Electrical Building Services Design

Design Notes

2020-03-13T23:20:00.002+03:00

DESIGN INFORMATION:

1. What is the power absorbed by a Motor load?

Power absorbed is the input power drawn by the motor. It should be noted that a motor’s rated kW or name plate rating is the output rating of the motor (also known as the shaft power).

When we are calculating the total load requirements of a system, we have to always use absorbed power, which is the actual power drawn by the motor.

Power absorbed= input power=kW rating/ efficiency
It is also represented as “ P1” in pump data sheets.

Also note that the Input current drawn by a motor= kW/(1.732 *V* pF)

Powerfactor correction

2020-03-09T23:44:00.002+03:00

What is power factor?

Power factor is defined as the ratio of real power to total power

ie, pF=Real power/Apparent Power= kW/ kVA

or kW= kVA *power factor

Real (active power) power does the useful work. Apparent power(reactive power) is non working power, which is mainly utilized for magnetization of cores of transformer, induction motors etc. Real or true power kW is the power available after reactive power requirement has been satisfied.

In an AC circuit, it is also defined as the cosine of the angle between voltage and current vector.

power factor triangle is given below

The power factor can get values in the range from 0 to 1.When power factor is close to One, it is considered as goof pF

Effects of low power factor

If power factor is low, a system would draw more current for a given kW from the source, which will cause more voltage drop. By improving the pF, one can reduce voltage drop and thus increase the voltage level.

Utilities will charge a higher cost to industrial and commercial clients having a low P.F.

What is power factor correction?

Power factor correction is an adjustment of the electrical circuit in order to change the power factor near 1. Mos electricity authorities requires that that power factor should be corrected to 0.9 or 0.95

How to correct power factor in an AC distribution system?

Power factor compensation can be defined as a process of correcting the lagging current by producing leading current so that the angle between voltage and current reduces.

This is achieved by connecting a capacitor panel to the distribution system.

A capacitor panel shall consist of many capacitors connected in parallel to create the required kVAR.There will be a power factor control relay which will sense the voltage and current of the system and then calculate the required value of capacitors to be connected. This assures that only the needed capacitor is energized depending on the load and power factor of the network to ensure the power factor remains above a selected value

Indicative image of a APFC Panel

Power factor control relays

These are electronic relays which sense the voltage /current from the system and give output signals to switch contactors ON to switch on required number of capacitors. Based on number of outputs given, 6 steps, 12 steps relays are available. For more precise control of pF, higher number of steps would be required.

Indicative image of a Power factor control relay

Calculating Required Capacitor kVAR

Calculating Required Capacitor kVAR to improve P.F from 0.85 to 0.90

Required Capacitor kVAR = kW (tan θ₁– tan θ₂)

where kW is the connected load of the panel
(many electricity authorities agrees to apply a diversity factor on the kW value for sizing the capacitor value

so if the connected load is 1200 kW and diversity factor is 0.8
demand= 1200x 0.8 kW

tan θ₁– tan θ₂ works out to be about 0.28

kVAR required= 1200 x 0.8 x 0.28=268 kVAR say 300 kVAR which is the nearest standard value.

following combination of capacitors units could be used to achieve this;

50x 5 kVAR
25x 2 kVAR

A 10 step relay could be used to connect this capacitor bank

Rated current of a capacitor

I_{N}={\frac {Q}{{\sqrt {3}}.U}}

where Q is the kVAR value and U is the phase-phase voltage in kV

for 300 kVAR,

In= 300/0.4*1.732= 300x1.44 =433.02 Amp

Breaker size required= 630 Amp (nearest standard value)

