Electrical Equipment Selection Guidelines

 Electrical Equipment Selection Guidelines 

#Part_01

1. Power Cable Selection 

Voltage Drop: 

Maintain ≤ 3% for feeders. 

≤ 5% for branch circuits. 

Note: This is in line with BS 7671:2020 and IEC 60364 guidelines for voltage drop in final circuits (≤ 5%) and for feeders (≤ 3%). 

Current Carrying Capacity: 

Cable should support 1.25–1.5 times the full load current. 

Note: This is consistent with both BS 7671 and IEC 60364 which consider a safety margin for temporary overload conditions. 

Cable Size: 

For 3-phase: 1.5–2.5 mm² per kW. 

For 1-phase: 1.0–2.0 mm² per kW. 

Note: BS 7671 does not specify this exact ratio but recommends cable sizing based on load current, environmental conditions, and safety factors. The provided values are reasonable but should be verified against specific load and environmental factors.

 

Cable Insulation Level: 

600V/1000V for Low Tension (LT) systems. 

Note: These insulation levels align with BS 7671, IEC 60038, and common industry standards for LT and HT systems. 

2. Earthing Cable Selection 

Earth Fault Current: 

Design for 10–20 times the full load current. 

Note: This is in line with BS 7671 and IEC 60364, which consider high fault current ratings for safety.

 

Earthing Cable Size: 

Select 50–70% of the phase conductor size. 

Note: While this is a common guideline, BS 7671 recommends ensuring the earthing conductor is capable of carrying the fault current safely. The actual size may vary based on fault current, installation conditions, and other factors. 

Earth Resistance: 

≤ 1 Ω for LT systems. 

≤ 5 Ω for HT systems. 

Note: These values align with BS 7671 and IEC 60364 for maximum acceptable earth resistance.

3. Motor Selection 

Motor Capacity: 

Rated power should be 1.25–1.5 times the load kW. 

Note: BS 7671 and IEC 60364 do not specifically recommend this multiplier, but it’s a common practice to ensure motors can handle short-term overloads and ensure longevity. 

Efficiency: 

≥ 90% for IE2 motors. 

≥ 95% for IE3 motors. 

Note: These values are consistent with the IEC 60034 efficiency standards for motors (IE2 and IE3). 

Power Factor: 

Maintain ≥ 0.8 for induction motors. 

Note: This is consistent with BS 7671 and general motor efficiency requirements. 

Starting Current: 

Typically 6–8 times the full load current. 

Note: This is typical for motor starting characteristics and aligns with BS 7671 and IEC 60034. 

4. Generator Selection 

Generator Capacity: 

Rated power should be 1.25–1.5 times the load kVA/kW. 

Note: This guideline is consistent with the BS 7671 and IEC 60034 standards for selecting generators based on peak load considerations.

 

Efficiency: 

Maintain ≥ 90%. 

Note: This is aligned with typical BS 7671 efficiency expectations for power generation equipment. 

Power Factor: 

Maintain ≥ 0.8. 

Note: This is in line with standard practices for generators as per BS 7671