How to Choose Correct Cross-Section of Cables

Refer BS 7671 or manufacturer's literature.

Calculate it!

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Are you measuring them or selecting them? Measurement can be done using length measuring tools. Selecting them is something a qualified Electrician does using your national standard.

Reported

But nothings been done about it.

What's to calculate or determine. Contact the cable manufacturer with the part number and look up the outer dimension in their catalog.

From first principles, following British Standards:

1. Cables serve electrical loads, therefore, cable ampacity Iz shall commensurate with design load current, Ib at least; Therefore, know your design load for a start.

2. Current flow causes voltage drop, Vd, mV per Ampere per unit length of cable - cable impedance may be obtained from cable manufacturers/design codes

Voltage drop shall not be greater than that stated in the design code.

3. Faults may occur so cables shall be protected against overcurrents - In (nominal rating of overcurrent protection device - OCPD)

4. Tabulated cable ampacity, It varies with cable types, and in accordance to factors that affect heat transfer to/from cable - C, correction/adjustment factors, eg.

• Ca - ambient temperature
• Cg - grouping of cables
• Ct - thermal insulation, etc. (refer design code)

Iz = It x C

Select your cable type, method of installation, etc. Refer to tabulated cable ampacity, cable impedance & correction factors

5. Therefore, size your cables as follows:

Iz ≥ In ≥ Ib ;

Vd ≤ max volt drop in design code.

In addition where OCPD affords protection against overload, I2 operating current of overload protection device ≤ 1.45 Iz

Rgds.

Select the correct size of the cable considering the following

1)continuous current rating

2)Max. voltage drop permissible

3)short cct rating of the cable checking the system fault level

4)Cable laying condtions

5)max ambient Temperature

6)Single core or Multi Core

7)Earthed cable or unearthed cable

The cable manufacturer's catalogue will provide all the details for the above points

V.Natarajan

Use the Force. Just as everyone here has done!

Usually one has to choose the cable according to a national electric code-NEC, BS, VDE and so on. You may chose the insulation material-as PVC, XLPE EPR and other- and so to limit the conductor maximum permissible temperatures-for continuous load, short time emergency load and short-circuit. You may chose the conductor-usually copper or aluminum. You have to increase the cross section conductor area if the voltage drop-for continuous load or starting-will require, and you have to check if the cable withstand the presumed short-circuit current[you may use IEC 60949 for non-adiabatic phenomenon to increase a bit the withstand current].

But this in case in which the cable cost is your interest.If you are worrying about losses cost in this cable you have a further increasing of the cross section area up to the point where the total cost of the cable and the losses cost is the minimum.

Dear Friend,

Pl. refer the following links, and you will find good amount of data/details about choosing cables. This topic was discussed several times in this Forum. You can search and see that also. You can google and find more details.

http://www.electrical-installation.org/enwiki/General_method_for_cable_sizing

• 1- Possible methods of installation for different types of conductors or cables
• 2- Possible methods of installation for different situations:
• 3- Examples of wiring systems and reference methods of installations
• 4- Maximum operating temperature:
• 5- Correction factors
• 6- Admissible current as a function of nominal cross-sectional area of conductors

http://www.openelectrical.org/wiki/index.php?title=Cable_Sizing_Calculation

http://www.openelectrical.org/wiki/images/2/2b/Cable_waterfall.jpg

http://www.polycab.com/blog/2012/11/28/polycab-guidelines-for-selection-of-cables

http://www.belden.com/resourcecenter/tools/cablefinder/upload/DataTuff-Cable-Selection-Guide.pdf

http://www.aeicables.co.uk/literature/CurrentRatings.pdf

DHAYANANDHAN.S

As I expected, people are again focusing on solely the cross sectional area of the presumably copper conductors without any understanding of where or how these cables will be used. Nothing about any attribute of the insulating material is being considered at all. Nothing about the thermal conductivity of the insulation or media the cable rests in matters one iota to this forum. The difference between continuous, peak, or even fault mode operation parameters cannot come into the conversation. Suspended cable across the Sahel is considered identical to cable submerged under Puget Sound. Nobody ever worries about any loading effects until a fly-back and reflection of a load triples the supply voltage at one point causing repeated insulation breakdown at just one point in a transmission line.

Did I rant about the unknown frequency of the current? No. Frequency! Frequency! Frequency! Duty cycle! Frequency! Skin effect! Frequency!!!

You are right to be red Fred. But slow down a bit. The open post asks how to choose the "size" of a cable "correctly and efficiently". Nothing more. Correctly: see the Code- or calculate it if the Code does not answer. Efficiently: the total cost could be involved: installation and losses.

Usually one cannot state the cable raceway but it is dictated. For most of the cases the Code has an answer.

Dear Mr.redfred,

I appreciate your views and I feel, the topics referred by you is not receieving due attention.

What I want to mention is that at least one third or half of the points expressed by you is taken care to extent in my Reply (Refer Post No.16 above) in the 1st Web-Link http://www.electrical-installation.org/enwiki/General_method_for_cable_sizing has 6 sub-topics which takes care for Temp.Rise, Installation method, with several of cables, in Trenches, Cable Trays Earthing Resistance etc.

Perhaps, the PERFECTION which you expect has to (or will) come by PERSEVERANCE and continued efforts, by the ENGINEERS in the days to come.

DHAYANANDHAN.S