This is a thumb rule-based approach; use it as a starting point before going into extensive calculations and guidelines for appropriate electrical design.
1. Load (kW)
Load (kW) = √3 × V × I × PF / 1000 To calculate 3-phase power 3Φ, 400V, 50A, PF=0.8 → Load = 27.71 kW
2 . Current (I)
Current (I) = Load (kW) × 1000 / (√3 × V × PF) To estimate current from known power 30 kW, 400V, PF=0.9 → I ≈ 48.1 A
3 . Horsepower
1 HP = 0.746 kW Convert motor horsepower to kilowatt 5 HP motor → 3.73 kW
4 . KVA
1 kVA = 0.8 kW (for PF=0.8) Estimate real power from apparent power 50 kVA DG → 40 kW (approx.)
5 . Demand Factor
Demand Factor ≈ 0.6 to 0.8 Total connected load × DF = estimated actual demand 100 kW connected → 70 kW (DF=0.7)
6 . Diversity Factor
Diversity Factor = Total Connected Load / Maximum Demand Used in panel and feeder sizing 200 kW connected / 120 kW demand = 1.67
7 . Voltage Drop (V)
Voltage Drop (V) = √3 × I × L × R / 1000 Voltage drop in 3Φ system; R = resistance (Ω/km) I=100A, L=50m, R=0.3 Ω/km → Vdrop = 2.6V
8 . Max Allowable VDrop
3–5% As per standard for final circuits 400V system: Max Vdrop = 20V
9 . Current Carrying Capacity
1 sq.mm copper cable ≈ 6-7 A Current carrying capacity (rule of thumb) 10 sq.mm cable → ~65 A
10. Cable Derating Factor
0.7–0.9 Adjust cable ampacity in hot areas or bunching 70 A × 0.8 = 56 A actual.
