Electrical Thumb Rules  (Part 6)

1. Transformer earthing wire / Strip size

Size of T.C or DG	Body Earthing	Neutral Earthing
<315 kva="" strong="">	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	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="" strong="">	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 td="" 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