Determining cable length

[sprint1]

Hi,
Im working on site in worthing and have been asked to find out the length of an installed cable.
Cable size 150mm 4 core
r1+r1=0.3
Ze=0.06
Any help on how to calculate the length of cable would be much appreciated.

 

[tony mc]

Find out the resistance of the 150mm cable from the regs.

Example. A ring final cct R1+R2 = 0.55 ohms. It is wired in 2.5 twin and earth

2.5 =7.41 m/ohms/m
1.5 = 12.10 m/ohms/m

7.41 + 12.10 = 19.51

0.55 x 1000 / 19.51 = 28.19 meters.

Hope this helps!

 

[telectrix]

roughly the mohms/m (R1+R2) for 150mm Cu is 0.260. so L = (0.3 x 1000)/0.26 = 1154m

 

[sprint1]

Cheers for that,could you point me to the page in the regs book that i need.

 

[MarkieSparkie]

We need further information to be able to help you sprint. Cable type? Copper or aluminium conductors? Did you mean R1+R2 = 0.3 Ohms?

 

[telectrix]

table 9A in osg goes up to 50mm.

 

[sprint1]

Ok.
Cable is a 150mm swa/lsf copper conductors.
We were only allowed to do an r1+r1 across the yellow and blue phases as access to the 70mm cpc was unobtainable.i was told that we would be able to determine the cable length from these readings.

Regards Sprint

 

[telectrix]

so what you have is the loop resistance of RL2+RL3, both 150mm. so find the value of R1+R2 for 150mm in mohms/m ( i calculated it from the 50mm in table 9A by dividing by 3) then calculate the length.

L= (Rtx1000)/mohms/m

 

[oxocube]

Why not use a tape measure?

The cable resistance is dependent on temperature in every part of the cable so using this method could be very inaccurate.

 

[Silly Sausage]

The cable resistance is dependent on temperature in every part of the cable so using this method could be very inaccurate.

True, but he's measured the R on a dead cable so you could "assume" it's knocking around 20degC.

Resistance of 150mm copper core @ 20degC is 0.1240 ohms/km, got from this table...
http://www.eland.co.uk/documents/SWA Cables - BS5467.pdf

 

[telectrix]

True, but he's measured the R on a dead cable so you could "assume" it's knocking around 20degC.

Resistance of 150mm copper core @ 20degC is 0.1240 ohms/km, got from this table...
http://www.eland.co.uk/documents/SWA Cables - BS5467.pdf

so, that would make his 0.3ohms ( for 2 conductors) 2.42km, or 1210m each close to what i said in post #3.

 

[Big n Daft]

Working it out is only an educated guess due to the many variables and its length.
If you cant measure the run then its only method open to you.
Do you need to know the length to work how much beer money is involved:drunk:arty:

 

[telectrix]

at todays scrap prices i reckon 150mm stripped clean is worth around 3 pints/metre

 

[johnboy6083]

to be honest mate, measuring the resistance is not going to be an accurate way of working out the length of that cable, unless you have a very accurate meter, or you opnly require to find the length tyo withing say 10m on a very long run.

This is due to the fact that on a cable of that size, the resitance is going to be very low, and yopur meter rounding up to the nearest decimal place will have a massive influence on your calculation.

 

[SW1970]

Why not use a tape measure?

The cable resistance is dependent on temperature in every part of the cable so using this method could be very inaccurate.

And joints and poor connections etc.