MCB Current Calculation

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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

KW   = 20 x 0.746 = 14.92 (15KW)

 MCB Current Calculation

FLC = 19.1Amps

 MCB Current Calculation

Fuses Current Value Calculation as per NEC

Single Phase (1 Ï†)    = 300 % of FLC
Three Phase (3 Ï†)    = 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 Ï†)    = 300 % of FLC
Three Phase (3 Ï†)    = 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