LT CABLE SIZING FOR DG & TRANSFORMER
Proper electrical cable sizing is crucial for the reliable and safe operation of plants. Below are some examples to help understand the calculations:
1) Cable between Transformer Panel and Main LT Panel:
Rating Factor for variation in Ambient Temperature (at 50°C): 0.87
Total Derating Factor: 0.87
Actual Load: 67 kW
Full load Current: Total Load (kW) / (1.732 x Motor Efficiency x Power Factor x Voltage (kV))
= 67 / (1.732 x 0.8 x 0.85 x 0.415) = 137 Amps
Length of cable: 50 meters
Cable size selected: 3.5C x 120 mm² XLPE cable (Current carrying capacity is 223 amps in the ground)
Derated capacity of cable: 223 x 0.87 = 194 amps
Cable runs: 137 / 194 = 0.71 (Rounded to 1 run)
This cable can easily handle the load of 137 amps and is suitable.
Voltage Drop (%): 1.732 x Full Load Current (A) x Cable Length (m) x 100 x (Cos φ x Resistance (ohms/km) + Sin φ x Reactance (ohms/km)) /
(Rounded Cable Runs x 415 x 1000) = 1.732 x 137 x 50 x 100 (0.85 x 0.325 + 0.52 x 0.0712) / (1 x 415 x 1000) = 2.48% (Well within the permissible limit of 5%).
2) Cable between DG Panel and Main LT Panel:
Rating Factor for variation in Ambient Temperature (at 50°C): 0.87
Total Derating Factor: 0.87
Actual Load: 67 kW
Full load Current: Total Load (kW) / (1.732 x Motor Efficiency x Power Factor x Voltage (kV))
= 67 / (1.732 x 0.8 x 0.85 x 0.415) = 137 Amps
Length of cable: 50 meters
Cable size selected: 3.5C x 120 mm² XLPE cable (Current carrying capacity is 223 amps in the ground)
Derated capacity of cable: 223 x 0.87 = 194 amps
Cable runs: 137 / 194 = 0.71 (Rounded to 1 run)
This cable can easily handle the load of 137 amps and is suitable.
Voltage Drop (%): 1.732 x Full Load Current (A) x Cable Length (m) x 100 x (Cos φ x Resistance (ohms/km) + Sin φ x Reactance (ohms/km)) /
(Rounded Cable Runs x 415 x 1000) = 1.732 x 137 x 50 x 100 (0.85 x 0.325 + 0.52 x 0.0712) / (1 x 415 x 1000) = 2.48% (Well within the permissible limit of 5%).
