Electrical Thumb Rules Part-02

Eight rules to follow:

1. Substation Capacity and Short Circuit Current Capacity (As per GERC)
2. Substation Capacity and Short Circuit Current Capacity (As per Central Electricity Authority)
3. Minimum Ground Clearance and Fault Clearing Time
4. Busbar Ampere Rating
5. Busbar Spacing
6. Sound Level of Diesel Generator (ANSI 89.2 and NEMA 51.20)
7. IR Value of Transformer
8. Standard Size of MCB, MCCB, ELCB, RCCB, SFU and Fuse

1. Substation Capacity and 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)

2. Substation Capacity and 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

3. 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	–

4. Busbar Ampere Rating

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

5. Busbar Spacing

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

6. Sound Level of Diesel Generator (ANSI 89.2 and 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

7. 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

8. Standard Size of MCB, MCCB, ELCB, RCCB, SFU and Fuse

MCB, MCCB, ELCB, RCCB, SFU, Fuse – Standard Ratings
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

Size of Capacitor for Power Factor 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 & KEB)

Voltage Limit (As Per CPWD)
240V	< 5 KW
415V	<100 KVA
11KV	<3  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

Electrical 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

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

Contracted Load in case of High-rise Building Useful 

Electrical 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 (I_av) = 0.637 x Im (Im = Max.Value)
• RMS Value of Sinusoidal Current (I_rms) = 0.707 x Im (Im = Max.Value)
• A.C Current = D.C Current/0.636.
• Phase Difference between Phase = 360/ No of Phase (1 Phase=230/1=360°, 2 Phase=360/2=180°)
• Short Circuit Level of Cable in KA (I_sc) =
  (0.094 x Cable 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 Al = 105, K for steel = 58 , K for GI = 80
• Most Economical Voltage at given Distance = 5.5 x √ ((km/1.6) + (kw/100))
• Cable Voltage Drop (%) =
  (1.732 x current x (RcosǾ+jsinǾ) x 1.732 x Length (km) x 100) / (Volt(L-L) x Cable Run.
• Spacing of Conductor in Transmission Line (mm) = 500 + 18 x (P – P Volt) + (2 x (Span in Length)/50).
• Protection radius of Lighnting Arrestor = √h x (2D-h) + (2D+L).
  Where h= height of L.A, D-distance of equipment (20, 40, 60 Meter), L=V x t (V=1m/ms, t=Discharge Time).
• Size of Lightning Arrestor = 1.5x Phase to Earth Voltage or 1.5 x (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) x 100)/Average Voltage
  Ex: 462V, 463V, 455V, Voltage Variation= ((460 – 455)  x 100)/455 = 1.1%.
• Current Variation (10%) = ((Average Current – Min Current) x 100)/Average Current
  Ex: 30A,35A,30A, Current Variation = ((35-31.7) x 100)/31.7 = 10.4%
• Fault Level at TC Secondary
  = TC (VA) x 100 / Transformer Secondary (V) x Impedance (%)
• Motor Full Load Current = Kw /1.732 x KV x P.F x Efficiency