Change: Electric Shower Feed to Attic Radial Circuit

[mike2016]

I plan to pull out an electric shower and want to convert it's power supply to feed a couple of sockets in my attic instead. The Shower isolator is fed mostly likely with 6mm square cable.

 

I'll get the sparks to replace the Consumer Unit fuse/RCD/RCBO with a 32amp one, assume it's much higher than that now. How do I arrange the power sockets coming off the junction where the electric shower isolator used to be to avoid any mistakes and overloading the 2.5mm square electrical cable I plan to use...? Should I run each socket back to this junction? Anything else I should consider?Thanks! 

[Jeremy Harris]

For a radial feeding outlets where part of the cable run is 2.5mm² the overload protection needs to be reduced from 32 A.  Depends on the cable runs (derating if in insulation, etc) so the MCB needs to be either 16 A or 20 A most probably.  Clipped direct 2.5mm² T&E is only rated at 27 A, which is OK for a 32 A ring final, where the assumption is that current will be shared around the ring, but not OK for a radial.

[mike2016]

So maybe upgrade to 4mm square would be more appropriate? 

[Carrerahill]

You have a couple of options, 2.5mm would really need a breaker of 20A - you could go 32A and use 4mm cable (clipped direct) if you think your loads are likely to exceed this, however unless your planning some heavy duty electric loads up there I suspect you will be fine with 20A.

 

My whole ground floor main ring is 20A (no idea why it was put onto a 20A) and I have never had an issue.

 

Also note, that just because you exceed 20A you won't automatically trip a 20A breaker. Over current protection at this level is about heat generated in cables, 6242Y cable (twin and earth) has a max operating temp of 70° - so let's say you load up the circuit a bit and it sits at 45°C - that is fine, the the breaker is designed to permit this, this is thermal overload protection, so it is actually perfectly acceptable to, for example, load a 20A breaker at 25A for a period and nothing will happen. 

 

The thermal protection of a B curve breaker is that at 1.25 x the rated current it will take about 15 minutes to trip, 22A for about an hour and a half etc.

 

 

 

 

Edited April 15, 2020 by Carrerahill

[mike2016]

Thanks - not planning on running much up there, CCTV NVR, 12v DC power supply for Alarm input, Vent-Axia PIV with 300w heater, Solar PV mains detection circuit, Vent-Axia bathroom fan, that's it really.....

[ProDave]

Another option that some of us have done that seems to generally be approved of, is to wire a 2.5mm ring circuit from the end of the 6mm.  It has become known as a "lollipop circuit"

[mike2016]

Oh, I like the sound of that....! The things I never knew existed until now.....!! 

[Carrerahill]

22 minutes ago, mike2016 said:

Thanks - not planning on running much up there, CCTV NVR, 12v DC power supply for Alarm input, Vent-Axia PIV with 300w heater, Solar PV mains detection circuit, Vent-Axia bathroom fan, that's it really.....

10A would do you!!

 

I'd just go for a 20A MCB, 6mm radial up to a suitable junction box, 2.5mm out and then create a ring or radial. It is certainly future proofed!