2.5mm or 4.0mm

[jabezz]

I have three sockets off a 4.0mm radial circuit serving three bedrooms. Can I add an extra socket in each bedroom using 2.5mm cable. (The circuit breaker is 30amp.)

[ryanj]

No.

Use 4mm^2.

[ban-all-sheds]

http://www.diynot.com/forums/viewtopic.php?p=81696#81696

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[jabezz]

Thanks Ryan J for your quick response. I'll havta buy 4.00mm cable -- I had loads of 2.5mm leftover from a previous (ring main) job.

[ban-all-sheds]

If it had been a 2.5mm² ring final, on a 30A breaker, he could have spurred off a socket in each bedroom with a single piece of 2.5mm² cable.

As it is a 4mm² radial, on a 30A breaker, he cannot add a branch with a single piece of 2.5mm².

Discuss.

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[ban-all-sheds]

ryanj wrote:

No. Use 4mm^2.

Alternatively, yes if you use 2 x 2.5mm² of the same length.

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[jabezz]

Hi Ban-all-sheds -- thanks for the link in response to my question about 2.5mm or 4.00mm cable -- however this link has a lot of info -- which particular thing should I be looking at?

ban-all-sheds wrote:

http://www.diynot.com/forums/viewtopic.php?p=81696#81696

[ban-all-sheds]

That link should take you straight to the reference post called SIZING MCBs & CABLES - that's the one to read regarding this question....

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[andy]

ban-all-sheds wrote:

If it had been a 2.5mm² ring final, on a 30A breaker, he could have spurred off a socket in each bedroom with a single piece of 2.5mm² cable. As it is a 4mm² radial, on a 30A breaker, he cannot add a branch with a single piece of 2.5mm². Discuss.

because on the radial, you can continue to add more sockets to the extra socket, so 2.5 would overheat. where as on a ring main, your only susposed to take 1 spur which should not exceed 13A so 2.5 is OK

[ban-all-sheds]

10/10

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[jabezz]

Greatly appreciate each one who posted replies. I not only know what to do, but also the WHY!

It so much more reassuring to learn the reason behind a standard, knowing it's not just some arbitrary rule.

Thanks!

[streethawk]

ryanj wrote:

No. Use 4mm^2.

but i can't get 4mm anywhere anymore... what am i meant to use - 6mm? bit excessive, surely!

[Spark123]

Have you tried:
http://www.tlc-direct.co.uk/Main_Index/Cable_Index/Twin_and_Earth/index.html

BTW the post you are replying to is over 12months old.

[fireman22]

jabezz, if you are only feeding a few bedroom sockets and you have central heating i imagine the load will be quite low so you could run 2.5mm radials and lower the amp rating of the mcb to 20A. but if you had an overload problem later this could be a pain. if you can get back to the fuse board you could change this circuit to a 32A ring using the 4mm cable as the first leg of the ring then from the last socket in 4mm round the rest in 2.5 then back to the board.

[streethawk]

Spark123 wrote:

Have you tried: http://www.tlc-direct.co.uk/Main_Index/Cable_Index/Twin_and_Earth/index.html BTW the post you are replying to is over 12months old.

I hadn't, no just the obvious suspects - wickes, screfix etc.

thanks for that

age is in the mind btw