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Electrical Thumb Rules You MUST Follow, part 5
Electrical Thumb Rules You MUST Follow, part 5 (photo by engineeringourfreedom.blogspot.com)

Continued from: Electrical Thumb Rules You MUST Follow (Part 4)


Eight rules to follow:

  1. Standard Size of Transformer (IEEE/ANSI 57.120)
  2. Standard Size of Motor (HP)
  3. Approximate RPM of Motor
  4. Motor Line Voltage
  5. Motor Starting Current
  6. Motor Starter
  7. Impedance of Transformer (As per IS 2026)
  8. Standard Size of Transformer

1. Standard Size of Transformer (IEEE/ANSI 57.120)

Single Phase TransformerThree Phase Transformer
5KVA, 10KVA, 15KVA, 25KVA, 37.5KVA, 50KVA, 75KVA, 100KVA, 167KVA, 250KVA, 333KVA, 500KVA, 833KVA, 1.25KVA, 1.66KVA, 2.5KVA, 3.33KVA, 5.0KVA, 6.6KVA, 8.3KVA, 10.0KVA, 12.5KVA, 16.6KVA, 20.8KVA, 25.0KVA, 33.33KVA3KVA, 5KVA, 9KVA, 15KVA, 30KVA, 45KVA, 75KVA, 112.5KVA, 150KVA, 225KVA, 300KVA, 500KVA, 750KVA, 1MVA, 1.5MVA, 2MVA, 2.5MVA, 3.7MVA, 5MVA, 7.5MVA, 10MVA, 12MVA, 15MVA, 20MVA, 25MVA, 30MVA, 37.5MVA, 50MVA, 60MVA, 75MVA, 100MVA

Go back to Rules ↑


2. Standard Size of Motor (HP)

Electrical Motor (HP)
1, 1.5, 2, 3, 5, 7.5, 10, 15, 20, 30, 40, 50, 60, 75, 100, 125, 150, 200, 250, 300, 400, 450, 500, 600, 700, 800, 900, 1000, 1250, 1250, 1500, 1750, 2000, 2250, 3000, 3500 and 4000

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3. Approximate RPM of Motor

HPRPM
< 10 HP750 RPM
10 HP to 30 HP>600 RPM
30 HP to 125 HP500 RPM
125 HP to 300 HP375 RPM

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4. Motor Line Voltage

Motor (KW)Line Voltage
< 250 KW>440 V (LV)
150 KW to 3000KW2.5 KV to 4.1 KV (HV)
200 KW to 3000KW3.3 KV to 7.2 KV (HV)
1000 KW to 1500KW6.6 KV to 13.8 KV (HV)

Go back to Rules ↑


5. Motor Starting Current

SupplySize of MotorMax. Starting Current
1 Phase< 1 HP6 X Motor Full Load Current
1 Phase1 HP to 10 HP3 X Motor Full Load Current
3 Phase10 HP2 X Motor Full Load Current
3 Phase10 HP to 15 HP2 X Motor Full Load Current
3 Phase> 15 HP1.5 X Motor Full Load Current

Go back to Rules ↑


6. Motor Starter

StarterHP or KWStarting CurrentTorque
DOL<13 HP (11KW)7 X Full Load CurrentGood
Star-Delta13 HP to 48 HP3 X Full Load CurrentPoor
Auto TC> 48 HP (37 KW)4 X Full Load CurrentGood/ Average
VSD0.5 to 1.5 X Full Load CurrentExcellent
Motor > 2.2KW Should not connect direct to supply voltage if it is in Delta winding

Go back to Rules ↑


7. Impedance of Transformer (As per IS 2026)

>MVA% Impedance
< 1 MVA5%
1 MVA to 2.5 MVA6%
2.5 MVA to 5 MVA7%
5 MVA to 7 MVA8%
7 MVA to 12 MVA9%
12 MVA to 30 MVA10%
> 30 MVA12.5%

Go back to Rules ↑


8. Standard Size of Transformer

Standard Size of TransformerKVA
Power Transformer (Urban)3, 6, 8, 10, 16
Power Transformer (Rural)1, 1.6, 3.15, 5
Distribution Transformer25, 50, 63, 100, 250, 315, 400, 500, 630

Go back to Rules ↑

About Author //

author-pic

Jignesh Parmar

jiguparmar - Jignesh Parmar has completed M.Tech (Power System Control) ,B.E(Electrical). He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 13 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", "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.

7 Comments


  1. sangdeep walde
    Sep 24, 2015

    dear Mr.Jignesh,
    Good day,
    what did you think of applying negative sequence overcurrent relay(51Q) with zero time delay at incoming feeders in GIS substation of 230KV parallel line and after stepdown to 13.8KV applying in all outging feeders to all distribution transformers.
    confused in 51Q and 46/47 for negative sequence protection.
    Thanks and warm regards


  2. Joseph Auo
    Jul 08, 2015

    Dear Sir,

    It seems that Power Transformer Sizes mentioned are wrong. These should be in MVA.

    Please correct

    Regards

    Joe Auo


  3. Ramakanta
    Nov 24, 2014

    Power Transformer (Urban) 3, 6, 8, 10, 16
    Power Transformer (Rural) 1, 1.6, 3.15, 5
    Distribution Transformer 25, 50, 63, 100, 250, 315, 400, 500, 630

    sir, I am confusing about Standard Size of Transformer – how Power Transformer less capacity than Distribution Transformer ???


  4. kamyar elyasi
    Jan 27, 2014

    thank you


  5. SYAMLAL
    Nov 23, 2013

    very usefull .thank you


  6. Ahmed Imam
    Nov 21, 2013

    It’s very good and quick reference
    thanks


  7. deepakdj18
    Nov 21, 2013

    grt info jignesh……..thank you

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