Electrical Thumb Rules (Part 5)
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="" td=""> | 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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