Equipment Ground Conductor (EGC) size for Medium Voltage Feeder

Status
Not open for further replies.

philly

Senior Member
I am trying to determine the Equipment Grounding Conductor size for a 13.8kV feeder circuit based on Table 250.122. The feeder breaker being used is a 13.8kV 1200A frame breaker with the relay overcurrent pickup setting set to 160A. It has always been my understanding that you base the Ratting or setting of the overcurrent device based on the actual relay pickup setting and not the breaker frame size when using 250.122. I believe 250.190(C)(3) provides confirmation that this is correct.

So in my case with a relay pickup setting of 160A I would use the 200A setting value in table 250.122 and thus require a EGC size of #6AWG or larger.

Am I correct in my thinking here, or am I missing something or some other exception in the code.

Thanks
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
In my opinion Table 250.122 is suitable for low voltage systems only. A rough appreciation of the EGC cross section area it could be calculated for a trip current of 6 times the setting.[160*6=0.96 kA].For PVC insulation from 40 to 60 oC 0.5 sec.adiabatically].
In a medium voltage system the short-circuit power may be about 500 MVA and for 13.8 kV it is Isc=500/sqrt(3)/13.8=21 kA.
A simple calculation for this current and 0.25 sec clearing it gives 2/0-3/0 copper conductor [from 40oC ambient to 250oC-XLPE insulation-short-time- rated ].:weeping:
 

Smart $

Esteemed Member
Location
Ohio
I agree with your assessment of the NEC requirement, provided the Informational Note is taken into consideration.
 
Status
Not open for further replies.
Top