sizing cables from site only

kennyjb · Jan 19, 2009

Hi everyone

I'm new to this forum so thought I'd ask a simple question and see if anyone can answer this with an easy solution.

I've just installed 3 new circuits to a renavation job (originally a butchers workshop now being used as a studio) the client wanted to keep the 3 phase supply and board, although now i've ripped everything out that's no longer needed we have only 9 ways being used on a 36 way merlin gerin board.

One ring final, one radial plus studio lights, sockets on 32A and 20A rcbo's.
the question I have is can anyone tell me an easy way to size the incoming supply cable and earth, sounds stupid but i started filling in the installation certificate after testing and I couldn't tell what size cable was being used.

before people ask yes i am a qualified electrician i just usually work as a contractor doing a lot of first fix work containment etc so never really needed to know cable sizes in too much detail (this is a private job)

Is there a way of knowing by the cable diameter maybe, some cables have it marked on the outer insulation of the cable but this one doesn't neither does the earth

any answers appreciated guys

thanks ken

Spudnik · Jan 19, 2009

Hi Ken and welcome!

Got yourself a tricky one there!

There is no easy way to tell, only from experience really.

Are there any other sparks on site that may have an idea?

Its really difficult without seeing them in situ.

You could always hazard a guess with 25mm & 16mm.

They are unlikely to be any smaller on a 3ph incommer.

Worse case scenario is that they will probably be 35mm.

Reg Man · Jan 19, 2009

If its any help at all, I knew a bloke who kept 2 inch cuttings of all the most common sizes of cable upto around 35mm and stuck them to a board. He would just compare. Came in handy sometimes.

More complicated way is measure one copper thread of the cable with vernier and do the equation 3.14 x the measured diameter squared and divide by 4.
This gives total area of one thread. Then multiply by number of threads in the one core (not all the cores). Gives overall size of cable. Nice and simple !!!!

WarrenG · Jan 19, 2009

Reg Man said:
If its any help at all, I knew a bloke who kept 2 inch cuttings of all the most common sizes of cable upto around 35mm and stuck them to a board. He would just compare. Came in handy sometimes.

More complicated way is measure one copper thread of the cable with vernier and do the equation 3.14 x the measured diameter squared and divide by 4.
This gives total area of one thread. Then multiply by number of threads in the one core (not all the cores). Gives overall size of cable. Nice and simple !!!!

Wow! you would think that something would have been invented now for this wouldn't you?

Trouble is different manufacturers different cables types.

Afraid no easy answer!

Guest123 · Jan 19, 2009

Hey.

No substitute for experience really. Pop into the wholesaler and ask for a couple of 2inch off cuts off the different size cables and keep em in your van.

Cheers

ezzzekiel · Jan 20, 2009

i find cable clips are ideal for this, the perfect fit clip is the cable size

ian.settle1 · Jan 20, 2009

Can't remember off hand which reg it is but the maximum size protection for a lighting circuit is 16A not 20A as you have.

Spudnik · Jan 20, 2009

ian.settle1 said:
Can't remember off hand which reg it is but the maximum size protection for a lighting circuit is 16A not 20A as you have.

kennyjb · Jan 20, 2009

ian.settle1 said:
Can't remember off hand which reg it is but the maximum size protection for a lighting circuit is 16A not 20A as you have.

Hi Ian

You've slightly mis-read my original thread there.
'3 new circuits' I installed a 32A rcbo on the ring for the main studio, a 20A rcbo on the radial circuit for 4 sockets in the office (in 3 compartment trunking)
The lights are on a 10A mcb (was left spare in the board)

thanks for your replies everyone, glad to see its not just me on that one great idea though, keeping short lengths of different size cables, think i'm gonna give that little formula above a try, thanks again

Ken