MCB Current Calculation, OLR, Fuses and Cable size Calculations for 20 Hp Three phase (TP) Induction Motor. You can use this formula for any load, to choose MCB (Miniature circuit breaker), Fuses and Over Load Relay (OLR) calculation.
Type of Load : Three Phase Induction Motor 20 Hp, Line to Line voltage - 400V, Power Factor 0.88 PF as shown in Motor name plate
Full Load Current (FLC) Calculation
KW = 20 x 0.746 = 14.92 (15KW)
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| MCB Current Calculation |
FLC = 19.1Amps
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| MCB Current Calculation |
Fuses Current Value Calculation as per NEC
Single Phase (1 φ) = 300 % of FLC
Three Phase (3 φ) = 300 % of FLC
Synchronous motor = 300 % of FLC
Wound motor = 150% of FLC
Direct Current = 150% of FLC
300 % of 1 = 3
Time delay fuses = 3 x 19.1 = 57.3 Amps (Choose above the available rate.)
Non Time delay fuses = 1.75% x 19.1 = 33.4 Amps (Choose above the available rate.)
Circuit Breaker (MCB) calculation as per NEC
Single Phase (1 φ) = 300 % of FLC
Three Phase (3 φ) = 300 % of FLC
Synchronous motor = 300 % of FLC
Wound motor = 150% of FLC
Direct Current = 150% of FLC
Circuit Breaker (MCB) = 3 x 19.1 = 57.3 Amps (Available 63 Amps)
Circuit Breaker (MCB) Instantaneous Trip Current Capacity as per NEC
Single Phase (1 ph) = 800% of FLC
Three Phase (3 ph) = 800% of FLC
Synchronous motor = 800% of FLC
Wound motor = 800% of FLC
Direct Current = 200% of FLC
Synchronous motor = 800% of FLC 8 x 19.1 =152.8 Amps
Circuit Breaker (MCB) Invers Trip Current Capacity as per NEC
Single Phase (Iph) = 250% of FLC
Three Phase (3ph) = 250% of FLC
Synchronous motor = 250% of FLC
Wound motor = 150% of FLC
Direct Current = 150% of FLC
Synchronous motor = 250% of FLC 2.5 x 19.1 = 47.75 Amps
Circuit Breaker (MCB) Instantaneous Trip Current = 800% x 19.1 = 152.8 Amps
Circuit Breaker (MCB) Invers Trip Current = 250% x 19.1 = 47.75 Amps
Starting Current : 6 or 7 times x FLC (IL) = 7 x 19.1 = 133.7 Amp
Contactor Value as per NEC:150% of FLC = 1.5 x 19.1 = 28.65 Amps (Available 40 Amps)
OLR Rating Calculation as per NEC
OLR Min = 70% of FLC (IL) = 0.7 x 19.1 = 13.37 A (14 A)
OLR Max = 120% of FLC (IL) = 1.2 x 19.1 =22.92 A (23 A)
Should be use above the value of 23A OLR (15A to 25A)
Conclusion
I hope you have learned from this article how to calculate full load current (FLC) and MCB per load.
