Wire Size Calculation
Step-1: Calculate Current
Single phase: I = P / (V x PF) = 20000 / (230 x 0.8) ≈ 109A
Three phase: I = P / (√3 x V x PF) = 20000 / (1.732 x 400 x 0.8) ≈ 36A
Step-2: Apply Derating Factors
Ambient temperature correction.
Grouping factor (if multiple cables).
Installation method.
Step-3: Select Cable
For three phase 36A (more common for 20kW):
After applying derating (assume 0.8 factor): 36/0.8 = 45A required current capacity.
Cable size: 10mm² copper (rated ~57A in typical conditions).
For aluminium: 16mm²
For single-phase 109A:
Cable size: 35mm² copper or 50mm² aluminum.
MCB Rating
Select MCB rating ≥ full load current but ≤ cable capacity
Three phase: 40A (or) 50A MCB (Type C / D for motors)
Single phase: 125A MCB
Use Type B for resistive loads, Type C for general use, Type D for motors
ELCB/RCCB Rating
Current rating should be equal to or higher than MCB
Three phase: 40A (or) 63A, 4-pole RCCB
Single phase: 125A, 2-pole RCCB
Sensitivity: 30mA for personnel protection, 100-300mA for equipment protection.
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