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greenmagnetic (Electrical) (OP)
21 Feb 12 23:10
What is the difference between single core and multi core power cable? I mean when we deciding to use single core and multi core power cable? For example 3x1x240sqmm for 3,3 KV or 1x3x240sqmm for 3,3KV

 
GEORGE2311 (Electrical)
22 Feb 12 6:38
If i was in your position i would have chosen the three single core 240mm2 as it is easiest in installation and of course it has 10% more current capability. As a matter of fact over 185mm2 i prefer single core.  

G.F.
www.engineering-services.gr

GEORGE2311 (Electrical)
22 Feb 12 6:53
Of course for small cross sections (less than 150mm2) the multi core are preferred as the cost of installation is smaller than the single core (i.e. you you don't have to care about trefoil and similar concerns), so in my opinion labor costs are critical for that kind of comparison.    

G.F.
www.engineering-services.gr

GEORGE2311 (Electrical)
22 Feb 12 7:35
Let me also give some typical numbers in order to help you get an idea on what i mean when i am talking about installation easiness:
 1)A typical 6kv N2XSY 1x240 cable has aprox. 3kg/m and maximum bedding radius of 15x40mm=0,6m while,
 2)A typical 6kv N2XSEY 3x240 cable has aprox. 11,5kg/m and maximum bedding radius of 15x75mm=1,12m.
So taking in mind your cable data and depending on the cable routing you can decide what is the best from cost point of view.

Please note that in general one three core cable is cheaper than three single cores.
   

G.F.
www.engineering-services.gr

7anoter4 (Electrical)
22 Feb 12 10:08
If voltage drop is not  important single core cables would be preferred namely if you could keep a certain distance between each other, so you could increase the ampacity.
If the cables run in the same conduit no difference will be.
 
waross (Electrical)
22 Feb 12 10:53
There are induction hysteresis and circulating current issues with single core. Not to say don't use single core but be aware.

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

Helpful Member!  rcwilson (Electrical)
22 Feb 12 11:42
Ongoing argument and discussion in our company!  At sizes below #1/0 AWG (70mm2) three and four core are the standard. Larger sizes are usually single core, especially for 5 kV and 15 kV circuits.

If the cable routing includes underground duct banks and/or conduits, a three core cable with earth wire is too large to fit in commonly available conduits or to be able to negotiate a conduit bend during pulling.  Three core cable costs more than single conductors for the cable but single core usually takes more manhours to install since there are three or four reels to position for each pull.  

Installation costs are proportional to weight of installed cable and devices. Single core reels are lighter and easier to handle. Pulling and handling three lighter single conductors may be less expensive than one heavy 3/c. (Our historical installation costs are not granular enough to determine the better method).

The counter argument is that the three core cable is difficult to handle in the cable tray or vault and the connection fitting costs are expensive. (No special cable glands for single core). If the project is in a high cost labor area, we may use more multi-core.

But in Africa, our affiliated company's project used all 3 and 4 core cables for 600V, 6.6 kV and 11 kV, including a lot of 1200 mm2, 3/c with armor. Labor was inexpensive and cable routing was designed to accommodate the wide bends.

In North America, there is a perception that all cables must be enclosed or protected in a conduit.  Open wiring, as is common in the rest of the world, is frowned on by owners, inspectors and especially the electricians who are very good at bending and installing steel conduit. That requirement makes single core the design choice even though a multi-core routed in cable tray would be just as rugged and reliable.
 
greenmagnetic (Electrical) (OP)
22 Feb 12 19:45
Thanks for the answer. It will be my consideration to choose the type of cable, between single core or multi core. For information, this cable will be installed underground. This cable is N2XSEKBY.
healyx (Electrical)
22 Feb 12 20:29
If you are considering paralleling a supply, I would not recommend doing it with multicore cables in excess of about 35mm2 (AWG 2).  Above this, reactance becomes a larger proportion of cable impedance and it is too hard to guarantee phase geometry with a multicore cable.
jghrist (Electrical)
23 Feb 12 1:17

Quote:

it is too hard to guarantee phase geometry with a multicore cable.
Really?  You can get more consistent phase spacing pulling single core cables than with the phases factory installed in triplex?
 
healyx (Electrical)
23 Feb 12 2:43
jghrist

I am referring to the phase geometry inter-set not intra-set.

When you parallel 2 cable sets in trefoil (?triplex) they should be arranged as:

 A      A
B C    C B

Much easier to guarantee this formation if they are multicore instead of single core.

 
healyx (Electrical)
23 Feb 12 2:46
sorry, last bit should have been:

Much easier to guarantee this formation if they are single core instead of multicore.
dpc (Electrical)
23 Feb 12 11:24
healyx,

That's interesting. I've been doing this a long time and never heard that argument before.  

There should be very little flux outside the triplex cable, so why would the interposed phases between the two cables really matter much.  

In the 480 V commercial world, it is common to see feeders consisting of up to 10 conductors per phase pulled into 10 conduits using triplex or three single conductors pulled in at random.  This is allowed under the NEC and I'm not aware of any serious issues that have occurred.   
7anoter4 (Electrical)
23 Feb 12 12:39
First of all I agree with dpc.The three-phase cable cores are laid-out with a pitch of 2-3 ft. so all 2-3 ft. each phase is in the other position.
In this case a very small amount of the magnetic flux will flow outside. So you don't need to carry of parallel cables current unbalance.
Straight laid single-core cables even pulled close each other a current unbalance could take place if they are not in a certain order.
A good indication how to lay the phases is in the Prysmian
http://www.cooprg.org.ar/documentacion/Errores%20en%20el%20trabajo%20con%20cables.pdf
The text is in Spanish, indeed, but the sketch is very clear.
Second, the cable reactance will increase with the cores centerline distance. On the other hand the temperature will decrease with
the distance between cores[ and cables, of course]
 
healyx (Electrical)
23 Feb 12 17:55
Sorry, my bad.

You're right, multicore cables twist the phases together - so no issue there.

dpc:

I would have thought paralleling single core cables with random arrangement as you described would cause some rather nasty current imbalances?

 
dpc (Electrical)
23 Feb 12 18:21
As long as all three phases are in each conduit, it seems to generally work - the conductors are in close proximity regardless of the twisting, so flux cancellation is pretty complete.  The code requires that all conductors be the same length (in theory).  Very short runs are more of a concern than long runs.  It's ugly, no doubt at that.     

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