Rotating Machines Back Protection

The selected cable size should match with the circuit breaker size.

I agree.

Reason being that specially in situations the motor starter is installed near rotating machine equipment far from the circuit breaker protecting the circuit,the cable should be matching with circuit breaker size.

.Assuming the starter over load did not trip at overcurrent,either due to defects or wrong selection of overload or overload is dismantled for replacement awaiting delivery or loose connection on the input terminals of contactor then the role of circuit breaker is needed to operate at overload.This necessates that cable feeding the stater should withstand 1.45 times the circuit breaker rating .

That is certainly not correct in the NEC (U.S.), and I doubt that it is correct in any Code.

Joshi is correct for most applications. The NEC will allow the over current device to be up to 250 % of the FLA. The reason is because the overloads will protect the cable and motor. The breaker protects against fault current.

This will allow the breaker to with stand the starting amps with out tripping.
The conductor size needs to be 1.25 x FLA. The conductor size does not need to match the breaker size.

Please reply back to let us know if you understand.

You are telling that the over current device to be up to 250% of FLA to protect cable and motor and the cable to be selected for 125% of FLA.The cable might get overheated due to problems on the motor side and the circuit breaker does not trip still.

As you stated overcurrent and over load,I understood that you meant overcurrent in circuit breaker and over load in starter unit respectively.However,you are assuming the starter unit is in MCC and cable commencing from MCC to motor isolator next to motor.What will be the case if a local control panel including starter is located next to motor and cable commencing from circuit breaker in switchboard to the local control panel?will the overload be protecting the cable and motor? It will just protect the final connection from starter to motor in addition to motor.

Question 1...

Neither... According to the CEC (the NEC is similar) the motor conductors should be sized at not less than 125% of the full load current of the mortor.

Quyestion 2...

The answer for cirrcuit breakers has alot to do with what type of circuit breaker you plan on using.

If you have additional details... provide them.

NEC-US is well defined code for each and every item of electrical.

For your question, you did not define the type of motor whether it is Squirrel cage motor or Wound rotor.

Please refer the NEC Table 430.52. It gives CB rating for different kinds and conditions of the AC motors. The CB rating is given in percent of the motor full load current.

The branch feeder cable size shall be 125% times full load motor current as per Article 430.22.

It is squirrel cage motor.As we know,the selected circuit breaker rating differs if it is protecting a direct on line started motor or reduced voltage starting unit.We follow tables provided by manufacturers for this purpose,

!4 minutes before, you were following the BS and IEC, and at some other time the NEC. These sources all differ; you can't follow them all.

I just want to know what NEC tells about these issues.I follow the BS standards but sometimes I come across such issues in projects design based on NEC.

Post 2 summarized decently.

Mushtag

Do you use the NEC at your location?

We follow BS and IEC

Is BS7671 unclear on this topic, then?

Dear Mr. wareagle

I am working in Saudi Arabia. I have been using NEC in design since 1980.

Thank you

BS7671 435-01-01 recomends following BS EN60947-4-1. Which basicly boils down to "follow the manufacturers instructions".

It is advisable to use local code for motor ptotection by CB and Cable sizing or if motor is a special type use the manufacturer recommendation.

If you do not find the local code or supplier recommendation then go to other location code to minize the error.

Abdel Quote " You are telling that the over current device to be up to 250% of FLA to protect cable and motor and the cable to be selected for 125% of FLA.The cable might get overheated due to problems on the motor side and the circuit breaker does not trip."

As stated in the second post, the breaker does not protect the cable from overloads. The motor overloads take the motor off line and protect it and the conductors.

Abdel Quote "I just want to know what NEC tells about these issues.I follow the BS standards but sometimes I come across such issues in projects design based on NEC."

As stated in the second post.