IEC conductor and breaker for motor

[timm333]

I am not able to find the rules to size the conductor and breaker for a motor using IEC standards. (In NEC you simply take 1.25 times the FLA); I will appreciate if someone helps me in sizing IEC conductor and breaker for the motor.

 

[ScottyUK]

Most of the manufacturers will provide type-tested combinations for standard IEC motor sizes, certainly Schneider, Siemens and ABB all do. Cables are sized based on the upstream protective device, be it an MCCB, fuse, ACB or whatever, and also on acceptable volt-drop at the motor. The latter is somewhat dependant on the load - a centrifugal fan is more tolerant of volt-drop than a reciprocating compressor for example.

Be careful if the manufacturer has type-tested the contactor you want to use with (say) a 100A frame size breaker and you are using a 160A frame size because of the higher fault rating available with the bigger breaker: the higher energy let-through may force you to choose a larger contactor to just to maintain a type-tested combination. You need to ask for type 2 coordination; type 1 coordination allows a fault to be cleared but permits the contactor and overload relay to be destroyed in the process.


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If we learn from our mistakes I'm getting a great education!

 

[timm333]

Thanks ScottyUK for the help. The problem is that the recommended starters in IEC don’t provide allowance for motor starting current. For example, the 160kW motor has a FLA of 297A, and the Eaton’s recommended starter for 160kW motor is 300A which is only 3A higher than the FLA.

If I size the conductor for 300A, it will not provide any allowance for the motor starting current. (In NEC, if the FLA is 297A, then the conductor is sized for 297*1.25=371.25A.)

 

[ScottyUK]

Sure they do, you just need to know where to look. If you look at the contactor ratings you'll find there are at least two listed: AC-1 and AC-3. AC-1 is a resistive load, AC-3 is a DOL start. The AC-3 rating is considerably lower than the AC-1 rating. Other ratings include AC-4 for reversing or plugging duty, AC-6a for trasformer switching and AC-6b for capacitor switching. They are all listed in IEC 60947-4-1, as are the equivalent DC-x ratings for DC contactors.


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If we learn from our mistakes I'm getting a great education!

 

[timm333]

Thanks ScottyUK...You are talking about AC1 which is for non-inductive loads with power factor greater than or equal to 95%. In my case the load is motor, so I should look only under the AC3 column. I am not able to understand how you are relating the "non-motor AC1 column" to the motor starting current. Please clarify.

 

[Kiribanda]

timm333,
It is my understanding that the current given in the starter is the "Rated Opeartional Current" (Ie)which is not necessarily to be the motor FLC.If you select your contactor from the manufacturer's table according to the motor kW rating,voltage and Type 2(or 1)coordination, then you are covered.Also the manufacturer's tables are available to select the rating and type of either the MCCB or fuse depending on your wish based on Type 1 or 2 coordination.Per IEC, you cannot use the MCCB(fuse) from one manufacturer and the contactor from another manufacturer,because it will violate the Type 2(or 1) coordination.The sizing of the motor cable for its continuous current and short circuit rating should be done per IEC 600364 or your local code.

 

[ScottyUK]

timm333,

I wasn't really trying to relate the AC-1 rating to motor starting, just to say that the motor starting current is accounted for within the AC-3 rating, which is part of the reason why for any given contactor the AC-3 rating is considerably lower than the AC-1 rating.

A qualifier to Kiri's post: some manufacturers with complementary ranges of products have type-tested together: for example A-B have type-tested their contactors and O/L relays with Terasaki's breakers. A-B's IEC breaker range isn't that good, and Terasaki don't make contactors, hence the mutually beneficial relationship.


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If we learn from our mistakes I'm getting a great education!