Tricky radial

[Observer]

I have a somewhat odd radial circuit serving one side of a kitchen - circuit diagram below (sorry if it strains the eyesight). I'd appreciate some expert (constructive) advice on the best way to remove the potential overload.



Although, AIUI, the 32A MCB is suitable for the runs of [edit]2 x[/edit] 2.5mm and 4mm, the circuit then continues in single 2.5mm and is overloaded if all the high load appliances potentially connected are drawing power at the same time. This is, to be honest, quite likely. The washing machine takes quite a battering in a family of five and the tumble dryer is quite likely to be on at the same time. And the steam generator/iron is kept busy as well.

Clearly, one option is to downrate the MCB to 20A but that's not going to take the load of w/machine and dryer together so will keep tripping. My best idea so far is to take separate feeds from the first connection unit (where the 4mm run ends) to the existing w/machine and dryer FCUs and take the onward feed through a 13A FCU so protecting the rest of the 2.5mm circuit. This means that the high load appliances would be drawing through the 2 x 2.5mm/4mm runs of cable and I can live with limit of 13A on the rest of the circuit. The new cable runs would have to be on the surface, likewise the new FCU for the rest of the circuit, but that's bearable because they'll be tucked behind the machines.

I understand that the feeds to the FCU should ideally be in 4mm but I'm unsure about physical viability of terminating 2 x 4mm conductors. As that part of the cable will be protected anyway by the 13A fuse, would it be OK to run them in 2.5mm?

Hope this makes some sense and looking forward to comments.

Thanks to ImageShack for Free Image Hosting

 

[Adam_151]

what you suggest sounds ok, the 2.5mm to the FCUs, I see it as being no different to a spur on a 32A ring final, the adiabatic equation could be used to check that the upstream protection offers adiquate protection against shorts, but you'd need to take measurements with a loop/PSCC tester to use that (and I'm guessing you don't have one of them!), not sure what connection you have at the end of the 4mm², but if the termianls are the same size as sockets and FCUs then they will take 2*4mm², so you could take 4mm² to each of the FCUs in turn

If it was me, I'd probably downrate the MCB to 20A and install a separate 32A 4mm² radial circuit serveing the washing appliances.

Who is responsible for this circuit being like this anyway?

 

[JohnD]

Would it be feasible to link from the socket where the LCD TV is, back to the CU, thus making a Kitchen Ring? The cable could then be in 2.5mm. As you know, the "ring" doesn't have to be ring-shaped, it can double back in any convenient position (as long as people can work out where it is so they don't nail through it very often. As the circuit starts out in a pair of 2.5mmm T&E perhaps the installer origibnally planned a ring then changed his mind?

Another idea - is the cable run in conduit or anything where you could conveniently replace with 4mm and keep it as a radial?

Is there an RCD on that outside socket?

Nearly forgot.. it's in a kitchen so notifiable to Building Control.

 

[Adam_151]

As you know, the "ring" doesn't have to be ring-shaped, it can double back in any convenient position (as long as people can work out where it is so they don't nail through it very often.

A ring circuit should however be designed in such a way that the load is spread around it as evenly as possible, if you stuck all the load close to one end, then you'll overload that leg of the ring.

If you convert it to a ring, there looks like there will be some spurs off spurs that'll have to be sorted (looking at the FCUS near the wahsing appliances)

 

[Observer]

If it was me, I'd probably downrate the MCB to 20A and install a separate 32A 4mm² radial circuit serveing the washing appliances.

Not possible to bring in a new circuit from the CU without vey major upheaval. This is "making the most of a bad job" territory.

Who is responsible for this circuit being like this anyway?

Well... how long have you got? The 4mm section came from the original CU in meter cupboard to feed what was a 'lean to' utility area. Then we built a major extension and I commissioned new CU in garage in new extension. So sparks ran 16mm SWA from meter/isolator under house to new CU with cables back under house to original CU position for various original circuits to mate with original cabling in JBs. Why he ran 2 x 2.5mm to mate with the existing 1 x 4mm I don't know. Perhaps had no 4mm on the van?

Anway, next phase of house makeover was the kitchen and existing 4mm cable run was utilised because cable run back to JB was difficult. There was already some cable in position for external socket and I made the join between 4mm and the external socket run with 1 x 2.5mm without calculating likley loads. So - my bad (as they say). The bit that confused me was the section of 4mm. If it had been run as a ring I'd almost certainly have continued it as a ring. Anyway - it's done. The question is how to make the best of it.

