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sj
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Join Date: Mar 2008
Posts: 10

MV Cable Sizing For Sckt

03/06/2008 3:25 AM

In case of MV cable sizing ( eg 33kV ),

* Considering the short circuit level, the minimum crossection required is 350 sqmm ( aluminium conductor, XLPE insulated ).

* As per the current carrying requirement, the minimum cable size required is 3Cx400 sqmm.

So if I were to consider multiple runs of multicore cable ( 3C ) is it OK that I consider

2 runs of 240sqmm cables ( ok for the current carrying capacity )

Or , is it that each run has to meet the minimum cross section of

350 sqmm and hence I cannot use 2 runs 240 sqmm.

Pl clarify.
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Sparkstation
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#1

Re: MV cable sizing for Sckt

03/06/2008 5:01 AM

Hello sj

A) You ask us to clarify the unknowns, and that is not possible.

B) You have not stated the length of run. - Specify Please.

C) Nor have you specified the installation conditions:

Maximum/minimum temperatures at Site

1. Cable Tray
2. Steel Conduit above ground
3. Steel Conduit below ground
4. Direct buried
5. Other - Specify please.

It is normal to use the actual Cable Maker Charts, for their particular Cable and installation temperature also position.

Reply here, with

Kind Regards....

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manirul
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#2

Re: MV Cable Sizing For Sckt

03/07/2008 3:33 AM

Steps to size a MV cable as follows

1) Determine Total Load current to carry

2) De-rate the current as par actual installation conditions and select a cable based on this current from mfr table

3) Calculate Short Circuit current for this cable as Isc=K*A/sqrt t or from mfr table

4) Compare this Isc to system max Isc, cable Isc should greater than system Isc to withstand, if not go for higher size of cable.

5)Calculate % Voltage Drop as [{1.732*route length of cable*total Load current*(r cosphi + x sinphi)}/V] *100

check the acceptable level of Vd with your local standard. if it is more than acceptable limit, go for higher size and repeat calculation.

** In case of no of run, you have to do economical analysis of installation i.e. what is the cost of single run cable + cost of energy lost and what is the cost of multiple run cable + cost of energy lost AND site suitability.

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Anonymous Poster
#3
In reply to #2

Re: MV Cable Sizing For Sckt

03/07/2008 12:13 PM

Thank you for the reply.

But my query is incase multiple runs of cable are being used to meet the current requirement, is it required that each of the run has to have c/s equal to or greater as required as per the sckt level.

Pl. clarify
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manirul
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#4
In reply to #3

Re: MV Cable Sizing For Sckt

03/08/2008 12:01 AM

Yes it is, each run has to withstand at that fault level within the duration to clear the fault.

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Yasser
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#6
In reply to #2

Re: MV Cable Sizing For Sckt

03/09/2008 10:12 PM

hai..Manirul;

where I can get this mfr table??

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Sparkstation
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#7
In reply to #6

Re: MV Cable Sizing For Sckt

03/09/2008 10:55 PM

Hello Yasser

I have answered your Post,

Generally best to use the Cable Maker of your particular cable, but try:

http://relemaccables.com/XLPEcables.html

If you are still needing help, reply with

Kind Regards....

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manirul
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#8
In reply to #6

Re: MV Cable Sizing For Sckt

03/10/2008 12:44 AM

Assalamualikum Yasser

you can contact any mfr for the catalog. if you have IEE 7671, BS6622, you will get all those data.

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nambiathiri
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#5

Re: MV Cable Sizing For Sckt

03/08/2008 9:17 AM

each cable shall be of min. 350sq.mm considering sckt current withstand capability
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sj
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#9
In reply to #5

Re: MV Cable Sizing For Sckt

03/14/2008 11:57 PM

Can you suggest / send a link/email on any reference ( book / standard / paper ) that elaborates this criteria ? Thanks
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Sparkstation
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#10
In reply to #9

Re: MV Cable Sizing For Sckt

03/15/2008 12:13 AM

Hello sj

The Post made earlier by nambiathiri

Refer Post: http://cr4.globalspec.com/comment/194099/Re-MV-Cable-Sizing-For-Sckt <"each cable shall be of min. 350sq.mm considering sckt current withstand capability">

The reason the minimum for any conductor in the circuit to be of 350 square mm is simple:

Refer your own Post #1: http://cr4.globalspec.com/thread/18588/MV-Cable-Sizing-For-Sckt

<"* Considering the short circuit level, the minimum crossection required is 350 sqmm ( aluminium conductor, XLPE insulated ).">

And because if there is a fault, there may be a short circuit on ANY conductor at the total fault current which you specified yourself, then all conductors no matter how many are in parallel, each of those conductors must be able to withstand that short circuit current.

So ALL conductors connected to a circuit must be able to withstand the short circuit current safely, without injury, fire or explosion, and that ensures protective equipment, (circuit breaker/fuse/other), shall disconnect without damage occurring to persons or the installation.

It is a matter of simple reasoning, and does not require any reference to elaborate.

Trust that explains it for you.

Kind Regards....

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