Electrical Thumb Rules-(Part-4)


Sub Station Capacity & Short Circuit Current Capacity:

As per GERC

Voltage Sub Station Capacity Short Circuit Current
400 KV Up to 1000 MVA 40 KA  (1 to 3 Sec)
220 KV Up to 320 MVA 40 KA  (1 to 3 Sec)
132 KV Up to 150 MVA 32 KA  (1 to 3 Sec)
66 KV Up to 80 MVA 25 KA  (1 to 3 Sec)
33 KV 1.5 MVA to 5 MVA 35 KA (Urban) (1 to 3 Sec)
11 KV 150 KVA to 1.5 MVA 25 KA (Rural) (1 to 3 Sec)
415 V 6  KVA to 150 KVA 10 KA  (1 to 3 Sec)
220 V Up to 6 KVA 6 KA  (1 to 3 Sec)

 Sub Station Capacity & Short Circuit Current Capacity:

As per Central Electricity Authority

Voltage Sub Station Capacity Short Circuit Current
765 KV

4500 MVA

31.5 KA for 1 Sec
400 KV

1500 MVA

31.5 KA for 1 Sec
220 KV

500 MVA

40 KA for 1 Sec
110/132 KV

150 MVA

40 KA or 50 KA for 1 Sec
66 KV

75 MVA

40 KA or 50 KA for 1 Sec

 Minimum Ground Clearance and Fault Clearing Time:

Voltage Min. Ground Clearance Fault Clear Time
400 KV 8.8 Meter 100 mille second
220 KV 8.0 Meter 120 mille second
132 KV 6.1 Meter 160 mille second
66 KV 5.1 Meter 300 mille second
33 KV 3.7 Meter  
11 KV 2.7 Meter  

 Bus bar Ampere Rating:

For Phase Bus bar Aluminium 130 Amp / Sq.cm or 800Amp / Sq.inch.
For Phase Bus bar Copper 160 Amp / Sq.cm or 1000Amp / Sq.inch
For Neutral Bus bar Same as Phase Bus bar up to 200 Amp than Size of Neutral Bus bar is at least half of Phase Bus bar.

 Bus bar Spacing:

Between Phase and Earth 26mm (Min)
Between Phase and Phase 32mm (Min)
Bus bar Support between Two Insulator 250mm.

 Sound Level of Diesel Generator (ANSI 89.2&NEMA 51.20):

KVA

Max. Sound Level

<9 KVA 40 DB
10 KVA to 50 KVA 45 DB
51 KVA to 150 KVA 50 DB
151 KVA to 300 KVA 55 DB
301 KVA to 500 KVA 60 DB

IR Value of Transformer:

IR Value of Transformer

Voltage

30°C

40°C

50°C

>66KV

600MΩ

300MΩ

150MΩ

22KV to 33KV

500MΩ

250MΩ

125MΩ

6.6KV to 11KV

400MΩ

200MΩ

100MΩ

<6.6KV

200MΩ

100MΩ

50MΩ

415V

100MΩ

50MΩ

20MΩ

Standard Size of MCB/MCCB/ELCB/RCCB/SFU/Fuse:

MCB Up to 63 Amp (80Amp and 100 Amp a    per Request)
MCCB Up to 1600 Amp (2000 Amp as per Request)
ACB Above 1000 Amp
MCB Rating 6A,10A,16A,20A,32A,40A,50A,63A
MCCB Rating 0.5A,1A,2A,4A,6A,10A,16A,20A,32A,40A,50A,63A,80A,100A (Domestic Max 6A)
RCCB/ELCB 6A,10A,16A,20A,32A,40A,50A,63A,80A,100A
Sen. of ELCB 30ma (Domestic),100ma (Industrial),300ma
DPIC (Double Pole Iron Clad) main switch 5A,15A,30 A for 250V
TPIC (Triple Pole Iron Clad) main switch 30A, 60A, 100A, 200 A For 500 V
DPMCB 5A, 10A, 16A, 32A and 63 A for 250V
TPMCCB 100A,200A, 300Aand 500 A For 660 V
TPN main switch 30A, 60A, 100A, 200A, 300 A For 500 V
TPNMCB 16A, 32A,63A For 500 V, beyond this TPNMCCB: 100A, 200A, 300A, 500 A For 660 V
TPN Fuse Unit (Rewirable) 16A,32A,63A,100A,200A
Change over switch (Off Load) 32A,63A,100A,200A,300A,400A,630A,800A
SFU (Switch Fuse Unit) 32A,63A,100A,125A,160A,200A,250A,315A,400A,630A
HRC Fuse TPN (Bakelite) 125A,160A,200A,250A,400A.630A
HRC Fuse DPN (Bakelite) 16A,32A,63A

