Here are some electrical thumb rules for selecting various electrical equipment:
Power Cable Selection:-
1. Voltage drop: 3% for feeders, 5% for branch circuits
2. Current carrying capacity: 1.5-2 times full load current
3. Cable size: 1.5-2.5 mm²/kW for 3-phase, 1-2 mm²/kW for 1-phase
4. Cable insulation: 600V/1000V for LT, 6.6kV/11kV for HT
Earthing Cable Selection:
1. Earth fault current: 10-20 times full load current
2. Earthing cable size: 1/2 to 2/3 of phase conductor size
3. Earth resistance: ≤ 1 ohm for LT, ≤ 5 ohms for HT
Motor Selection:
1. Motor size: 1.5-2 times load HP/kW
2. Motor efficiency: ≥ 90% for IE2, ≥ 95% for IE3
3. Power factor: ≥ 0.8 for induction motors4. Starting current: 6-8 times full load current
Generator Selection:–
1. Generator size: 1.5-2 times load kVA/kW
2. Generator efficiency: ≥ 90%
3. Power factor: ≥ 0.84. Frequency: 50/60 Hz
Transformer Selection:-
1. Transformer size: 1.5-2 times load kVA
2. Efficiency: ≥ 95% for distribution, ≥ 98% for power transformers
3. Voltage regulation: ≤ 4%
4. Insulation level: 600V/1000V for LT, 6.6kV/11kV for HT
UPS Selection:-
1. UPS size: 1.5-2 times load kVA/kW
2. Efficiency: ≥ 95%3. Power factor: ≥ 0.84. Backup time: 15-60 minutes
Inverter Selection:-
1. Inverter size: 1.5-2 times load kW
2. Efficiency: ≥ 95%3. Power factor: ≥ 0.84. Frequency: 50/60 Hz
Other Equipment Selection:-
1. Circuit Breaker (CB): 1.5-2 times full load current
2. Contactors: 1.5-2 times full load current
3. Relays: 1-2 times full load current
4. Fuses: 1.5-2 times full load current
General Thumb Rules:-
1. Derating: 20-30% for temperature, 10-20% for altitude
2. Overloading: 10-20% for motors, 5-10% for transformers
3. Efficiency: prioritize high efficiency equipment
4. Redundancy: consider N+1 redundancy for critical systems
Note: These thumb rules serve as guidelines and may vary depending on specific application, industry standards, and local regulations.
