Motor Load Protection

No harm in using much larger cables and breaker, just a waste of money and space. Please note that the breaker MUST be set to the rated motor current, so it trips when that current is exceeded....that is, even if you use a 250A breaker, you must set the overload trip to 100A. However, please also note that the breaker's thermal trip curve is much coarser than that of an overload relay, which is part of a motor starter, which is used to swtch on/off the motor. The breaker should not be used to do this....

You have some way to go in motor protection and control i am afraid...

Good luck.

thanks very much for the comment, it enlightens me a lot --> "MUST be set to the rated motor current". However, for a general application, does overdesign poses no safety issues especially for breakers that do not have setting options?

Motors must have starters to switch them on and off, they can have breakers or fuses for short-circuit protection only, not for overload protection. The IEC starndards recommend that the breaker/fuse and starter be coordinated so that each protection acts in its zone properly. So, if you have too large a breaker, which cannot be set at the motor's rated current, you may end up with an unprotected zone of medium-short-circuit currents which can destroy the starter and/or the motor. Not a good practice.

Agreed! KVS! But, doesn't IEC recognise & permit damage to starter components under Type-1 Co-ordination?

Even though it may not be a good practice, it still is acknowledged in IEC as well as in IS. Anybody practices so is not violating the Standards.

Quite right.i remember that US practice required replacement of the starter after a short circuit, and that is how Type 1 came into being in IEC. However, i do believe that Type 2 is becoming more preferred nowadays even in USA. i don't know for sure.

I'm sorry, I forgot to tell you at first that I'm a novice. So now, does this tells me that overdesign/overrated breakers indeed has safety issues? How about for building wiring, does overrated breakers may also have issues? Sorry, this may sound silly but thanks though.

For a motor rated 100 Amp there will be starting inrush current hence selection of MCCB of less than 225 Amps is a design failure please be aware of that.

As for the motor circuit a MCCB just gives short circuit protection, thermal overload relay is there to protect the motor and the magnetic contactors.

Nobody has indulged in over design, in engineering practice 'factor of safety' is a necessary evil, for general load cable selection you have appropriately quoted: "calculated load x factor of safety = 120 Amps".

Codes do however bind that the rating of cable in general applications should not be less than the rating of MCCB; this is to protect the cable from overheating.

I'm pretty sure that you don't have to go back through and upsize the wiring to match .say. a 225A CB.

(Also, in the NEC, wiring ampacity ≥ 125% FLA of motor.)

Remember too that "overdesign" usually equals "higher cost." Your employer will not be happy if you regularly overdesign and break his budget. He will reduce his budget by eliminating YOU.

Those code references are to provide protection for the conductors and allow for startup. Remember most motors have a service factor of 1.15. It is usually best to provide separate motor overload protection.

ianeil18,

We all started as novices. There are a number of good handbooks that explain and walk you through the code rules and exceptions. When I was beginning, I read the code section on motor control about 5 times before it began to make sense. With motors you are trying to provide two different types of protection--against short circuits and against running overloads. At the same time, the supply equipment must be able to safely handle and be unaffected by the inrush current while the motor is getting up to speed. This starting interval will be short or long, depending on the inertia of the load and the design of the motor.

The starting current requires (often) a larger setting or selection for the short-circuit and ground-fault protective CB (or fuses)--if their time delay is short, the size is larger. However, as others have said, too big a size does not provide any protection.

The starter has two parts--the contactor to turn the electricity on/off and the overload relay that provides running overload protection. With an overload, the enemy is heat, so the overload relay tries to mimic the heat produced inside the motor, and then turn the contactor off when it gets too hot. That is why a careful selection of conditions and size is important with it.

The contactor can be oversized, as can the conductors. There are no theoretical limits on these. However, oversizing the overload relay or the CB/fuse can cause you the have no protection. Starters are labeled with the acceptable size(s) of CB's or fuses--this is for protection against damage from short circuits, etc. So, read the instructions and follow their limits.

--JMM

JMM, "too big a size does not provide any protection" --> this is what I've been wanting to find out or to hear from you guys - the experts. I was just wondering, from any codes that I've read, they only specify the minimum requirements. Not just for motor applications, even for simple building or residential applications, but now, the fact is -> "too big a size does not provide any protection". Should the codes also specify the maximum limit? But I guess nobody would indulge in over-design, surely their employer would eliminate them! What if, a novice like me would happen to have a spare which is too big a size and use it, would any inspector let it pass?

For motors (any form of starter) above 50HP and upto 150HP (like in your kase), as per the regulation in here is that maximum permissable starting current, I = 1.5 X Full Load Current :)