Learner query on resistance tables for copper conductors and resistivity

[zempo]

Hi everyone

Table A.1 in Guidance Note 3 shows various values of resistance/metre for copper conductors at ambient temperature, including e.g:

18.10 milliohms/metre for one 1mm sq. conductor

0.387 milliohms/metre for one 50mm sq. conductor

But these given values of resistance do not exactly follow an indirectly proportional relationship with csa (i.e. 18.10 / 50 = 0.3620 and not 0.387).

Does anyone please know why this is? It looks like the table takes into account another (physical? chemical?) property apart from length and csa.

Thank you for any help. Zempo :cheesy:

 

[SirKit Breaker]

The only other thing i can think of which would affect resistance is temperature. High temp higher resistance, lower temp lower resistance. The tables in the regs work off an ambient temp of 20 degrees C if my memory serves me/

Cheers...........Howard

 

[PC Electrics]

The tables take into account the standard tolerances for the csa of the individual wires according to BS and give figures for the top of the tolerance band, hence the apparent lack of linearity.

At least, that's my theory and I'm sticking to it. Don't actually remember where I got that nugget from.

 

[zempo]

The only other thing i can think of which would affect resistance is temperature. High temp higher resistance, lower temp lower resistance. The tables in the regs work off an ambient temp of 20 degrees C if my memory serves me/

Cheers...........Howard

Hi Howard

Thank you for your reply. Yes, the table I was looking at was giving figures at 20 degrees celsius (Table A.1 'Values of resistance/metre for copper and aluminium conductors and of R1 + R2 per metre at 20 degrees C in milliohms/metre' on p.104 IEE Guidance Note 3 Inspection & Testing).

I have to admit I am still confused. If the tabulated figures are all given for 20 degrees celsius I can't imagine where the increase in temperature comes from. Maybe they are using another formula for something like heat dissipation as well?

Thanks for your quick reply again!

 

[telectrix]

could it be that 1mm cable is solid , single strand crap, whereas the larger cables are stranded.

 

[johnboy6083]

its reactance. this is nelgible for smaller cables(<16mm) but plays a part in larger cables, or very long runs. The skin effect also has its part to play, but how much i dont know. ive just been doing some academic research on the design of the job im doing at the mo, and reactance has its part to play in this too.

 

[JUD]

could it be that 1mm cable is solid , single strand crap, whereas the larger cables are stranded.

its reactance. this is nelgible for smaller cables(<16mm) but plays a part in larger cables, or very long runs. The skin effect also has its part to play, but how much i dont know. ive just been doing some academic research on the design of the job im doing at the mo, and reactance has its part to play in this too.

It doesn't work out for 2.5mm² either so it can't be nothing to do with it being stranded.

Table A.1 -- 2.5mm² = 7.41mΩ/m (18.1 ÷ 2.5 = 7.24).

@johnboy Table A.1 is for resistance/meter. If it was reactance that was making the difference surely it would be impedance​/meter.


P.S. What does nelgible mean??? :cheesy:

 

[johnboy6083]

very true JUD. my mistake.

Just to clarify, this is the reason that current carrying capacities dont add up either. Eg, a 120mm cable is rated at 485A (3ph, horizontal on tray), but 240mm cable is rated at 715A for the smae conditions and install method.

 

[JUD]

very true JUD. my mistake.

Reactance was my first thought too. I was thinking more of the volt drop tables which include reactance at 25mm² and above.

 

[Inteificio]

Skin effect and reactance are negligible for smaller cables, but the difference between the tabulated values and calculated are also negligible.

Most of the maths in the regs guide is extremely dumbed down, normally based on DC too, expect discrepancies.

Compare the normal resistance calc with the skin effect one and you will understand why.

Who cares if your answer is 1% out if you save 10 mins to work out the answer; your instrument deviation will skew the results more than that.

 

[Des 56]

This is what I found

Solid single cores have a greater ampacity ( that's a nice word)than solid cores in a strand
The difference gets less and less after 7 or more strands are wound together

So on a small 1.0mm, there is a noticeable difference comparing size and resistance to larger cores ,as the size increases ( + stranded cable) the difference decreases, because the stranded gets better ampacity (there it is again

) the more strands that are wound on the bunch


Maybe that's why stranded was always- imperial equivalent 7/.029
Better ampacity ( there it is again) with flexibility