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#142570 02/02/05 06:02 PM
Joined: Jul 2002
Posts: 8,407
Likes: 2
Trumpy Offline OP
Member
Guys,
How do you go about calculating the Current-carrying capacity of two cables when they are parallled up?.
I'm trying to work out the CCC of two 90mm2 PVC Sheathed Aluminium cables.
I've forgotten how the formula goes and I'm pretty sure that the CCC is not double the single cable rating, but is a definite percentage.
Can anyone help?.

#142571 02/02/05 09:02 PM
Joined: May 2004
Posts: 186
A
Member
Mike, Your right its not just a double up of current rating, Need to apply a grouping factor quite rigidly as both cables would be carrying equal current and subject to simultaneouse overload condition. Believe its also important that both cables are of equal length otherwise Earth loop impedance calcs get screwed up! Have used paralell cables in the past but cheated I have always used a computer program and had it verified by cable manufacturer,s.
Put some details up Mike and lets see what we can come up with. Whats the CPD? Length of run? Installation Method? Max ambient Temperature?

#142572 02/04/05 04:06 AM
Joined: Dec 2004
Posts: 354
K
Member
G'day Trumpy. If you email or phone General Cables or Olex in Ch Ch they can punch your details in to their software and give you the specs for their cables. This is much easier than trying to figure out parallels for yourself, and cheaper than buying the software. It seems to me that a circuit can be uprated 65% to 75% when using the same conductors in parallel with the right grouping factor.
You're right though it's hard to find details of parallel cable calculations in NZ.

#142573 02/06/05 12:43 AM
Joined: Jul 2002
Posts: 8,407
Likes: 2
Trumpy Offline OP
Member
Hi there Alan and kiwi,
This sort of thing is very rare here, as it is looked upon as bad practice to parallel cables up.
This only came about after an apprentice Electrician was sent out to do his first Industrial job and mucked it up from start to finish.
Instead of using a single cable for a Submain to a new building, he ran two seperate cables, in conduit to the panel.
I was asked to come in and take over as an Electrician, as Faults work is a little slow at the moment.
Here's an outline of what I have:
  • Circuit length is 65 metres.
  • Supply cables are 95mm2 4C Aluminium (shaped) with XLPE insulation on both cores and sheath.
  • Current draw required is 300A per phase.
  • Cables are buried in PVC Ducting.
  • Soil temperature is 15C.
  • Protection will be a Merlin Gerin NS series 400A moulded case Breaker, with Electronic Trip Unit.

I personally would have used a single 150mm2 cable and saved all the hassle.
The apprentice must have had a telling off from higher up, as the last time I saw him, he wasn't saying much. [Linked Image]

#142574 02/06/05 09:40 PM
Joined: May 2004
Posts: 186
A
Member
Mike, ran details through computer program
2 x 95mm2 alum xlpe in parralel assumed ze .01 16ka fault level. max cable length = 76 meters. voltage drop = 6.78
at 300 amps zs at load end = .048 ohms
circuit protective device Merlin Gerin NS400H MA320 320amps 50mm2 CPC required.

Check it out with cable manufacturers
Alan

#142575 02/06/05 10:02 PM
Joined: Jul 2002
Posts: 8,407
Likes: 2
Trumpy Offline OP
Member
Hey thanks very much Alan!. [Linked Image]
Which software did you use to work that out?.
I rang Olex this morning and they gave me the exact same figures!. [Linked Image]
Cheers mate.

#142576 02/07/05 04:44 PM
Joined: May 2004
Posts: 186
A
Member
Hi Mike, I use a suite of programs called Amtech covers all aspects of design and testing. Glad to have been of help mate.

#142577 02/16/05 05:47 AM
Joined: Mar 2003
Posts: 52
D
Member
Hi Mike,

Do you use AS/NZS 3008 for your cable calculations?

There are tables there for derating parallel cables.


Regards
Gray

#142578 02/16/05 06:20 AM
Joined: Jul 2002
Posts: 8,407
Likes: 2
Trumpy Offline OP
Member
Gidday Gray,
Yes I was using AS/NZS 3008, it was on a CD ROM,along with a whole heap of other useful Standards in PDF form, until some egg-head at work decided to use the CD as a coffee coaster.
Grrrr!. [Linked Image]


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