MCB/MCCB/ELCB Termination Wire / Cable

Up to 20A MCB Max. 25 Sq.mm
20A to 63A MCB Max. 35 Sq.mm
MCCB Max. 25 Sq.mm
6A to 45A ELCB 16 Sq.mm
24A to 63A ELCB 35 Sq.mm
80A to 100A ELCB 50 Sq.mm

Electrical Thumb Rules-(Part-3)


Size of Capacitor for P.F Correction:

For Motor

Size of Capacitor = 1/3 Hp of Motor ( 0.12x KW of Motor)

For Transformer

< 315 KVA 5% of KVA Rating
315 KVA to 1000 KVA 6% of KVA Rating
>1000 KVA 8% of KVA Rating

 Earthing Resistance value:

Earthing Resistance Value

Power Station 0.5 Ω
Sub Station Major 1.0 Ω
Sub Station Minor 2.0 Ω
Distribution Transformer 5.0 Ω
Transmission Line 10 Ω
Single Isolate Earth Pit 5.0 Ω
Earthing Grid 0.5 Ω
As per NEC Earthing Resistance should be <5.0 Ω

 Voltage Limit (As per CPWD & Kerala Elect.Board):

Voltage Limit (As Per CPWD)

240V < 5 KW
415V <100 KVA
11KV ❤  MVA
22KV <6 MVA
33KV <12 MVA
66KV <20 MVA
110KV <40 MVA
220KV >40 MVA

Voltage Variation

> 33 KV (-) 12.5% to (+) 10%
< 33 KV (-) 9% to (+) 6%
Low Voltage (-) 6% to (+) 6%

 Insulation Class:

Insulation

Temperature

Class A

               105°C

Class E

120°C

Class B

130°C

Class F

155°C

Class H

180°C

Class N

200°C

 Standard Voltage Limit:

Voltage IEC (60038) IEC (6100:3.6) Indian Elect. Rule
ELV < 50 V    
LV 50 V to 1 KV <=1 KV < 250 V
MV   <= 35 KV 250 V to 650 V
HV > 1KV <= 230 KV 650 V to 33 KV
EHV   > 230 KV > 33 KV

 Standard Electrical Connection (As per GERC):

As per Type of Connection

Connection

Voltage

LT Connection

<=440V

HT connection

440V to 66KV

EHT connection

>= 66KV

As per Electrical Load Demand

Up 6W Load demand

1 Phase 230V Supply

6W to 100KVA(100KW)

3 Phase 440V Supply

100KVA to 2500KVA

11KV,22KV,33KV

Above 2500KVA

66KV

HT Connection Earthing

H.T Connection’s Earthing Strip

20mmX4mm Cu. Strip

CT & PT bodies

2Nos

PT Secondary

1Nos

CT Secondary

1Nos

I/C and O/G Cable+ Cubicle Body

2Nos

 Standard Meter Room Size (As per GERC):

Meter Box Height Upper level does not beyond 1.7 meter and Lower level should not below 1.2 meter from ground.
Facing of Meter Box Meter Box should be at front area of Building at Ground Floor.
Meter Room / Closed Shade 4 meter square Size

 Approximate Load as per Sq.ft Area (As per DHBVN):

Sq.ft Area Required Load (Connected)
< 900 Sq.ft 8 KW
901 Sq.ft to 1600 Sq.ft 16 KW
1601 Sq.ft to 2500 Sq.ft 20 KW
> 2500 Sq.ft 24 KW
For Flats :100 Sq.ft / 1 KW
For Flats USS /TC: 100 Sq.ft / 23 KVA

 Contracted Load in case of High-rise Building:

