conductor sizing

[jfplaniczka]

I have a piece of machinery which is suppose to have a 100 amp main breaker in the control panel.
This will be fed from a 480 volt 3 phase bus duct.
would i be correct in assuming that the wire size will be 3 awg thhn with a 8 awg ground?

also they originally told me that the machine was 54kw. so 54kw divided by 480= 112.5amps do you multiply that by 125%. doesn't seem to me that it would be 100 amp breaker inside there you know what i mean?

 

[ryan_618]

Re: conductor sizing

It looks like you missed the square root of three in your calc's

 

[jfplaniczka]

Re: conductor sizing

so then it would be about 65 amps then ok makes sense now , thought that i missed something.


THANK YOU

 

[charlie b]

Re: conductor sizing

You also missed the conversion of ?KW? to ?KVA.? They might be the same, if you have a resistive load. But I suspect there is a motor involved (else you might not have called it ?machinery?). Taking 54KW, dividing it by an assumed 0.85 power factor, then dividing it by 480 and again by the square root of 3, and finally tacking on an extra 25% (for the continuous load), I get a result of 95.5 amps. So a 100 amp breaker works for this situation. I concur with #3 conductors and a #8 EGC. This presumes, of course, that the run is not so long as to cause a concern over voltage drop.

 

[don_resqcapt19]

Re: conductor sizing

If the conductors running from the busway to the machine are tap conductors the #8 EGC will be too small. If there is an OCPD at the busway for the feeder conductors that has a rating of 100 amps or less, then the #8 is ok.
Don

 

[jfplaniczka]

Re: conductor sizing

so if it would be say 64Kva i would work the math the same?

sorry for all the questions trying to learn a little more about all of this. you all have been a great great help.

THANK YOU