×
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

Paralleling Different Cable size

Paralleling Different Cable size

Paralleling Different Cable size

(OP)
Hi,

What will happen if 4/O and 240mm2 underground medium voltage 33kV cable is paralled? it's used as a part of overhead line 33kV at the beginning of the circuit.

Regards,
Eskim

RE: Paralleling Different Cable size

They'll both still conduct electricity...

You'll need to be a little more specific about what you're after...

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Paralleling Different Cable size

Ampacity is not linear with respect to cross sectional area. The current may not split in proportion to the ampacity ratings of the cables. If you are near maximum ampacity with respect to the sum of the cable ampacities one cable may take enough current to be overloaded.
I believe that it is a violation of North American codes to parallel dissimilar conductors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Paralleling Different Cable size

As waross stated this is against the National electrical code.

RE: Paralleling Different Cable size

The code rules – end of story.

Just for fun, here is an attempt to think through the physics fwiw:

IF we have two very small conductors of different size, the impedance is dominated by resistance which is inversely proportional to cross sectional area.   The current in parallel conductors would be inversely proportional to resistance and therefore directly proportional to cross sectional area. In other words amps/mm^2 would be the same in each conductor and heat generated per volume would be (approximately*) the same in each conductor.  The smaller conductor has more heat-dissipation surface area per cross-sectional area, and therefore would run cooler (assuming the same insulation thicknesses and ambient conditions etc).
 
IF we two very large conductors of different size, the impedance is dominated by inductance, which is roughly constant.   So both conductors would carry roughtly the same constant.  The smaller of the two conductors will generate more I^2*R and tend to run at higher temperature.  

So the result is different depending on whether we're comparing too large conductors or two small conductors.  (for small conductors, the larger of the two runs hotter, for large conductors the smaller of the two runs hotter).   

* In both cases the temperature coefficient of resistance effects would cause more current in the cooler conductor and tend to bring the temperatures slightly closer together.

=====================================
(2B)+(2B)'  ?

RE: Paralleling Different Cable size

Quote:

* In both cases the temperature coefficient of resistance effects would cause more current in the cooler conductor and tend to bring the temperatures slightly closer together.
Actually this is more relevant to the small-conductor case which is dominated by resistance... not so relevant for the large-conductor case which is dominated by inductance.

=====================================
(2B)+(2B)'  ?

RE: Paralleling Different Cable size

(OP)
Chapter 3 article 310.4 states "the paralled conductors in each phase ... shall: a)be the same length b)have the same condcutor material c)be the same size d) have the same insul type e) be terminated in the same manner"

Which one is correct for the following:

- Phase A =OK to have two different sizes conductors as long as phase B & C are the same.  

- Phase A =OK to have 2 conductors but both must be identical and Phase B & C must be the same.

Rgds,
Eskim
 

RE: Paralleling Different Cable size

The second one.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

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