GENERATOR SIZING FOR PUMP STARTING

2015-09-03T15:32:00.001+03:00

GENERATOR SIZING FOR PUMP STARTING
DETAILS OF LOAD
Consider 4 Pump motors of 60 kW , 400 V, 50 Hz each is connected to a Panel fed from a Generator. Pump motors are started using soft starters. Running conditions are ;a) All pumps could be running one time b) Pumps could start one by one c) 3 Pumps running and one could be starting.
SIZING CRITERIA
1. Running kW or kVA should be less than the selected Generator capacity 2.Wosrt case starting scenario: Total kVA or kW required should be within the sKVA rating of the Generating set.( sKVA rating shall be available in the Generator catalog) otherwise consider 1.5 times the Generator kVA rating.
RUNNING
Maximum running load= 4 pumps running
Pump rating=60kW, efficiency=91.8%
running kW=60/0.918=65.35 kW
running kVA=65.35/0.8=81.68 kVA
Total kW( 4 pumps running)	261.4	kW
Running pF=0.8 ( lowest valve from pump catalog)
Total kVA required while running= 245 kVA	326.75	kVA
STARTING
Worst case could be 3 pumps running and 4th starting
1) 3 Pumps running kVA= 81.68 * 3	245.04	kVA
2) 4th Pump starting kVA

Full load current of One pump	107	A
Starting current with Soft starter=2.5 FLC	267.5	A
Starting kVA of One pump	185.324	kVA

Total starting kVA = 3 pumps running kVA + One pump Starting kVA=245+185.324	430.364	kVA

Generator selection:
Running kVA required	326.75	kVA
Starting kVA required	430	kVA

Generator required ,considering 80% loading of Generator	408.4375
Nearest available size	400	kVA
400 KVA PRIME RATED GENERATOR SUITS THE REQUIREMENTS

Cable Sizing Calculations

2015-09-02T15:25:00.002+03:00

In electrical system design, cable sizing is one of the major activity. Lets see how to size the cable for a load.

CABLE SIZING CALCULATIONS

( This article is draft stage and is under modification..........)

In electrical system design, cable sizing is one of the major activity.
Simplified Procedure for cable sizing;
• Determine the design current
• Decide the Protective device rating
• Decide the installation method of cable
• Identify the de-rating factors
• Find the Calculated current rating of required cable
• Select the cable
• Check the voltage drop
• Check the short-circuit capacity

STEP-1 : CALCULATE THE DESIGN CURRENT
Use # phase power formula to calculate the design current

Let's assume P=50 kW, v=415 V, and pf=0.8
• Where P is 3 phase load in kW and V is the Line to Line voltage.
• pf is the power factor of the load which may be taken as 0.8 , if otherwise not known.

we would get Ib= 50/ (1.732 * 0.8 * 0.415 ) = 86.95 Ampere

If it is a motor load, we have to divide by efficiency so as to get the input current drawn.

STEP-2 : SELECT THE PROTECTIVE DEVICE RATING

Protective device ( MCB/MCCB circuit breaker) is selected to protect the cable against damage from thermal overload.
Protective device must be selected such that Nominal rating of the device is more than the design current, i.e

Diversity and Demand factor..etc

2006-08-10T11:00:00.000+03:00

DEMAND FACTOR, DIVERSITY FACTOR AND TRANSFORMER SIZING CALCULATION

a) Demand factor - the ratio of the load that a piece of equipment actually draws when it is in operation to the load it could draw (which we call full load).

For example, an oversized motor - 20 HP - drives a constant 15 HP load whenever it is on. The motor demand factor is then 15/20 = 75 %.

Demand factor= Maximum demand/ Connected load.

This is generally confused with diversity factor. Demand factors should be ideally applied to individual loads and diversity factor to a group of loads. When you talk about ‘diversity’, there is naturally more than one (many, in real case scenarios) loads involved.

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Modified for further clarity and as per reader's comments: 06-09/2011
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b) Diversity factor -Gives an indication about the simultaneous maximum demad of a system considering various loads operating at a time and the highest ' coincident' Maximum demand of all those loads.

Diversity factor =
Sum of individual maximum demands of various sub-loads / Maximum demand of the total system.
This will be less than 1.

Note: Reciprocal of the above ratio ( will be more than 1) also is used in some other countries.

c) Where to use which factor!

Demand factor can be applied to calculate the size of the sub-main which is feeding a Sub panel or a fixed load like a motor etc.

If the panel (Say SSB 1) total load is 250 kVA , considering a Demand factor of 0.8, we can size the feeder cable for 250 x 0.8= 200 kVA, which is equal to 280 Ampere on 415 V 3 phase supply.

