×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

THE NEC 1999 HAS AN EQUIPMENT GROUN

THE NEC 1999 HAS AN EQUIPMENT GROUN

THE NEC 1999 HAS AN EQUIPMENT GROUN

(OP)
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..

RE: THE NEC 1999 HAS AN EQUIPMENT GROUN

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.  

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources