cable ampacity rating question

[antigfk]

I have this situation:

panelboard-bkr1
|
cable
|
|
bkr2
|
contactor
|
overload relay
|
motor

bkr2 is a magnetic-only MCP. (100A continuous, 150A frame)
motor is a 25hp induction machine (31A full load, 480V)
bkr2, contactor, and overload relay form a packaged stand-alone motor starter

I know that panelboard-bkr1 must be rated at a minum of 31A*1.25, and must protect cable while not tripping when motor starts due to current inrush.

overload relay is protecting the circuit from an overload, that is too much current drawn by motor. If bkr2 is opened, then cable cannot be 'overloaded'... it can only have a 'short circuit'.

My problem stems from having to use a 200A thermal-mag breaker for 'panelboard-bkr1' to allow motor inrush current without tripping... BUT I can't terminate cable rated for 200A at the 'bkr2'.

Is it allowable in this instance, per NEC, to use a cable rated for less than 200A, so long as 'panelboard-bkr1' protects the cable from short-circuit?

Thank you

 

[rbulsara]

Yes. Motor branch circuit is one of the exceptions permitted in NEC where a breaker rating can exceed cable ampacity as long as there is a separate motor overload protection provided in the circuit.

 

[resqcapt19]

It looks to me like the largest permitted breaker in the first panel would be 90 amps. 250% of T430.250 current for a 3 phase 25 hp, 480 volt wire. The minimum wire size would be #8 copper.

 

[rbulsara]

resqcapt19 is right. I was not looking at the breaker ratings compared to the motor. I was only commenting on what NEC permits.

 

[antigfk]

Resqcapt19,

430.52(C)(1)(exception No. 2)(c) states:

"The rating of an inverse time circuit breaker shall be permitted to be increased but shall in no case exceed 400 percent for full-load currents of 100 amperes or less or 300 percent for full-load currents greater than 100 amperes."

It looks like I could use (31amps)*400% = 124amps
Next standard breaker size is 125amps.

You guys agree?

 

[rbulsara]

I guess not, it applies only if 250% breaker is not sufficient for allowing to start the motor.

Why do you want larger breaker?

If you have to keep the existing 200A breaker, feed a 200A fusible disconnect and install a smaller fuse in it.

 

[antigfk]

We tried using a 100A thermal-mag breaker and it tripped on motor inrush current.

 

[rbulsara]

If you can back up your claim, you can apply the exception, I would think.

 

[pwreng4]

"We tried using a 100A thermal-mag breaker and it tripped on motor inrush current."

Then you have some other problem, most likely. a 480V, 3phase, 25hp motor should work fine with a 50 amp thermal-magnetic breaker.