Paralleling Different Cable size

[eskim]

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

 

[MacGyverS2000]

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

 

[waross]

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

 

[mcgyvr]

As waross stated this is against the National electrical code.

 

[electricpete]

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)' ?

 

[electricpete]

* 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)' ?

 

[eskim]

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

 

[xnuke]

The second one.

xnuke
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