Question Trouble ( need help )

[mh10acj]

Hello everybody,
This is my first post names Micky. I was wondering if anyone could help me out with this question I’ve have been struggling with it for a long time. As a result I’ve lost a large part of my hair, mainly from the frontal region .

Question: - Given that the resistance per meter of 2.5mm^2 cable is 7.41MΩ/M at 20(degrees), what would the approximate R1 + R2 reading be at the extreme end of a 25(meter) long 20A radial socket outlet circuit

I would like to know step by step how this is worked out and what you have to do to get the answer.

Thanks in advance for any help or assistance

 

[Adam W]

Ah yes I thought that looked strange - it might sound pedantic but you've put "7.41MΩ/M", but it's actually "7.41mΩ/m" as in milliohms, not megaohms. Appendix 9 of the onsite guide (page 166) gives you the resistance of R1+R2 per metre of copper conductors at 20°C.
All you do is select the right cable size (if it's T&E with the correct CPC) and multiply that by the length to give you the resistance at 20°C,
eg:
2.5mm² with 1mm² CPC is 25.51 mΩ/m
25.51 x 25m = 637.75mΩ, or 0.64Ω.

 

[Adam W]

Welcome to the forum btw!

 

[telectrix]

but who uses 2.5mm cable with 1.0mm cpc.

 

[Adam W]

Well that was just an example, but if you like I'll demonstrate with 1.5mm² CPC:

2.5mm² with 1.5mm² CPC is 19.51 mΩ/m
19.51 x 25m = 487.75mΩ, or 0.49Ω.

BTW the first value given for each size cable is for R1 only, ie the line conductor with no CPC.

 

[V]

sometimes they just give theory questions for understanding and not for real onsite working

 

[IQ Electrical]

sometimes they just give theory questions for understanding and not for real onsite working

Yes but calculating R1+R2 is a rather important theory question wouldn't you say?

While testing, any suspect measured figure can be cross-checked against the approximate distance and cable type to reference the expected reading with the measured value.

 

[V]

i was saying in general sometimes for classroom work they give questions that are not used onsite

 

[Adam W]

i was saying in general sometimes for classroom work they give questions that are not used onsite

Like doing cable calcs for BS3036 fuses - as if you're going to do that in the real world!

 

[V]

i luv rewireable fuses

 

[Mr.Si]

i luv rewireable fuses

Isn't that something that should be in the "Things you'll never hear an electrician say" topic?

 

[V]

no you mean " i luv the way city and guilds chance part1 and part2 numbers from 2360 to 233333333336778888 " or what ever it is this week

 

[mh10acj]

Thanks lads you really helped me on that question but I may need help with other questions in the future

 

[VoltzElectrical]

Fair play for posting, but when these questions come up, I always find myself asking 'why don't you ask the lecturer?' Unless you're doing a distance learning course I suppose.

 

[Adam W]

I agree that's what lecturers should be there for and if you have a good one they won't mind at all, but I understand it can be a bit daunting asking in front of the class or having things explained verbally, which is where t'internet comes into it's own.