×
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

Gounding of System Neutral

Gounding of System Neutral

Gounding of System Neutral

(OP)
We have a 5 MVA 46kV/4160V transformer which is impedance grounded with a resistor. The secondary conductors are parallel 500 kcmil.

How do you size the neutral conductor which feeds the grounded resistor?

If you could also tell me which section of the code gives the sizing for this installation that would be great.

Thank you

RE: Gounding of System Neutral

See NEC 250.186, but I don't think it is going to give you a sizing criteria.  You should size it based on the resistor rating.  So for a 400 A resistor, I would size the conductor for 400 A.  Since the resistor is only rated for probably 10 sec, this will be very conservative.  A smaller conductor could be used, but you would need to look at the short-time thermal rating of the conductor.

The conductor should be insulated for full line-to-line voltage, at least in my opinion.  

RE: Gounding of System Neutral

(OP)
Is there a minimum size which the conductor should be rated at?

250.190 sounds like the conductor should not be sized for less than 6 AWG copper.

Thanks

RE: Gounding of System Neutral

I was working on a project where a similar question came up regarding high resistance grounding of the same size transformers (5MVA) but lower voltages. I agree that conductor should be sized based on the current flow and short time thermal rating. In addition although a smaller conductor may work I typically would use some thing that is physically rigid enough to also afford some additional protection.

In reference a NEC section that addresses refer to 250.36. This however only applies to systems rated 480V to 1000V so that would not be applicable to your installation. It may offer some guideance though.

RE: Gounding of System Neutral

For 5 kV cable, you are not going to find anything much smaller than #2 AWG anyway.  As I said, I would size for 400 A.  This  cable run is very short, so you may just want to use whatever you are using for the phase conductors.

 

RE: Gounding of System Neutral

(OP)
In our case the resistor is 25 A. Are you saying it should be sized for 25A then? That would but us at a #8 or so.

RE: Gounding of System Neutral

If you can find a 5 kV shielded cable with a #8 AWG conductor, that would probably work OK.  But I think you are going to end up with #2 because that is generally the smallest size available.

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