‐ Methods of Electrical load estimation

There are (5) methods for Electrical Load Estimation, which are:

A‐ Preliminary load calculation

This method is subdivided into (3) sub‐methods as follows:

1. Space by space (functional area method),
2. Building method.
3. Area method.

B‐ NEC load calculations.

C‐ Final load calculations.

Note:

In this course, I will explain the preliminary load estimation methods, and the two
other methods; NEC load calculations and Final load calculations will be explained
later in course " EE‐3: Basic Electrical design course – Level II ”, because the
preliminary load estimation methods are used in the early design phase while the
other two methods are applied in the final stages of design.

5‐ Preliminary Electrical Load estimate

5.1 Difference between preliminary and final load estimate

before going through the calculation steps for Preliminary Electrical Loads, we need
to highlight the main differences between the load estimation or calculation by the
preliminary and final methods. The following table shows these differences as
follows:

S/N Preliminary load calculations Final load calculations

1 Units of Loads will be in (W/ft2) Units of Loads will be in KW (kilo‐watt),
watts per square foot or/and or/and KVA (kilo‐volt‐ampere), or/and HP
(VA/ft2) volt‐amperes per square (horse power)
foot

Page 19 of 41


2 units are used interchangeably Units can’t used interchangeably. So, Hp
because unity power factor is will be converted to kVA; and kVA may be
assumed 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 Demand and load factors values  are Real
will be selected from tables values that will document and reflect the
based on the designer estimation number, the type, the duty rating
and they will be Used to (continuous, intermittent, periodic, short
calculate the transformer and time, and varying), and the wattage or
service size. volt‐ampere 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 Actual demand load will be calculated
estimated based on area or based on summation of individual
population 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

5.2 Preliminary load calculations sub‐methods:

As I indicated before, this method is subdivided into (3) sub‐methods as follows:

1. Space by space (functional area method),
2. Building method.
3. Area method.

Note:

A particular design may use one Preliminary load estimate method or a combination
from two or even the three methods.

Page 20 of 41


5.3 Space‐by‐Space Method (functional area method)

In the Space‐by‐Space 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.

5.3.1 Usage conditions of Space‐by‐Space Method

• The Space‐by‐Space Method is used only for individual spaces in the building.
• The Space‐by‐Space Method may be used for any building or portion of a
building.

5.3.2 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.

And the sum of the Gross Interior Area equals the total Gross Area of the building.

5.3.3 Method of estimation by using Space‐by‐Space 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:

Method of estimation by using Space‐by‐Space Method will be as follows:

1‐ Divide the building into different space based on its function (for example, office,

Page 21 of 41

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