100% CB used as required disconnect

[jaiken@trane.com]

A water chiller has a manufacturer supplied UL Listed unit mounted compressor motor starter with a 100% rated circuit breaker as its supply connection. Can this circuit breaker be used as the 440.12(A) required disconnecting means or does a separate disconnect sized to 115% of the nameplate rated load current need to be added? The rated load current of the compressor motor is 1119 and the circuit breaker is 100% rated at 1200 amps.

 

[templdl]

Re: 100% CB used as required disconnect

This is the way I look at it:
A 100% rated breaker can only be installed in a minimum enclosure size as specified by the breaker manufacturer as part of its 100% listing. The size of the breaker is chosen by sizing the cable based upon 100% on the non-continuous load + 100% of the continuous loads. The cable must carry that current based upon the 75degC column of NEC 310-16 but the cable must be rated 90dgC.
A 100% rated breaker with almost all certainty has a solid state trip init and is not any better than a breaker that can be only applied at 80% of its rating. The only thing that is gained by applying a 100% rated breaker is that it is possible to reduce the size of the cable as you don't have to use that 125% of the continuous load in sizing the cable.
So you must do the calculation in order to see if the 100% rating can save you some cost by allowing to use a smaller cable.
Remember that a common breaker is automatically derated when to size the breaker based upon the cable its protecting and the cable is sized 125% of the continuous load to begin with.
Now the big question is what is the objective in using the 100% rated breakered as you have described? The trip curves are the same and there is no added protection benefit.

 

[infinity]

Re: 100% CB used as required disconnect

Don't manufacturers list there equipment with 125% as part of the nameplate rating? Wouldn't the 1119 amps have this already included as part of the nameplate listing for the minimum circuit ampacity?

 

[jaiken@trane.com]

Re: 100% CB used as required disconnect

The compressor motor starter is shipped with, and mounted on the water chiller by the chiller manufacturer. The starter is listed by UL to comply with UL standard 508A. The 100% rated CB application was investigated and tested by UL for its use in this starter enclosure. The reason the 100% CB, 1200A in this case, is used in this application is that the manufacturer has designed the starter to get its highest ampere rating possible in this size of enclosure. The next frame size CB, 1600A, will not physically fit in this chiller mounted enclosure. My question is, can this CB be used by the chiller owner as the disconnecting means for the chiller installation? Or does the disconnecting means need to be rated at least nameplate rated load amps x 1.15? In this case, 1287A (1119 X 1.15).

 

[wirenut1980]

Re: 100% CB used as required disconnect

Don't manufacturers list there equipment with 125% as part of the nameplate rating? Wouldn't the 1119 amps have this already included as part of the nameplate listing for the minimum circuit ampacity?

Yes that is true, but I think the question has to do with the disconnecting means and not the circuit ampacity.

I see your problem, jaiken. NEC 440.12(A)(1) I believe clearly describes your situation and clearly states that the disconnecting means needs to be 115% of the nameplate current rating. If the nameplate says 1119 Amps, then IMO, you need to size the disconnect for at least 1287 Amps. So I think you know what you have to do...you can use the 1200 Amp circuit breaker for short circuit protection (hopefully you do not get any nuisance tripping from inrush current), but if it was listed by UL for this specific application, it should work. But I believe you will need to add a disconnect sized per 440.12(A)(1). That is my interpretation of your situation.

 

[templdl]

Re: 100% CB used as required disconnect

I was referring to molded case circuit breakers. Power air and insulated case circuit breakers available today are commonly 100% rated and meet applicable UL, ANSI, NEMA,and CSA standards.