NEC 250.118, 250.122(F), 392.60

D

Dale001289

Senior Member
Location
Georgia
We have a client who thinks the embedded ‘grounding loop’ that ties into the MCC bus is the EGC for parallel 500 kCMIL cables ran entirely in cable tray from a switchboard feeder CB to the MCC MCB. He doesn’t recognize the cable tray represents the EGC and needs to be tied at each end for low impedance fault path. Not sure how to convince them that the tray itself is the EGC per 392.60 and doesn’t appear in 250.118 and the buried ground loop is only an equipotential grid - not a fault path for the circuits…suggestions?
 
Maybe 300.3(B) -- all conductors, including equipment grounds, generally need to be run together. Embedded grounding loop isn't run with the other circuit conductors.
 
As I recall there is a table in the IEEE Green book that shows a separation of 18" between the EGC and the ungrounded conductors increases the impedance of the fault return path by about 50% and a 30" separation doubles the impedance as compared to a 2" separation.
 
I have the 5th printing (1989) version of the IEEE Green Book. On page 51 (section 2.2 Fundamental COncepts of Equipment Grounding), it shows impedances of 0.0182 when EGC is 2" from phase conductor, 0.0226 when 8" from phase conductor, and 0.0275 when EGC is 30" from phase conductors. These were large 500 KCMIL phase conductors and 4/0 EGC and 200' long.
 
suemarkp said:
I have the 5th printing (1989) version of the IEEE Green Book. On page 51 (section 2.2 Fundamental COncepts of Equipment Grounding), it shows impedances of 0.0182 when EGC is 2" from phase conductor, 0.0226 when 8" from phase conductor, and 0.0275 when EGC is 30" from phase conductors. These were large 500 KCMIL phase conductors and 4/0 EGC and 200' long.
Click to expand...
They want to use the embedded grounding ‘loop’ which is approximately 30 feet from the phase conductors…this client is the most ignorant I’ve ever seen
 
You must log in or register to reply here.
Top