Continued from: Electrical Thumb Rules You MUST Follow (Part 4)
Eight rules to follow:
- Standard Size of Transformer (IEEE/ANSI 57.120)
- Standard Size of Motor (HP)
- Approximate RPM of Motor
- Motor Line Voltage
- Motor Starting Current
- Motor Starter
- Impedance of Transformer (As per IS 2026)
- 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 |
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 |
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 |
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) |
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 |
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 | |||
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% |
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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Jignesh Parmar
Electrical Middle management professional having more than 22 years rich and dynamic experience in Project Execution / Project Management / Designing / Maintenance diversifies from Electrical Power Transmission (400KV/220KV/66KV)- Distribution(11KV/220V) to Lifts-HVAC-Ventilation-Fire Fighting-Fire Alarm-Lifts-CCTV-Stack Parking Works (High Rise Buildings, Townships, Shopping Complex, Commercial Complex, School, Temple).Profile: Jignesh Parmar
May be there is mistakenly written KVA ana mva of transformers.
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
Dear Sir,
It seems that Power Transformer Sizes mentioned are wrong. These should be in MVA.
Please correct
Regards
Joe Auo
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 ???
thank you
very usefull .thank you
It’s very good and quick reference
thanks
grt info jignesh……..thank you