For Domestic Load 500 watt per 100 Sq. foot of the constructed area.
For Commercial 1500 watt per 100 Sq. foot of the constructed area
Other Common Load For lift, water lifting pump, streetlight if any, corridor/campus lighting and other common facilities, actual load shall be calculated
Staircase Light 11KW/Flat Ex: 200Flat=200×11=2.2KW

Sanctioned Load for Building

Up to 50 kW The L.T. existing mains shall be strengthened.
50 kW to 450 kW (500 kVA) 11 kV existing feeders shall be extended if spare capacity is available otherwise, new 11 kV feeders shall be constructed.
450 kW to 2550 kW (3000 kVA) 11 kV feeder shall be constructed from the nearest 33 kV or 110 kV substation
2550 kW to 8500 kW (10,000 kVA) 33kV feeder from 33 kV or 110 kV sub station
8500 kW (10,000 kVA) 110 kV feeder from nearest 110 kV or 220 kV sub-station

Electrical Thumb Rules (Part-2)


Useful Equations:

  • For Sinusoidal Current : Form Factor = RMS Value/Average Value=1.11
  • For Sinusoidal Current : Peak Factor = Max Value/RMS Value =1.414
  • Average Value of Sinusoidal Current(Iav)=0.637xIm (Im= Max.Value)
  • RMS Value of Sinusoidal Current(Irms)=0.707xIm (Im= Max.Value)
  • A.C Current=D.C Current/0.636.
  • Phase Difference between Phase= 360/ No of Phase (1 Phase=230/1=360°,2Phase=360/2=180°)
  • Short Circuit Level of Cable in KA (Isc)=(0.094xCable Dia in Sq.mm)/√ Short Circuit Time (Sec)
  • Max.Cross Section Area of Earthing Strip(mm2) =√(Fault Current x Fault Current x Operating Time of Disconnected Device ) / K  
  • K=Material Factor, K for Cu=159, K for Alu=105, K for steel=58 , K for GI=80
  • Most Economical Voltage at given Distance=5.5x√ ((km/1.6)+(kw/100))
  • Cable Voltage Drop(%)=(1.732xcurrentx(RcosǾ+jsinǾ)x1.732xLength (km)x100)/(Volt(L-L)x Cable Run.
  • Spacing of Conductor in Transmission Line (mm) = 500 + 18x (P-P Volt) + (2x (Span in Length)/50).
  • Protection radius of Lighting Arrestor = √hx (2D-h) + (2D+L). Where h= height of L.A, D-distance of equipment (20, 40, 60 Meter), L=Vxt (V=1m/ms, t=Discharge Time).
  • Size of Lighting Arrestor= 1.5x Phase to Earth Voltage or 1.5x (System Voltage/1.732).
  • Maximum Voltage of the System= 1.1xRated Voltage (Ex. 66KV=1.1×66=72.6KV)
  • Load Factor=Average Power/Peak Power
  • If Load Factor is 1 or 100% = This is best situation for System and Consumer both.
  • If Load Factor is Low (0 or 25%) =you are paying maximum amount of KWH consumption. Load Factor may be increased by switching or use of your Electrical Application.
  • Demand Factor= Maximum Demand / Total Connected Load (Demand Factor <1)
  • Demand factor should be applied for Group Load
  • Diversity Factor= Sum of Maximum Power Demand / Maximum Demand (Demand Factor >1)
  • Diversity factor should be consider for individual Load
  • Plant Factor(Plant Capacity)= Average Load / Capacity of Plant
  • Fusing Factor=Minimum Fusing Current / Current Rating (Fusing Factor>1).
  • Voltage Variation(1 to 1.5%)= ((Average Voltage-Min Voltage)x100)/Average Voltage
  • Ex: 462V, 463V, 455V, Voltage Variation= ((460-455) x100)/455=1.1%.
  • Current Variation(10%)= ((Average Current-Min Current)x100)/Average Current
  • Ex:30A,35A,30A, Current Variation=((35-31.7)x100)/31.7=10.4%
  • Fault Level at TC Secondary=TC (VA) x100 / Transformer Secondary (V) x Impedance (%)
  • Motor Full Load Current= Kw /1.732xKVxP.FxEfficiency
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