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:



Class A


Class E


Class B


Class F


Class H


Class N


 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



LT Connection


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


Above 2500KVA


HT Connection Earthing

H.T Connection’s Earthing Strip

20mmX4mm Cu. Strip

CT & PT bodies


PT Secondary


CT Secondary


I/C and O/G Cable+ Cubicle Body


 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

About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical). He is member of Institution of Engineers (MIE) and CEng,India. Membership No:M-1473586.He has more than 16 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in “Electrical Mirror”, “Electrical India”, “Lighting India”,”Smart Energy”, “Industrial Electrix”(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

19 Responses to Electrical Thumb Rules-(Part-3)

  1. Kuldip Singh says:

    excellent ! I have kept electrical thumb rules part 1 to 3 on my desk top ready to refer at any time. Mr. Jignesh is doing a good service to electrical engg. community. Appreciating and all the best.


    kuldip Singh

  2. HI. Can you help me with this harmonic distortion problem? We have THD>30% on 3a, 5a, 15a, 17a, 20a and 22a harmonic . Is it necessary to improve all these levels? Can I only improve the 3a, 5a and 15a harmonics to solve this?


  3. Deepak Agrawal says:

    very much interesting data

  4. murali sir says:

    I feel new energy com to my eee cpt and gpt polytechnic thank you yery much

  5. aniket says:

    dear sir plz provide me 11KV/415V pole mounted substation and Overhead Line technical Specification including pole erection also

  6. namita says:

    Hi jignesh,
    pl upload some stuff on ups and battery sizing calculation.

  7. Akshay Bhokare says:

    Very much informative and useful compilation for the practising Engg, who are not from electrical engg side but have to carry out such work particularly in Govt Deppt. Electrical Notes wonderfull.

  8. PREMKUMAR S says:

    Dear sir,
    i need the formula of current carrying capacity of LT UG Armoured Aluminium and copper cables.
    i view the ur Thump rule calculations but dosn’t mach.
    so kindly reply.

  9. Renjith says:

    Hi Jignesh,
    Does the contracted load for High Rise Building includes AC & Heater also OR it includes only Lighting & Small power.


  10. Priyank says:


    Can you provide us a Calculator for Annual CO2 Savings ( kg C02) by power factor correction?

    Thanks in advance.

  11. Iskandar says:

    As long as I need

  12. janak says:

    how many clearance of 220kv line

  13. Maulesh Pandit says:

    What is the thumb rule to require kw per sqfeet

  14. patel vinod says:

    Pl send calculation formula for ht cable as per bellow details.
    Cable-185*3core ALU
    2.5 MVA transformer
    Length-700 mtrs
    Under ground

  15. Uday kumar says:

    Good and Beautifull stuffs…….Thus helps a lot….YOU ARE DOING A GOOD JOB SIR…

  16. Anand says:

    Sir we have 3200KVA Load on dairy plant, Plz suggest transformer selection either one or two transformers ? which capacity ?

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