Continued from: Electrical Thumb Rules You MUST Follow (Part 5)
12 electrical thumb rules to follow:
- Transformer earthing wire / Strip size
- Motor earthing wire / Strip size
- Panel earthing wire / Strip size
- Electrical equipment earthing
- Earthing wire (As per BS 7671)
- Area for transformer room: (As per NBC-2005)
- Span of transmission line (Central electricity authority)
- Max. lock rotor amp for 1-phase 230V motor (NEMA)
- Three phase motor code (NEMA)
- Service factor of motor
- Type of contactor
- Contactor coil
1. Transformer earthing wire / Strip size
| Size of T.C or DG | Body Earthing | Neutral Earthing |
| <315 KVA | 25×3 mm Cu / 40×6 mm GI Strip | 25×3 mm Cu Strip |
| 315 KVA to 500 KVA | 25×3 mm Cu / 40×6 mm GI Strip | 25×3 mm Cu Strip |
| 500 KVA to 750 KVA | 25×3 mm Cu / 40×6 mm GI Strip | 40×3 mm Cu Strip |
| 750 KVA to 1000 KVA | 25×3 mm Cu / 40×6 mm GI Strip | 50×3 mm Cu Strip |
2. Motor earthing wire / Strip size
| Size of Motor | Body Earthing |
| < 5.5 KW | 85 SWG GI Wire |
| 5.5 KW to 22 KW | 25×6 mm GI Strip |
| 22 KW to 55 KW | 40×6 mm GI Strip |
| >55 KW | 50×6 mm GI Strip |
3. Panel earthing wire / Strip size
| Type of Panel | Body Earthing |
| Lighting & Local Panel | 25×6 mm GI Strip |
| Control & Relay Panel | 25×6 mm GI Strip |
| D.G & Exciter Panel | 50×6 mm GI Strip |
| D.G & T/C Neutral | 50×6 mm Cu Strip |
4. Electrical equipment earthing
| Equipment | Body Earthing |
| LA (5KA,9KA) | 25×3 mm Cu Strip |
| HT Switchgear | 50×6 mm GI Strip |
| Structure | 50×6 mm GI Strip |
| Cable Tray | 50×6 mm GI Strip |
| Fence / Rail Gate | 50×6 mm GI Strip |
5. Earthing wire (As per BS 7671)
| Cross Section Area of Phase, Neutral Conductor(S) mm2 | Minimum Cross Section area of Earthing Conductor (mm2) |
| S<=16 | S (Not less than 2.5 mm2) |
| 16<S<=35 | 16 |
| S>35 | S/2 |
6. Area for transformer room: (As per NBC-2005)
| Transformer Size | Min. Transformer Room Area (M2) | Min. Total Sub Station Area( Incoming HV,LV Panel, T.C Roof) (M2) | Min. Space Width (Meter) |
| 1 x 160 | 14 | 90 | 9 |
| 2 x 160 | 28 | 118 | 13.5 |
| 1 x 250 | 15 | 91 | 9 |
| 2 x 250 | 30 | 121 | 13.5 |
| 1 x 400 | 16.5 | 93 | 9 |
| 2 x 400 | 33 | 125 | 13.5 |
| 3 x 400 | 49.5 | 167 | 18 |
| 2 x 500 | 36 | 130 | 14.5 |
| 3 x 500 | 54 | 172 | 19 |
| 2 x 630 | 36 | 132 | 14.5 |
| 3 x 630 | 54 | 176 | 19 |
| 2 x 800 | 39 | 135 | 14.5 |
| 3 x 800 | 58 | 181 | 14 |
| 2 x 1000 | 39 | 149 | 14.5 |
| 3 x 1000 | 58 | 197 | 19 |
– The capacitor bank should be automatically switched type for substation of 5MVA and higher.
– Transformer up to 25 KVA can be mounted direct on pole.
– Transformer from 25 KVA to 250KVA can be mounted either on “H” frame of plinth.
– Transformer above 250 KVA can be mounted plinth only.
– Transformer above 100 MVA shall be protected by drop out fuse or circuit breaker.
