Sizing of Main CB controlled motors

[TMMB]

I have 3 motors each 400 KW, 660V , I did the following calculations, so please correct me if I am wrong

FLC for each motor = (400*1000)/(1.732*660*0.8)= 437 A

Conductor size = 1.25 * 437 =546 A , the conductor size 3 x 1C x 500 mm2 (1000 AWG/Kcmil) , the cable length about 50m
The motor C.B : 630 A

The main C.B for the panel controlled the 3 branch motors = (630 * 1.25) + (437+437)=1661 A , so we can use 1600A

The size of the required transformer (1.2 (3 * 400 KW))=1440 KW = 1800 KVA it mean 2000 KVA transformer

Please advice if something wrong

 

[TMMB]

75 viewers without any single reply !!!

 

[augie47]

One problem I have is addressing motors in kw. For calculations, NEC directs us to Tables listed by HP.
Secondly, it appears you are using 660 volts..unknown to most of us.
That aside, if we do use your motor FLA (437) and take your calculated conductor ampacity (546), the NEC required conductor would be 1000 kcmil as noted. (In real life probably 3-4/0s). THe C/B for one motor, using 430.52, would be more in the 1000 amp range, for (3) motors, the 1600 amp would be about right, (See 430.52).

I would imagine motors this size would not be ACL but on some type drive, in which case all the above is mute as the drive FLA takes precedent.

 

[TMMB]

Thank you for reply , but I have some comments, if we follow NEC for C.B sizing of one motor it will be at least 1.75 times the full rated current(437A) it means , C.B = 1.75 * 437=765 A , it mean 800 A , then the main C.B will be = 1.25*800+(437+437) =1874 A , SO main C.B will be at least 1800 A or 2000A , is that right??

 

[petersonra]

where did you get 175%

the code does not specify any particular value, only a maximum allowed.

you can always make it smaller as long as it does not trip.

 

[TMMB]

AS Stated in NEC 440.22

 

[petersonra]

AS Stated in NEC 440.22

your OP did not say it was an A/C compressor. there are slightly different rules for A/C compressors than for normal motors.

 

[TMMB]

sorry it is not A/C compressor it is Mill m/c

what is the right way?

 

[augie47]

Is it controlled by a drive or is it across-the-line start ?
If it's ACL read 430.52 (as noted twice) and 430.63.
If a drive is involved see 430 Part X.

also as noted, I don't see where 175% plays a part in Circuit Breaker selection.