1. Calculation of rated current of single-phase load
P = UI COSφ (power = voltage × current × power factor)
In the formula: U-single-phase 220V;
COSφ--load power factor, generally 0.8;
For example: the total capacity of single-phase load electrical equipment is 20KW, the line rated current is calculated according to the formula: I =20000/(220x0.8) = 113.6A
According to the rated current of 113.6A, a circuit breaker with a rated current of 140A should be selected. In case of poor heat dissipation conditions in the distribution cabinet or distribution box, the capacity should be reduced to 0.7-0.8 for use, and 160A circuit breakers should be selected. In this way, classification selection is made according to different rated currents.
2. Calculation of rated current of three-phase load
P = UICOSφ (power = voltage × current × power factor)
Where: U—Three-phase 380V
COSφ—load power factor, generally 0.8
For example: a three-phase motor with a rated power of 45KW, the calculated current I = 45000/( X380X0.8) = 85.5A
A circuit breaker with rated current In=100A can be selected. For places with poor heat dissipation conditions, a 120A circuit breaker may be selected. In this way, classification selection is made according to different rated currents.