Diverity factor can be used to estimate the total load required for a facility or to size the Trnasformer required.

Simultaneous maximum demand = (Sum individual maximum demands of different loads) x Div. Factor

( Note: if a value greater than 1 is used, divide with that factor)

Assuming there are 4 such SSB’s as per the chart below,

PANEL      LOAD        Demand Factor%       Max Demand
SSB 1        250 kVA           80                               200
SSB2        200 kVA            80                              160
SSB3        150 kVA             75                               112.5
SSB4        400 kVA            85                              340

                                                                      Total 812.50 kVA

The Sum of individual maximum demands is 812.5 kVA as given above.

To calculate the Main incoming capacity and Transformer size, we can use the Diversity factor which will take into account the fact that all the four SSB’s above won’t attain their individual maximum simultaneously.

Assuming a Diversity factor of 0.7, the simultaneous maximum demand would be

812.5 x 0.7 = 568.75 kVA

TRANSFORMER SIZING CALCULATION

Transformer could be sized after considering a loading factor of 80 % and future requirements of another 20 % extra.

Transformer size= ( 568.75 / 0.8 ) x 1.2 = 853 kVA ( Say 1000 kVA , the nearest available standard size)

Electrical building services Design-An intro

2006-08-10T10:44:00.000+03:00

Electrical services for a building generally includes the following activities:

1. Load estimation and Load calculations
2. Transformer sizing calculations
3. Circuits
4. Distribution planning
5. Choosing protective devices
6. Selection of cables, cable sizing calculations
7. Cable trunking and Tray sizing
8. Lighting design
9. Emergency lighting
10. Standby systems- Diesel Generator
11. Standby systems- UPS
12. Power factor compensation
13. Lightning protection
14. Earthing systems
In the following pages I am trying to give few details about each of these.
Usually, the design of a building starts with the load estimation, and then proceeds to equipment selection, lighting design, deciding small power/power socket outlet positions, mark up of plans for drafting etc.
Most of the design activities outlined above   can be done by manual hand calculations. It is better to use a software package for accurate design of Lighting.

Some useful packages would be
·        DiaLux,
·        ReLux,
·        Visual Lighting software etc..
For power calculations
·        AMTECH
·        HEVACOMP
·        ECODIAL
·        SIMARIS
are popular software available.

Formulas and rule of thumb for Electrical calculations

2006-06-07T17:15:00.000+03:00

kVA to Current

Current= 1.443 x kVA ( 3 phase 400 Volts)
Current= 1.39 x kVA ( 3 phase 415 Volts)

Eg:- 1000 kVA Transformer’s Full load current on LT side
1.443 x 1000= 1443 Amperes ( 3 phase 400 Volts)
1.39 x 1000 = 1390 Amperes ( 3 phase 415 Volts)

kW to Current ( power factor assumed as 0.8)

Current= 1.8 x kW (3 phase 400 Volts)
Current= 1.74 x kW (3 phase 415 Volts)

Power in 3 Phase circuit

kW= \/ 3 x V x I x cos φ
V= Line voltage ( 400 or 415 Volts)
I = Per phase current (Ampere)

Power factor correction Capacitor

Capacitor size= kW * (tan θ 2 - tan θ1) (kVAR)

cos θ1 = system power factor(say 0.8)

cos θ2 = Desired power factor( say 0.9)

Capacitor current

Ic = kVAr / \/ 3 x V ( V= 0.400 or 0.415 kV )

Breaker and Cable size for capacitor = 1.35 x Capacitor current (Minimum)

Prolouge

2006-06-07T13:47:00.000+03:00

The purpose of this blog is to share my experience and learning with others in the industry. I hope some of this information may be useful to someone out there who is trying to find out some technical information through a web search.

This blog is trying to give few technical information regarding distribution system design, with particular reference to Buildings. The information contained herein is generally based on practices followed as per British Standards(BS), IET Regulations 16th Edition and Indian Standards.

Disclaimer:

The information provided herein is given with the good intention of sharing of knowledge and the author is not responsible for any damage or loss whatsoever arising out of using the information contained herein.