Sizing Breaker Feeding Mechanical Unit

[Sparky1981]

As of recent Denver has been bringing this up in our reviews. I am wondering how you all size the breaker feeding a motor. In the mechanical schedule we get

FLA: 177A
MOCP:200A

To size the wire I take 177A*1.25=221.3A, so I use #4/o copper

To size the breaker out of my gear I use table 430.52, Inverse time breaker.
177A*2.5=442A.

I typically will size down and in this case I used 300A.

So I now have a 300A breaker, #4/O copper wire feeding this unit.

We got rejected because the reviewer says the mechanical schedule calls out for a MOCP of 200A. With a full load of 177A I feel like I am asking for trouble sizing my breaker that close to the FLA. Further I would assume the unit would be provided with overcurrent of 200A protecting my wire. Perhaps the reviewers are seeing a 300A breaker with 230A wire and having heart burn.

Would you provide the 200A breaker mechanical asks for? Or how do you go about sizing this stuff?

Again for years, review after review, this has never been brought up. But on the past two projects it has. So I am now baffled.

 

[david luchini]

As of recent Denver has been bringing this up in our reviews. I am wondering how you all size the breaker feeding a motor. In the mechanical schedule we get

FLA: 177A
MOCP:200A

To size the wire I take 177A*1.25=221.3A, so I use #4/o copper

To size the breaker out of my gear I use table 430.52, Inverse time breaker.
177A*2.5=442A.

I typically will size down and in this case I used 300A.

So I now have a 300A breaker, #4/O copper wire feeding this unit.

We got rejected because the reviewer says the mechanical schedule calls out for a MOCP of 200A. With a full load of 177A I feel like I am asking for trouble sizing my breaker that close to the FLA. Further I would assume the unit would be provided with overcurrent of 200A protecting my wire. Perhaps the reviewers are seeing a 300A breaker with 230A wire and having heart burn.

Would you provide the 200A breaker mechanical asks for? Or how do you go about sizing this stuff?

Again for years, review after review, this has never been brought up. But on the past two projects it has. So I am now baffled.

Are you sure the "FLA" isn't really the "MCA"? (I assume this is a "unit" not just a motor.)

You'd need #3/0 and a 200A c/b for an MCA of 177 and an MOCP of 200.

 

[Sparky1981]

FLA:177

MCA:182

 

[david luchini]

FLA:177

MCA:182

With an MCA of 182, you need a conductor with an ampacity of at least 182, or #3/0.

You can't exceed the MOCP with the breaker, so 200A max.

The MCA and MOCP calculations take care of all of the 125% for conductor sizes and 175% (assuming compressors) for the Overcurrent size.

 

[JoeStillman]

AC units are almost always multi-motor assemblies. To talk about the FLA of a multi-motor AC unit is not meaningful. You size the breaker according to 440.22 and the wire according to 440.33. If the unit is labeled with an MOCP, you can't exceed that.

 

[kwired]

AC units are almost always multi-motor assemblies. To talk about the FLA of a multi-motor AC unit is not meaningful. You size the breaker according to 440.22 and the wire according to 440.33. If the unit is labeled with an MOCP, you can't exceed that.

You will also find that if you do the calculations yourself that your results will be same or very close to the nameplate MCA and MOCP, so in effect they have done these calculations for you.