Calculating R2 from R1

[Superspark23]

Ive all ways for a bit of 2.5/1.5 Twin and Earth used this value 1.67 to calculate my R2 from my R1 values.

2.5/1.5= 1.66666666666666666667

so if i got my R1 as 0.34 i would expect my R2 to be 0.57

But the electrician i was working with disagreed with this and pulled out his onsite guide and said i should all ways be using the actual resistivity of the cable 12.10/7.41 = 1.63 meaning his calculated r2 would be 0.55

whose method is right?

 

[oxocube]

Its not an exact science and I would'nt get to worried about it.

For example, the resistances of 12.10/7.41 are for 20c but at full load the cables will be at 70c, and your 20c calc of R1/R2 should be multiplied by 1.2 (rule of thumb)

 

[Sparky Ninja]

He's wrong. The relationship works with the CSA not the resistivity values. In theory it would work similarly, as long as accurate resistivity value were at hand.

 

[Sparky Ninja]

Its not an exact science and I would'nt get to worried about it.

For example, the resistances of 12.10/7.41 are for 20c but at full load the cables will be at 70c, and your 20c calc of R1/R2 should be multiplied by 1.2 (rule of thumb)

that's not really on topic.

 

[Superspark23]

Thanks that's exactly what i thought. Every one who's ever told me this in the past has always said 1.67 rule. I think its a case that's he's taught himself a lot of various things as he's gone along he's nearing retirement. When he would have started there wouldn't have been hardly any testing maybe just a test lamp. There's several things he's told me that i don't agree with. I think as long as i don't listen to what he tells me as gospel then research the actual answers then I should be fine. I do that anyway. When it comes to testing i'm up to speed and will happily go and do periodic on my own.

 

[oxocube]

that's not really on topic.

Explain why not.

 

[Hawca]

Ive all ways for a bit of 2.5/1.5 Twin and Earth used this value 1.67 to calculate my R2 from my R1 values.

2.5/1.5= 1.66666666666666666667

so if i got my R1 as 0.34 i would expect my R2 to be 0.57

But the electrician i was working with disagreed with this and pulled out his onsite guide and said i should all ways be using the actual resistivity of the cable 12.10/7.41 = 1.63 meaning his calculated r2 would be 0.55

whose method is right?

both is right..

the method stated second by cable resistivity relies on factual numbers, like length and the copper resistivity and so is theory based physics.

the method you have used where by you test the R1 and multiply by 1.67 as the calculation 2.5/1.5...

first look at your calculation 2.5/1.5 is not 1.67 its 1.6666 recurring, also your test meter measures what is exacly there, where by the about relies on you getting the length perfect.
the last thing to think about is the temperature. the method above is based on 20c and your testing at what the temp is.

now we have a rule of thumb.. (how many have we got) our measured values should be around 0.05 of the calculated or we need to investigate.

you tested 0.57 he calculated 0.55 all is good

I hope that makes sense.

Now when it comes to using these methods, one is more for design and the other is more an aid for on site.