I have a 100 hp 460 volt three phase motor 124A flc and a 5kva xfmr 10.4A fed by a 250 amp main breaker. How do I size the service entrance conductors correctly. My calculation tells me to use 2/0 phase conductors and a #4 grounded conductor and my coworker says that I need 4/0 phase conductors and a #2 grounded conductor. Please advise.
Sizing Service Entrance Conductors
- Thread starter buggsy65
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Size it for the over current device for 250a. Off the top of my head that would work out to be #250 mcm copper. I am pretty sure a #4 is fine but double check per table 250.122. Make sure to use 250.66 to size the GEC. Be when you feed the xfrmr, whole mess of rules for that application.
I think you may have a situation that is, IMHO, a bit of a Code oddity.
Were you sizing a feeder to supply the motor and transformer
2/0 would be acceptable per 430.24.
Service conductors, on the other hand, are required to be sized by the OCP device (where there is a singular device) so you 250 amp breaker would require a 250 kcmil as a service conductor.
You grounded conductor to the service based on the 250 would be a #2 based on 250.24(C) & 250.66.
Your equipment ground to the load side could be reduced to a #4 per 250.122.
Note:
I went back to the NEC to substanuiate the above by Code reference as I was working on what I had previously learned, however, Exception 1 to 230.90 leads me to believe the above info is incorrect. As I read that exception, you could size the service conductors the same as the feeder.
Hopefully we will get some more insight.
Were you sizing a feeder to supply the motor and transformer
2/0 would be acceptable per 430.24.
Service conductors, on the other hand, are required to be sized by the OCP device (where there is a singular device) so you 250 amp breaker would require a 250 kcmil as a service conductor.
You grounded conductor to the service based on the 250 would be a #2 based on 250.24(C) & 250.66.
Your equipment ground to the load side could be reduced to a #4 per 250.122.
Note:
I went back to the NEC to substanuiate the above by Code reference as I was working on what I had previously learned, however, Exception 1 to 230.90 leads me to believe the above info is incorrect. As I read that exception, you could size the service conductors the same as the feeder.
Hopefully we will get some more insight.
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I always get a chuckle out of that requirement, i.e. 230.42(B) Specific Installations. Not chuckling about the safety aspect... rather about a lack of definition for a "Specific" installation. As such, who makes or by what criteria is the decision made: specific or non-specific???
Generically, all installations are specific to that service!
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- Logan, Utah
Gus, I agree with your interpratation of Exception #1 to 230.90.
When you have a service that is supplying a motor load or motor loads then the Exception to 230.90 would permit you to size the service entrance conductors the same way you size the feeder conductors for a motor feeder.
The above calculations look good to me.
Chris
When you have a service that is supplying a motor load or motor loads then the Exception to 230.90 would permit you to size the service entrance conductors the same way you size the feeder conductors for a motor feeder.
The above calculations look good to me.
Chris
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