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Home / Technical Articles / Electrical Thumb Rules You MUST Follow (3)
Electrical Thumb Rules You MUST Follow
Electrical Thumb Rules You MUST Follow (photo by megstewart via Flickr)

Continued from second part: Electrical Thumb Rules You MUST Follow (Part 2)

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 KVA5% of KVA Rating
315 KVA to 1000 KVA6% of KVA Rating
>1000 KVA8% of KVA Rating

Earthing Resistance value

Earthing Resistance Value
Power Station0.5 Ω
Sub Station Major1.0 Ω
Sub Station Minor2.0 Ω
Distribution Transformer5.0 Ω
Transmission Line10 Ω
Single Isolate Earth Pit5.0 Ω
Earthing Grid0.5 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

Class A105°C
Class E120°C
Class B130°C
Class F155°C
Class H180°C
Class N200°C

Standard Voltage Limit

VoltageIEC (60038)IEC (6100:3.6)Indian Elect. Rule
ELV< 50 V
LV50 V to 1 KV<=1 KV< 250 V
MV<= 35 KV250 V to 650 V
HV> 1KV<= 230 KV650 V to 33 KV
EHV> 230 KV> 33 KV

Standard Electrical Connection (As per GERC)

As per Type of Connection
LT Connection<=440V
HT connection440V to 66KV
EHT connection>= 66KV
As per Electrical Load Demand
Up 6W Load demand1 Phase 230V Supply
6W to 100KVA(100KW)3 Phase 440V Supply
100KVA to 2500KVA11KV,22KV,33KV
Above 2500KVA66KV
HT Connection Earthing
H.T Connection’s Earthing Strip20mmX4mm Cu. Strip
CT & PT bodies2Nos
PT Secondary1Nos
CT Secondary1Nos
I/C and O/G Cable+ Cubicle Body2Nos

Standard Meter Room Size (As per GERC)

Meter Box HeightUpper level does not beyond 1.7 meter and Lower level should not below 1.2 meter from ground.
Facing of Meter BoxMeter Box should be at front area of Building at Ground Floor.
Meter Room / Closed Shade4 meter square Size

Electrical Load as per Sq.ft Area (As per DHBVN)

Sq.ft AreaRequired Load (Connected)
< 900 Sq.ft8 KW
901 Sq.ft to 1600 Sq.ft16 KW
1601 Sq.ft to 2500 Sq.ft20 KW
> 2500 Sq.ft24 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 Load500 watt per 100 Sq. foot of the constructed area.
For Commercial1500 watt per 100 Sq. foot of the constructed area
Other Common LoadFor lift, water lifting pump, streetlight if any, corridor/campus lighting and other common facilities, actual load shall be calculated
Staircase Light11KW/Flat Ex: 200Flat=200×11=2.2KW
Sanctioned Load for Building
Up to 50 kWThe 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

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Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 20 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.


  1. Gorakh Doshi
    Sep 14, 2023

    Hello Sir,

    Can you explain me that thumb rule for the selection of APFCR panel Capacity in LT distribution.

  2. ismarezaazmi
    Oct 18, 2019

    Hi Sir. Can you help me with the formula on calculating motor thermal overload

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