Motor Circuit Breaker Rating

It should be sized(selected)according to full load current,starting current and tripping current as well as starting time.

i am not very familiar with US standards, but still, from the application angle....

No matter which starting method is used, the MPCB will be rated for the line current at steady state. That is Motor FLA. They are designed to handle the DOL starting inrush without nuisance tripping generally, so will be impervious to the inrusah currents of all other starting methids, which will be lower than DOL.

Is that mean in the normal sizing method, it usually refer to DOL starter. But the MCCB size is same for all using star delta, auto transformer or others ? I known kind of sizing method is overload protection MCCB = 115 or 125%FLA, short circuit ground fault protection MCCB = 150 to 300% FLC. Is this formula one for all type starter ? Please share me your experience.

As i said before, the protection kicks in when steady state is reached, ie, when the motor is drawing FLA.The starting phase is transitory, and the MPCB just ignores it usually. SC protection and stall protection are anyway available at that time.

Perhaps i am not able to see what your problem is.

I think I have got the answer I am looking for. Thanks

When sizing MCB for small motors approx 2xIn is OK for DOL but 1.5xIn is enough for star-delta or auto transformer starters. The correct approach is to select correct type of MCB(C or D)for motor circuits or refer MCB manufacturers catalogue like Hager.

The OP is askig about US recommendations. So a European recommendation is not valid. He already has NEC recommendations. That one only specifies the kind of motor, not the starting method. Hence his quandary i think.

They really don't have MCBs in USA. He has specifically asked about MPCB.

In US consumer units made by Federal pacific there are single pole MCBs for lighting & sockets,why they do not make 3P MCBs for lighting incomers as in UK?.

We have gone far afield from what the OP asked.That was only about this table :

Yes. This is it. One more I need to clarify. Since line current for motor using star delta is 33% FLC, is that means during sizing of a MCCB, we use 33% FLC x the percentage as in the table ?

No.

The starting current may be 33%, but the steady state current will be FLA. If you use a 33% rated MCCB, it will nuisance trip.

If the starting method had a bearing on the MCCB selection, NEC would have said so, wouldn't you think? Please select as per NEC, don't complicate the issue.

After all, overload protection is available from the MPCB (a normal MCCB would be a coarse protection, MPCB would be better, an Overload Relay would be the best), and SC protection is adequate if you select based on the system fault level.

This mean overload protection shall use MPCB. However, short circuit protection shall use MCCB ?

Of course not. "MPCB" means "Motor Protection Circuit Breaker". It is basically a circuit breaker with a fairly good SC capacity, its overload protection area being tailor-made for motor protection. Please study the catalogues of MPCB to understand this product better. Here is a good one - Moeller MPCB

It is a German product by Moeller, but the company now belongs to Eaton USA, so they are bound to have a product for the NEC market

And to respond to your next question, if you use a good MPCB, you may not need the OLR. If you use an MCCB, or a set of fuses, you will need an OLR.

Pardon my saying so, but seems to me that you want to learn all about Circuit Breakers right here on this thread. Unlikely to happen. i would urge you to read up such catalogues, technical papers, etc...plenty on the web.

Thanks. This help me a lot.

If an overload relay shall be used for overload protection, it shall be a combination with MPCB or MCCB ?

...and do bear in mind that the circuit breaker is there to protect the wiring. A separate overload device is needed to protect the motor and the topic is referred to in British Standard 7671 as "co-ordination".