7. Span of transmission line (Central electricity authority)
| Voltage | Normal Span |
| 765 KV | 400 to 450 Meter |
| 400 KV | 400 Meter |
| 220 KV | 335,350,375 Meter |
| 132 KV | 315,325,335 Meter |
| 66 KV | 240,250,275 Meter |
8. Max. lock rotor amp for 1-phase 230V motor (NEMA)
| HP | Amp |
| 1 HP | 45 Amp |
| 1.5 HP | 50 Amp |
| 2 HP | 65 Amp |
| 3 HP | 90 Amp |
| 5 HP | 135 Amp |
| 7.5 HP | 200 Amp |
| 10 HP | 260 Amp |
9. Three phase motor code (NEMA)
| HP | Code |
| <1 HP | L |
| 1.5 to 2.0 HP | L,M |
| 3 HP | K |
| 5 HP | J |
| 7 to 10 HP | H |
| >15 HP | G |
10. Service factor of motor
| HP | Synchronous Speed (RPM) | ||||||
| 3600 RPM | 1800 RPM | 1200 RPM | 900 RPM | 720 RPM | 600 RPM | 514 RPM | |
| 1 HP | 1.25 | 1.15 | 1.15 | 1.15 | 1 | 1 | 1 |
| 1.5 to 1.25 HP | 1.15 | 1.15 | 1.15 | 1.15 | 1.15 | 1.15 | >1.15 |
| 150 HP | 1.15 | 1.15 | 1.15 | 1.15 | 1.15 | 1.15 | 1 |
| 200 HP | 1.15 | 1.15 | 1.15 | 1.15 | 1.15 | 1 | 1 |
| > 200 HP | 1 | 1.15 | 1 | 1 | 1 | 1 | 1 |
11. Type of contactor
| Type | Application |
| AC1 | Non Inductive Load or Slightly Inductive Load |
| AC2 | Slip Ring Motor, Starting, Switching OFF |
| AC3 | Squirrel Cage Motor |
| AC4,AC5,AC5a, AC5b,AC6a | Rapid Start & Rapid Stop |
| AC 5a | Auxiliary Control circuit |
| AC 5b | Electrical discharge Lamp |
| AC 6a | Electrical Incandescent Lamp |
| AC 6b | Transformer Switching |
| AC 7a | Switching of Capacitor Bank |
| AC 7b | Slightly Inductive Load in Household |
| AC 5a | Motor Load in Household |
| AC 8a | Hermetic refrigerant compressor motor with Manual Reset O/L Relay |
| AC 8b | Hermetic refrigerant compressor motor with Automatic Reset O/L Relay |
| AC 12 | Control of Resistive Load & Solid State Load |
| AC 13 | Control of Resistive Load & Solid State Load with Transformer Isolation |
| AC 14 | Control of small Electro Magnetic Load (<72 VA) |
| AC 15 | Control of Electro Magnetic Load (>72 VA) |
12. Contactor coil
| Coil Voltage | Suffix |
| 24 Volt | T |
| 48 Volt | W |
| 110 to 127 Volt | A |
| 220 to 240 Volt | B |
| 277 Volt | H |
| 380 to 415 Volt | L |
Related electrical guides & articles
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
Do we require separate GI wire for cable gland to earth strip?
Motor earthing wire / Strip size for < 5.5 KW should be 8 SWG GI Wire.
Hi sir iam working for railways, What are steps to follow to reduce energy consumption, we converted 100%LED energy efficient motors, pumps, rather than this what more can we do for save energy, we are using solar also in our station
Hi Jignesh,
We have to do motors as well electrical panels body earthing as per standard electricity rules.Can you please guide which is the applicable rule number so that we can refer to that.
Thanks
Rajesh
Dear mr.Rajesh,
As every electrical engineer aware the size of the earth strip/wire should able to carry current during short circuit. based on this we need to select he size of earthing conductor.
You are working very well and your website article is very helpful for electrical technology. i love this type of site and save this link for further visiting.
you can see by site by link
http://www.electricalonline4u.com/2015/10/wiring-of-distribution-board.html
Hi,
Every body i want to know about energy conservation in detail,so i am working as a power and electric chief engineer in Pakistan with the largest group incorporate in Pakistan House of Habib(HOH) Pakistan and also i am working on energy saving project.
Pl. arrange to give full technical detail about protection relay setting and co ordination
Superb..I was expect the same from you people… Also, Can i write these type articles for Electrical engineering portal.com…as i have wrote many articles like this for Electrical Technology
As you can see as a ref
Three Phase Electrical Wiring Installation in Home
http://www.electricaltechnology.org/2013/12/three-phase-electrical-wiring.html