Excellent explanation Dave Very useful information, the finger and thumb method is good I was tort that back in the 80s when I was at college still use it now . Fantastic video as always mate 👍👍👍❤️

Plenty of good tips there, thanks very much.

Great video. How to tell what cable size to use for a floodlight?

Its worth pointing out that conductor mm2 sizes are nominal values. Per BS6360 the critical factor is actually the resistance of the conductor (& thus the current carrying capacity) NOT the CSA. For 2.5mm2 conductors the maximum specified resistance is 7.41 ohm/km at 20oC (resistance rises with temperature) is likely to be achieved with an actual CSA less than 2.5mm2. It gets complicated with different cable specs as the BS6360 max resistance will be different for tinned copper conductors or (twisted) multicore cables or aluminium conductors even though they may all be 2.5mm" nominal. The difference between the nominal CSA and the actual CSA will be greater with larger conductor sizes (16 - 70mm2+)

Good stuff. Worth knowing

Hi dave excellent video thank you so much, can you do more and easy method to find the cable size, thank you

Thanks for the video. Any chance you have an SWA version in the pipeline. I struggle trying to see what’s installed to a shed or garage especially if I want a 32 amp feed and some has put a cable in and I want at least 4 mm

Great video. Can I ask if you can do a video similar but with singles. I find it so hard differentiating between 1.5mm and 2.5mm singles in particular. Thanks

what can be confusing is when you do a test on an old installation and there are 2.5mm t&e with either a 1.5mm cpc or indeed a 1mm cpc without taking a measurement with a meter it is sometimes hard to determine what size cable is used and even harder to tell if they have used mixed cables over the years. for instance you could have an r1 and rn of 0.57 and a r2 of 1.20 which if it is 2.5mm with a 1.5mm cpc would be a fail but if it was a 2.5 with a 1mm cpc would be a pass but the reading of 1.20 for the r2 would suggest that either you have a loose connection on the cpc or the cables are mixed or parallel earths....im waffling here but some of you will get my point Edit 0.57 x 1.67 =0.95 ...... 0.57 x 2.5 =1.42

loveing them practical exampels. using fingernail for a rough guestimate. brilliant!

Many thanks

I applaud the videos you’ve put up - a very useful resource, that you’ve put a lot of effort into. Thank you . I feel bad about criticising - I feel drawn to point out a couple of important things that make me uneasy in what you say - The fairly involved description of how to pronounce “mm2” I find totally bizarre. I say this as someone with a mathematics degree, and extensive high level physics knowledge. (I have a couple of physics / engineering parents) . I’m afraid you don’t seem to understand how dimensioning of units works. Despite what you say about “saying it the wrong way” would mean 4 mm2 was actually 16 is totally incorrect - because of the order of precedence in arithmetic. The square binds to the Mm, before the numbers, hence what you say is incorrect . If you wanted the version you assert is being communicated - you’d have to insert brackets to change the order of precedence. The confusion you give is not a a confusion and in fact you calculate the arithmetic incorrectly. I’m also very uneasy when you talk about measuring the cables. Metrology is always difficult, and you give some excellent rules of thumb later - thank you. However - measuring the outside of a cable is dangerous as the thickness of insulation is not controlled - so cables can be of different outside dimensions. Different plastics can be used for the insulation with very different thicknesses. Indeed especially thin insulation is used for flexes and cables that are expected to suffer bending during their service life (usually inside machines rather than in the sort of situations electricians come across) so as to minimise the shear stresses on the metal during cyclic bending eg through vibration - and increase the life of the connection . Such wires definitely do not give away their cross section by their external dimension. As you rightly state - the cables are all measured for cross sectional area of the conductors. Therefor - you have to measure the conductors - not the conductor and insulation package. How do you do that ? Clearly not with a conventional tape measure, as you rightly point out the dimensions are too small. Instead you use the tool for the job - a “mic”, a micrometer calliper , or these days I use a digital calliper (my tool Of choice being a mitutoyo absolute). Of course one has to be aware of the tolerance and measurement accuracy of such a device, but then you are measuring the copper. I usually measure in a few places and rotate it round, in case it’s been squished, and is not presenting a circular cross sectional area. This is much easier than it sounds - and I always keep some callipers in my bag as they are so useful when measuring small distances eg measuring the diameter of a metric bolt etc. Metrology is difficult esp for small measurements below a mesoscopic scale. Well done for listing your rules of thumb though which give a good pragmatic guide and I found very useful. I too have a board i siliconed examples of cable to years ago I could eyeball when inspecting stuff, suspicious as I was of my ability to remember repeatable absolute sizes by eye.

Hi. How can I tell if it have 16mm or 25mm tails. Could I measure the circumference. Need to know for dno upgrade for ev charger

I never knew about Line and Live - here we always call the L a Live as that is the current carrier. The Neutral is 0V or same as Earth.

Pythagoras (a Greek mathematician) has nothing to do with Pi ( the ratio of a circle’s circumference to its diameter).

Thanks Dave