Electrical Thumbs Rules (Part-10)

Economical Voltage for Power Transmission:

  •  Economic generation voltage is generally limited to following values (CBIP Manual).

Economic generation voltage (CBIP Manual)

Total Load

Economical Voltage

Up to 750 KVA

415 V

750 KVA to 2500 KVA

3.3 KV

2500 KVA to 5000 KVA

6.6 KV

Above 5000 KVA

11 KV or Higher

  •  Generally terminal voltage of large generators is 11 kV in India. Step up voltage depends upon Length of transmission line for interconnection with the power system and Power to be transmitted.
  • High voltage increases cost of insulation and support structures for increased clearance for air insulation but decreases size and hence Cost of conductors and line losses.
  • Many empirical relations have been evolved to approximately determine economic voltages for power evacuation. An important component in transmission lines is labor costs which are country specific.
  • An empirical relation is given below.
  • Voltage in kV (line to line) = 5.5x√0.62L + kVA/150

  • where kVA is total power to be transmitted;
  • L is length of transmission line in km.
  • American practice for economic line to line voltage kV (based on empirical formulation) is
  • Voltage in kV line to line = 5.5x√0.62L + 3P/100

  • For the purpose of standardization in India transmission lines may be classified for operating at 66 kV and above. 33 kV is sub transmission, 11 kV and below may be classified as distribution.
  • Higher voltage system is used for transmitting higher amounts of power and longer lengths and its protection is important for power system security and requires complex relay systems.

Required Power Transfer (MW)

Distance (KM)

Economical Voltage Level (KM)













 Factor affected on Voltage Level of system:

  • Power carrying capability of transmission lines increases roughly as the square of the voltage. Accordingly disconnection of higher voltage class equipment from bus bars get increasingly less desirable with increase in voltage levels.
  • High structures are not desirable in earthquake prone areas. Therefore in order to obtain lower structures and facilitate maintenance it is important to design such sub-stations preferably with not more than two levels of bus bars.

 Size of Cable according to Short circuit (for 11kV,3.3kV only)

  • Short circuit verification is performed by using following formula:
  • Cross Section area of Cable (mm2)S = I x√t / K

  • Where:
  • t = fault duration (S)
  • I = effective short circuit current (kA)
  • K = 0.094 for aluminum conductor insulated with XLPE
  • Example: Fault duration(t)= 0.25sec,Fault Current (I) = 26.24 kA
  • Cross Section area of Cable = 26.24 x √ (0.25) / 0.094= 139.6 sq. mm
  • The selected cross sectional area is 185 sq. mm.

Ground Clearance:

  • Ground Clearance in Meter = 5.812 + 0.305 X K

  • Where K= (Volt-33) / 33
Voltage Level Ground Clearance
<=33KV 5.2  Meter
66KV 5.49 Meter
132KV 6.10 Meter
220KV 7.0   Meter
400KV 8.84  Meter

 Voltage Rise in Transformers due to Capacitor Bank:

  • The voltage drop and rise on the power line and drop in the transformers. Every transformer will also experience a voltage rise from generating source to the capacitors. This rise is independent of load or power factor and may be determined as follows:
  • % Voltage Rise in Transformer=(Kvar / Kva)x Z

  • Kvar =Applied Kvar
  • Kva = Kva of the transformer
  • z = Transformer Reactance in %
  • Example: 300 Kvar bank given to 1200 KVA transformer with 5.75% reactance.
  • % Voltage Rise in Transformer=(300/1200)x 5.75 =1.43%

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.

11 Responses to Electrical Thumbs Rules (Part-10)

  1. PRASHANT SONI says:


    Your work gives so much precious knowledge to the whole electrical engineers who are freshers or with lack of practical knowledge..

  2. Ritaban Banerjee says:

    Thanks for such a compact valuable information.

  3. Sarvan Kumar says:

    thank you sir

  4. walid salman says:

    I tried to buy the electrical software but in the countries of paypall there is no lebanon country the system refuse the credit card.
    What should I do?

  5. dilshad ahmad says:

    Thanks! to send me notes & articles.but my feiled is distrybution transformer if related my job notes pl send me

  6. joe c. says:

    Please I have a 300kvar capacitor bank for a 800kva transformer.
    What would be the implication of the voltage raise to the Factory electrical systems
    and operations? Help please

  7. raja says:

    classics with best selection

  8. Eng.ahmed Hashem says:

    Dear Sir,

    Thank you very much for these valuable technical information.

    Regards, Ahmad H. Al-Sheikh Power System Engineer. Saudi Arabia

  9. ganeshkumar says:

    if we by mistaken extra capacitor in circuits .circuit voltage and transformer voltage level rise at that time problem will occur .right sir?

  10. bhagvat vala says:

    Plz send me mail related variable frequency drive

  11. eswaran says:

    thank for your guidance

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