Power Cables Arrangement in a Conduit

Well, you could refer to British Standard 7671 instead of asking me.

Well, by your description, you are running all phase "R" cables in the conduits. What happened to phases "S" and "T"?

To avoid problems with the electromagnetic fields, each conduit should have one of each of the phase cables, as well as your neutral and/or ground, in it. Is there some reason you cannot use larger cables so that you don't need 9 per phase? If you are planning to use only 3 conduits, you would have 3 - "R", 3 - "S", and 3 - "T" plus 3 - earth cables in each conduit. Sounds like a mess.

Either use larger cables, or more conduits, so that you limit to a single cable for each phase in a conduit.

Remember too that you have to account for (in other words, derate) the mutual heating between conduits when they are run close together in parallel. They are then not isolated, and prevent each other from cooling (whether in air or soil).

Use cable tray instead of conduit & maintain recommended formations & distances as per BS 7671

From whom did you receive this advice?

Is the person properly qualified to give such advice,and is this person familiar with

LOCAL CODES and procedures approved by the Authority Having Jurisdiction?

It is normal procedure to put all 3 phases plus ground in each conduit,thereby

cancelling self heating that would otherwise occur due to inductive currents.

Strive to keep all conductors the same length,paying attention to bends in the

conduit routing when they are running parallel to each other.

PVC conduit will not occur any inductive heating of the conduit itself.

The ground wire size is determined by the size of the upstream breaker.

There is much additional information needed to answer your question properly,and I

have only scratched the surface.

Please refer this issue to a local qualified electrical company,and especially, the

inspector in charge of the operation.

The inspector has the final word for the approval of means and methods.

Good luck.

Usually "eddy currents" are in shield or armor. These cables have to be low voltage unshielded and unarmored. Are they? Or do you mean "proximity effect"?

I don't think it will be much difference if in a pvc conduit all the cable will be of the same phase. As I know nor NEC neither BS7671 [nor IEC 60364-5-52] considers a certain order of cables in a conduit.

Your concern has to be unbalance between conductors of the same phase. If it will be close each other-no clearance between conduits at all- the unbalance could be up to 70%.In my opinion, if the clearance will be more than 1 conduit diameter the unbalance is negligible.

Anyway, it will be preferable to put them in 9 conduits each R-S-T-E phased.

Dear all

Thanks for your valuable advices, I think that there might be a misunderstanding, maybe I didn't explain the condition clear enough. We have to use 240sqmm for the power cables due to some purchasing deals.

The current capacity we have is about 3150Amps. And as per the cables data sheet and after reviewing the derating values we will be using 9 cables (240sqmm) per phase and 9 cables (120sqmm) for the neutral phase.

For the conduits we have to use (6 Inches) size, and it will be direct buried in the ground.

I just asked for the best arrangement for the cables, that's it.

Save yourself a lot of aggravation, instead of arranging the conduits horizontally, place them in a triangle, symmetric and balanced, just requires a bit more defrating due to the reduced heat transfer to the surrounding soil.

Okay, what do u recommend for cables arrangement per each conduit

what is the length of cable run.

Thanks for your help but 9 conduits !!! its an out of reach option.

The conduits we are planning to use is 6 inches diameter and we have reviewed the IEC standard to check the suitable filling degree.

The thing is that the conduits will be separated from each (as shown below)

The information that I am looking for is what is the proper arrangement for the cables inside the conduits.

We have

27 cables (240sqmm) "9 cables per phase" and

9 cables (120sqmm) "9 cable for neutral"

Our service contractor suggested to put in each conduit a complete phase cables + 1/3 of the neutral cables.

and to leave the 4th conduit as a spare.

I am still searching in the IEC & BS for the best solution and I hoped that you might help me in this issue.

For the record thanks for you all even for the passive replies

Thanks allot for your comment,

Okay I am getting it now, As I mentioned we have 9 cables per phase and 9 cables for ground, so can we arrange the cables as following:-

Conduit#1 : 3 cables of phase "R" + 3 cables of phase "S" + 3 cables of phase "T" + 3 cables of GROUNDS

Conduit#2 : 3 cables of phase "R" + 3 cables of phase "S" + 3 cables of phase "T" + 3 cables of GROUNDS

Conduit#3 : 3 cables of phase "R" + 3 cables of phase "S" + 3 cables of phase "T" + 3 cables of GROUNDS

___________________

Its all about listening carefully to those who can teach.

I don't think there is a standard for 9 cables in an underground duct-bank ampacity.

In my opinion, if the earth temperature is 25 oC and earth RHO 90 C-cm/watt then no more than 230 A will be cable ampacity for 9 single-core copper cables of 1/0.6 kV XLPE insulated 100% load factor.

You have to calculate it according to IEC60287.

It would be better to group in 3 bunches of 4 cables[R,S,T,N] in order to diminish the unbalance between the cable currents [of the same phase].However, since the temperature drop from cable to conduit, from conduit to duct-bank and from duct to Earth is the same for all cables, the temperature drop in the insulation depends only on cable individual current and could add another 1 degree C in part of the cables.

If this order is not easy to achieve I think it could be neglected.

I meant 9 single-core cable [loaded] in one pvc conduit and 3* 6" pvc ducts.

Even 3 loaded cables in 9 pvc conduits -one conduit diameter clearance, in one row-the ampacity could not be more than 300 A.