THE NEC 1999 HAS AN EQUIPMENT GROUN 1

[devlin]

THE NEC 1999 HAS AN EQUIPMENT GROUNDING SELECTION TABLE BASES THE GROUND WIRE ON THE TRIP OR FUSE RATING OF THE QVERCURRENT DEVICE. WHEN YOU GET TO THE 4000 AMP RANGE IT CALLS FOR A VERY LARGE GROUND WIRE AND EACH OF THESE SIZES HAS TO BE RUN IN EACH PARALLEL RUN OF CONDUIT. OVER THE YEARS I HAVE ALWAYS PARALLEL SETS OF 500MCM WITH A #1/0 GROUND AS MY MAXIMUM.IS THIS A CODE MISTAKE OR JUST OVERKILL? WHY WOULD ANYBODY WANT TO RUN SUCH LARGE GROUND CONDUCTORS IN EACH PARALLEL RUN OF FEEDER CONDUIT OR AM I MISSING SOMETHING??

FRANK S..

 

[REDDOG]

If I understand the question? The voltage is 480/277 3 Phase 4 wire. A 4000 feeder originates from a switchboard to serve a load. This feeder is in conduit and consist of 11 sets of 3-500 MCM copper and 1-500 MCM copper grounding conductor in a 3" conduit. Assuming a 3500 KVA Transformer 5%Z, fault current will be about 84000 amps.
A 4000 Amp Class L Fuse will go into "Current Limit" at about 15 times its rating is 60000 amps. A 4000 AMP Circuit Breaker one pole interrupting rating is only 15000 amps. A breaker would be a miss application. The withstand rating of 500 MCM thermoplastic insulation at 60000 amps is about 8 cycles. The withstand rating of 1/0 thermoplastic insulation at 60000 amps is about .5 cycles. For a line to ground fault the grounding conductors must be able to carry this current. Therefore the code requires 500 MCM per the table 250-122 for this configuration. Article 250.2(c)&(d) requires a effictive ground return path of low impedance
to insure proper operation of the overcurrent device.