Star Delta MCCB Sizing

I have a question about sizing the MCCB and the cable for a 55kW 3 phase motor.

I have done following calculations and please correct me if wrong.

FLC = 55000/(1.732*415*0.8) , (I have assumed the PF as 0.8),

FLC = 96A

Main,delta Contactor & Over load realy;

96*0.58 = 56A,

Star contactor;

96*0.33 = 32A

If above calculations are correct what would be the ratings of MCCB/MCB

MCCB little over 56A would be enough for the starter?
or
Do I need to consider about the high transient current.

And when selecting a cable, would cable which can handle 56A be enough for the starter or again Do I need to consider about the high transient current.

How long is the cable? Is the cable in the air or underground? etc.

This information is important to select the correct cable size.

about 4m and the cable is not in underground. cable is above ground but in a PVC conduit.

for that distance, a 2X10mm2 cable will bo OK.
The line cable should be about 25mm2.

what about the MCCB,contactor and overload relay. Have I chosen the correct values for those

sthn said:

what about the MCCB,contactor and overload relay. Have I chosen the correct values for those

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Your calculations are correct.

Your calculations for your star/delta contactors and overload are correct, you may want to consider the length of time it will take to start the load as you may require a different class of overload. Reconsider your mcb / MCCB sizing. This will have to cover the total load, not just the load of one contactor. You will also have to consider the startup class. A lot of motors will operate on a type C mcb but if it is a hard start you may require a type D. If you are going for an mccb you would be advised to get hold of the electrical characteristics data / chart. This will allow you to choose the correct item for your task.

On another note, a lot of the 55kW 415V 4 pole motors I come across are closer to 105 Amps than your calculated 96A. I always try to get the stamped rating of the motor from the supplier of the motor to size my gear as there can be a reasonable variation between manufacturers and number of poles.

My 2p anyway.

Jonny

This will have to cover the total load, not just the load of one contactor

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Since current flow through one contactor i.e either through star or delta contactor if I choose a MCCB which can handle the current of the delta contactor (i.e 56A) won't it be enough for the system.

sthn said:

Since current flow through one contactor i.e either through star or delta contactor if I choose a MCCB which can handle the current of the delta contactor (i.e 56A) won't it be enough for the system.

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It is OK to use a ~56A overload relay under the main contactor, but you should also use a MCB of about 125A before the Star-Delta contactor assembly,
or you can install only a ~105A overload relay before the Star-Delta contactor assembly.

Since current flow through one contactor i.e either through star or delta contactor if I choose a MCCB which can handle the current of the delta contactor (i.e 56A) won't it be enough for the system.

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In short, no. Your mcb protects all aspects of the power circuit and so must be sized to the full load and placed upstream of all the associated contactors. When a motor is starting in star configuration all the power to the motor must travel through the main contactor, the star contactor carries current but it is not a source of electricity. In delta the power is shared between the main and delta contactors.

On another note some designers like to place the overload on the delta contactor while others like to place the overload on the main contactor. There are arguments for and against either configuration. I always design a star delta motor starter with the overload placed on the mains contactor as I am of the opinion that the overload is there to monitor and react to overload currents and the mcb is there for short circuit protection. If a motor is very tight or has too large a load on it to start, my opinion is that the overload should be the protection device that triggers. An mcb should only activate if there is a short in the windings or cabling. If the mcb triggers other than in these conditions it could possibly do with upsizing or re-evaluation of the mechanicals.

Hope this helps.

Jonny

Thank you all. Now I got it

... but noone told you about breaking capacity of line fuse/mccb, which must be considered when cables are short.

Homer_BL then can you help me with that

Follow this link -

http://www.moeller.net/en/support/slider/motorstarter_en.jsp

whilst it is "based" on the manufacturers own equipment it will help with sizing.

Hi, a small notice!
"What ever" means up to 250 A for star contactor.