Parallel T&E Cables

[BJC]

I have 6mm T&E in steel conduit going to a cooker unit which is no longer used. Also in the conduit is a 2.5mm T&E, also not used. I intend to use the 6mm for kitchen wiring with a design current of 36A. Kevins cable calculator advises the 6mm is good up 38A but it appears a bit marginal (especially since I was going to protect with a 40A RCD). Is it permissible to parallel up the 6mm and the 2.5mm at both ends.

 

[andy]

I have 6mm T&E in steel conduit going to a cooker unit which is no longer used. Also in the conduit is a 2.5mm T&E, also not used. I intend to use the 6mm for kitchen wiring with a design current of 36A. Kevins cable calculator advises the 6mm is good up 38A but it appears a bit marginal (especially since I was going to protect with a 40A RCD). Is it permissible to parallel up the 6mm and the 2.5mm at both ends.

no. you can only double up if both cables are the same CSA

 

[Lectrician]

when you say 'kitchen wiring' - is this for the cooker, sockets, or a submain for both??

 

[BJC]

when you say 'kitchen wiring' - is this for the cooker, sockets, or a submain for both??

A submain for all fixed appliances (fan oven, fridge/freezer, dishwasher, gas hob, washing machine/drier, cooker hood, lights). Kitchen has a separate RCD protected ring main.

 

[mapj1]

Either (preferred) make it a 30A radial, and just use thr 6mm and then 4mm as daisy chain after it emerges, or use the wires as two limbs of a ring main, as if they were both 2.5mm but dont just paralell it up at both ends and say 6mm+2.5 mm - the current sharing will not work properly, and you may overheat the 2.5mm.
Both these options limit you to a 32A breaker - but how much do you really need?

 

[Adam_151]

32A submain, and 16A Radial (probably good for the oven?) back to the main CU

 

[ban-all-sheds]

Why a submain?

 

[Adam_151]

Why a submain?

Its what the original poster wanted, and also ensures that for example a burnt out element which shorts to earth in the dishwasher doesn't take out the freezer as it might if you have:

100A service fuse -> 32A B Breaker -> 6mm² Radial -> 13A Plug top fuse

In the above case its possible that both the breaker and plug fuse get taken out simulatanusly (I've seen it happen - more than once)

if you had

100A service fuse - 32A B submain breaker -> 16A B dishwasher breaker-> 2.5mm² radial -> 13A plug fuse

Then the 16A breaker should just take out the dishwasher, leaving the freezer on.

Also, you could use a split load sub main board to get rcd protection on some circuits, but not others, eg, you could have the freezer and lights on non rcd, and some or all of the rest on rcd.

That all said in practice a 32A 6mm² radial would probably be fine

 

[BJC]

Actually what I was planning to do was to use the 6mm as the input to a new CU (via suitably rated RCD) in a kithen cupboard then have individuallly fused (MCBs) radials to each appliance (via a central bank of DP switches).

 

[ban-all-sheds]

Use a grid plate with switch & fuse modules, switches with appropriate legends - much neater...

 

[BJC]

Planned to use MK grid plate switches with legends but wasn't aware you could get fuse modules (which is why I was stuffing everything in a hidden CU).

 

[Big_Spark]

BJC, what your proposing is uneccessary and in my opinon daft.

 

[BJC]

BJC, what your proposing is uneccessary and in my opinon daft.

I agree it is not necessary to do this to get the appliances to operate but "daft " is quite subjective and not very informative. Do you mean it is against the regulations or just that you wouldn't do it this way.