Preliminary electrical load calculation course share

2013

Ali Hassan

Certified Energy Manager – AEE ‐ USA

Electrical Load Estimation Course

Copyrights Reserved for www.Electrical‐Knowhow.com

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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 [email protected]


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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

S/N Item Page No.

1 Introduction 5

2 Importance of Electrical Load Estimation (Preliminary Load Calculations)

5

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)


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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

Feeder#2 demand = 350 kVA × 90% = 315 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:


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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, • 10 nos. fluorescent lamps on one circuit with 1 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, • 20 nos. fluorescent lamps on one circuit with 2 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)


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table#3: Factor of simultaneity according to circuit function

solution:

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

fig.2


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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:

Workshop Name

Load Type Load No. Apparent Power (KVA)

Utilization Factor Max.

Apparent Power Demand Max.KVA

Workshop A:

lathe No.1 5 0.8 4

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 10/16 A 18 1 18

30 nos. fluorescent lamps 3 1 3

Workshop B:

Compressor 15 0.8 12 3 nos. sockets outlets 10/16 A 10.6 1 10.6


Workshop C:

ventilation fan No.1 2.5 1 2.5

No.2 2.5 1 2.5

Oven No.1 15 1 15

No.2 15 1 15

5 nos. sockets outlets 10/16 A 18 1 18


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).


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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:

Workshop Name

Load Type Load No.

Apparent

Power (KVA)

Utilization

Factor Max.

Apparent

Power Demand

Max.KVA

Level‐1

simultaneity factor

Apparent

Power Demand

Max.KVA

Workshop A:

lathe No.1 5 0.8 4

0.75 14.4

No.2 5 0.8 4

No.3 5 0.8 4

No.4 5 0.8 4


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

Workshop B:

Compressor 15 0.8 12 1 12 3 nos. sockets outlets 10/16 A 10.6 1 10.6 0.4 4.3


Workshop C:

ventilation fan No.1 2.5 1 2.5

1 35 No.2 2.5 1 2.5

Oven No.1 15 1 15

No.2 15 1 15

5 nos. sockets outlets 10/16 A 18 1 18 0.28 5


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).


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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.

The Table of Calculation will be as follows:

Workshop

Name

Load Type Load No.

Apparent

Power

(KVA)

Utilization Factor

Max.

Apparent

Power

Demand Max.KVA

Level‐1 Level‐2

simultaneity factor

Apparent

Power

Demand Max.KVA

simultaneity factor

Apparent

Power

Demand Max.KVA

Workshop A:

lathe No.1 5 0.8 4 0.75 14.4 0.9 18.9

No.2 5 0.8 4

No.3 5 0.8 4

No.4 5 0.8 4


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

Workshop B:

Compressor 15 0.8 12 1 12 0.9 15.6


10.6 1 10.6 0.4 4.3

10 nos. fluorescent

lamps

1 1 1 1 1

Workshop C:

ventilation fan No.1 2.5 1 2.5 1 35 0.9 37.8

No.2 2.5 1 2.5

Oven No.1 15 1 15

No.2 15 1 15


18 1 18 0.28 5


2 1 2 1 2


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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. The Table of Calculation will be as follows:

Workshop Name

Load Type Load No.

Apparent

Power

(KVA)

Utilization Factor

Max.

Apparent

Power

Demand Max.KVA

Level‐1 Level‐2 Level‐3

simultaneity factor

Apparent

Power

Demand Max.KVA

simultaneity factor

Apparent

Power

Demand Max.KVA

simultaneity factor

Apparent

Power

Demand Max.KVA

Workshop A:

lathe No.1

5 0.8 4 0.75 14.4

0.9 18.9

0.9 65

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


18 1 18 0.2 3.6


3 1 3 1 3

Workshop B:

Compressor 15 0.8 12 1 12 0.9 15.6 3 nos. sockets

outlets 10/16 A 10.

6 1 10.

6 0.4 4.3

10 nos. fluorescent

lamps

1 1 1 1 1

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


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hop C: 1 8

No.2

2.5 1 2.5

Oven No.1

15 1 15

No.2

15 1 15


18 1 18 0.28 5


2 1 2 1 2

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


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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) watts per square foot or/and (VA/ft2) volt‐amperes per square foot

Units of Loads will be in KW (kilo‐watt), or/and KVA (kilo‐volt‐ampere), or/and HP (horse power)


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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 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 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

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.


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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,

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