Amps Calculation

The motor should have a nameplate. The FLA "Full Load Amps" should be present on this label. As far as minimum amps, consult the manufacturer or take a current reading with no load on the motor.

The actual amperage drawn by the motor is dependent on load connect to it.

Read section 430 of the NEC. Why would you want the minimum load? The wiring and breaker are based on the maximum from table 430.250.

okey thank you,I can forget the minimum current ,but what about the max.current?

Here I am working with PLC pannel Building team.From customer we are getting only the power (watts)and voltage only of the motor .How can I design my MPCB's and contactors correspondingly?

As a "rule of thumb" as you asked, I would take the full load amps on the motor nameplate and multiply by 1.25 (125%). That is the number I would use for the MPCB in the motor control center.

If the FLA on the nameplate is 7.5, multiply by 1.25. The result is 9.375. I would use a minimum of 10 amp breaker for that application.

Can you quote the relevant NEC which justifies the selection of 125% FLA for MCCB which was quoted as up to 250% FLA in this forum

MPCBs are typically the IEC style adjustable thermal trip circuit breakers used in conjunction with a contactor to make up a Motor Starter package. Being that the thermal trips ARE the overload protection device, they must be set to the motor Full Load Amps. if you are designing a control panel from afar and do not have the motor nameplate FLA available to you, you can estimate it as follows (assuming a 3 phase motor):

approx. Motor FLA = W/(V x pf x Eff x 1.732)

pf = Power Factor, assume .7 if not otherwise known

Eff. = Efficiency, assume .8 if not otherwise known

So for your 3kW motor, Approx. FLA = 3000 / (415 x .7 x .8 x 1.732)

FLA = 3000/402.52 = 7.45A

Cross checked against a motor mfr. chart, 6.5A; probably their pf and Eff were better. So if you use this estimate, you should be OK for initial component sizing, but select an MPCB that has an adjustment range where this value fits in the middle.

Maximum current will be FLA in the nameplate,any increase will decrease the life of motor while minimum current will be the noload current,approximately 25 to 40% of FLA. But donot forget the locked rotor current or starting current(6In) which should not last for long.