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Home / Technical Articles / Electrical Thumb Rules You MUST Follow (5)
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 Transformer Three 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, 33.33KVA 3KVA, 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

HP RPM
< 10 HP 750 RPM
10 HP to 30 HP >600 RPM
30 HP to 125 HP 500 RPM
125 HP to 300 HP 375 RPM

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4. Motor Line Voltage

Motor (KW) Line Voltage
< 250 KW >440 V (LV)
150 KW to 3000KW 2.5 KV to 4.1 KV (HV)
200 KW to 3000KW 3.3 KV to 7.2 KV (HV)
1000 KW to 1500KW 6.6 KV to 13.8 KV (HV)

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5. Motor Starting Current

Supply Size of Motor Max. Starting Current
1 Phase < 1 HP 6 X Motor Full Load Current
1 Phase 1 HP to 10 HP 3 X Motor Full Load Current
3 Phase 10 HP 2 X Motor Full Load Current
3 Phase 10 HP to 15 HP 2 X Motor Full Load Current
3 Phase > 15 HP 1.5 X Motor Full Load Current

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6. Motor Starter

Starter HP or KW Starting Current Torque
DOL <13 HP (11KW) 7 X Full Load Current Good
Star-Delta 13 HP to 48 HP 3 X Full Load Current Poor
Auto TC > 48 HP (37 KW) 4 X Full Load Current Good/ Average
VSD 0.5 to 1.5 X Full Load Current Excellent
Motor > 2.2KW Should not connect direct to supply voltage if it is in Delta winding

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7. Impedance of Transformer (As per IS 2026)

>MVA % Impedance
< 1 MVA 5%
1 MVA to 2.5 MVA 6%
2.5 MVA to 5 MVA 7%
5 MVA to 7 MVA 8%
7 MVA to 12 MVA 9%
12 MVA to 30 MVA 10%
> 30 MVA 12.5%

Go back to Rules ↑


8. Standard Size of Transformer

Standard Size of Transformer KVA
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

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author-pic

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 13 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.

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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