**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)**

**FLC = 19.1Amps**

__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**

I hope you have learned from this article how to

**calculate full load current**(FLC) and**MCB**per load.
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