# What exactly does cable description describe?

Discussion in 'Electricians' Talk' started by Trofim, Sep 9, 2013.

1. ### TrofimNew Member

Probably a very silly question, and obvious answer, but I can't quite understand how the size in mm relates to the actual cable. I've bought 2.5 T&E from different sources which appears to slightly vary in size depending on vendor. What does the 2.5 cm actually refer to? The diameter? Radius? And from time to time I come across lengths of cable disposed of in skips. I've got a good 6 metre length of cable that is twin and earth and looks as thick as that used on my cooker, and it says 2 x 10 + 4 on the side. It's 16 mm at its widest. What would this be?

2. ### Sean_orkScrewfix Select

you've found 10mm twin core which has a 4mm earth within

cable is specified by detailing the cross sectional area of the current carrying conductors - so 2.5 actually refers to a 2.5mm CSA of the conductors within

3. ### LectricianScrewfix Select

Your skip cable is a 10mm² Earth. A standard and recent sized cable.

² Symbol. It is 10mm squared. Cables are measured by cross sectional area.  The insulation and sheathing is not taken into account in any measurements.

4. ### TrofimNew Member

Sorry - CSA. My understanding is that a cross section of a cylindrical object, e.g wire, is the diameter. So theoretically, if you placed the three copper wires in a row and measured one side to another, that is the sum of their diameters, that is the CSA?

Update: I think the penny may have just dropped - 2.5 means two wires 1 mm in diameter, and the earth which is 0.5 mm?

5. ### retiredsparksWell-Known Member

Hi Trofim
Just to calrify.
2.5mm squared   is  an annotation which means that the round conductor has an 'Equivalent' cross sectional area of 2.5 square millimeters of copper.

A ' cross section' is a section formed by a plane cutting through an object, usually at right angles to an axis.  So if your were looking at a cylinder the CS would indeed be the diameter.  The AREA would be different.

Does this help ?
RS

PS
stop crawling around skips !

6. ### TrofimNew Member

Erm, so the 2.5 is 2.5 sq mm, that is the combined cross sectional areas of all three wires? I think I'm getting there!

7. ### joinerjohn1Screwfix Select

The 2.5mm refers to the cross sectional area of the live and neutral wires individually. Both these are 2.5mm sq. I think the earth wire will be either 1.5mm sq or even 1mm sq.

The formula for working out the area of a circle is Pi xR squared where R = the radius. For your example of 2.5mm twin and earth, the effective diameter of the live and neutral cables are approximately 0.9mm

8. ### Bazza-sparkWell-Known Member

Hi Trofim

No that isn't quite right.  It is the cross sectional area of the face of the conductor.

Hope that helps.

Kind regards

BS

9. ### unphasedScrewfix Select

I overheard a discussion in a DIY shed a few years ago where the knowledgeable assistant was carefully explaining to a customer that the reference to the size of a cable in mm2 was measured across the width of the sheath. Thus, the 10mm2 he was asking for to be used on a shower circuit was 2.5mm2 as measured across the width of the sheath. I wonder if the shower ever caught fire.

10. ### Bazza-sparkWell-Known Member

Its frightening how little people know and understand yet try to bullsh1t there way through isn't it?  And to think that this is barely scratching the surface.

Kind regards

BS

11. ### sinewaveScrewfix Select

Kinnel!  :'(

Ya not wrong there Bazza!

12. ### LectricianScrewfix Select

You're nnot getting there, lol.

2.5mm/sq is the area of the live OR neutral. The earth on this size cable is typically 1.5mm/sq.

For a perfectly cylindrical conductor, think about ² (Pi r squared). Half the diameter of the conductor, times it by itself, times it by Pi (3.14).  This gives the area (CSA).  Only works for perfectly "round" conductors though.

13. ### Mr. HandyandyScrewfix Select

....or square ones!

14. ### Mr. HandyandyScrewfix Select

"Hi Trofim
Just to calrify.
2.5mm squared is an annotation which means that the round conductor has an 'Equivalent' cross sectional area of 2.5 square millimeters of copper."

Nope.

Mr. HandyAndy - Really

15. ### retiredsparksWell-Known Member

..........or whatever material the cable conductor is made from.............but for the purposes of this topic I have all assumed Mr Trofin was referring to standard cables found in domestic premises..........Go ahead, Mr Handy...make my day !
RS

16. ### Mr. HandyandyScrewfix Select

2.5mm squared is 2.5mm X 2.5mm = 6.25 sq.mm.

2.5mm&#157;­² is 2.5mm x 1mm = 2.5 sq.mm. (square millimetres not millimetres squared)

Mr. HandyAndy - Really

17. ### TrofimNew Member

I get confused between 2 square miles and 2 miles square and the like. If I understand rightly now, 2.5 sq mm refers to the cross-sectional area of either of the main wires, live or neutral. If they DID make wires square in cross section, then each side would be the square root of 2.5, that is 1.581 mm. Right?
My confusion was compounded by my misreading of 2.5 as square centimetres.

;-)

19. ### retiredsparksWell-Known Member

Quite right Andy Pandy.
2.5mm2    is    2.5 square mm .......and the "D" makes all the difference.
Quick typing and the abuse of tramadol can have serious maths consequenses.
RS

20. ### joinerjohn1Screwfix Select

I'd like to see the look on a sparks face when his gaffer told him he's be first fixing some 2.5cm squared cables.