 

[Observer]

Would it be feasible to link from the socket where the LCD TV is, back to the CU, thus making a Kitchen Ring? The cable could then be in 2.5mm. As you know, the "ring" doesn't have to be ring-shaped, it can double back in any convenient position (as long as people can work out where it is so they don't nail through it very often. As the circuit starts out in a pair of 2.5mmm T&E perhaps the installer origibnally planned a ring then changed his mind?

Another idea - is the cable run in conduit or anything where you could conveniently replace with 4mm and keep it as a radial?

Is there an RCD on that outside socket?

Nearly forgot.. it's in a kitchen so notifiable to Building Control.

Nope. Impossible to make a ring out of it without major work. It would be possible to extend 4mm up to the second connection unit but not past there. The way I see it, the overload potential can be obviated by ensuring that the two high load appliances come from the 4mm run and protecting the rest of the circuit with a 13A fuse.

And no - the external socket isn't on RCD (and the two 2G 13A sockets are close to back door). The sparks persuaded me that I don't want freezer on RCD circuit but I think the external appliance protections over-rides the inconvenience/cost of losing a few bags of frozen peas. So the next job is reconfiguring the split load CU (that's a subject for another post).

And yes, thanks for Part P reminder. Happily I still have some red/black cable knocking around.

 

[Adam_151]

And no way to get 4mm² SWA to the washing appliances by going around the outside of the house? and then possibly along under the kitchen units?

If not, then I think more a less what you propose is the best solution, give me a minute in paint

 

[JohnD]

About the RCD for the outdoor sockets - to avoid putting the whole kitchen on, if you don't want to, you can get an RCD Double in waterproof enclosure at enormous expense; but you can also buy an indoor RCD 13A FCU and interpose that. I was surpised to see Argos selling an outdoor socket kit with a waterproof socket and the RCD FCU, I think it was about £25 for the kit.

 

[Adam_151]

http://imagehost.darkernet.co.uk/i/kitchenradialnow9iw2.JPG

This is what I recommend, not sure if its exactly what you were first proposing, but a picture speaks a thousand words and all that...

red = 4mm²
blue = 2.5m²

It might be a little tricky to loop the 4mm² cables between the supply side of the FCUs, but it shouldn't be too much of a problem as long as you do it neatly and cut the cores to the right length, etc

 

[Observer]

And no way to get 4mm² SWA to the washing appliances by going around the outside of the house? and then possibly along under the kitchen units?

If not, then I think more a less what you propose is the best solution, give me a minute in paint

If I can terminate 2 x 4mm on the 'output' side of the first flush connection unit, I can take 4mm to each of the FCUs for the w/machine and dryer. I'd then need to crimp some 2.5mm onto the onward cabling and bring it out into a surface mounted 13A FCU (I think I have to do that anyway). But I don't see the point in using 4mm to each when they're drawing <13A anyway.

If you mean take 4mm to the second connection unit then yes, that's possible (surface run better than outside?) but access to the second connection unit is tricky - I'd need to cut an access panel in the back of a cupboard (another bad )

 

[JohnD]

Looks neat, and will be OK until you buy a dishwasher!

 

[Observer]

http://imagehost.darkernet.co.uk/i/kitchenradialnow9iw2.JPG

This is what I recommend, not sure if its exactly what you were first proposing, but a picture speaks a thousand words and all that...

red = 4mm²
blue = 2.5m²

It might be a little tricky to loop the 4mm² cables between the supply side of the FCUs, but it shouldn't be too much of a problem as long as you do it neatly and cut the cores to the right length, etc

Yeah - thaks Adam. It makes sense. I should bite the bullet and cut an access to the second connection unit anyway. You're sure I can get 2 x 4mm conductors in the terminals of an FCU?

 

[Observer]

Looks neat, and will be OK until you buy a dishwasher!

Happily it's already there on a separate, proper ring with m/wave, hob ignition, extractor hood, etc.

 

[colport27]

Ok, this post has taught me something........I think.
All you guy`s had to learn somewhere right?, but if somebody with say 20 years more experience in the trade told you something........you might believe him?.
I have always been told that radials within kitchens are .......er.......frowned upon......yet after a little research after reading the comments in this post I cannot find anything to substansiate this comment?

 

[Adam_151]

I have always been told that radials within kitchens are .......er.......frowned upon......yet after a little research after reading the comments in this post I cannot find anything to substansiate this comment?

You sure thats not in repect to kictchen fitters doing work and splitting the ring in half, creating a radial branched at the CU, with inadequade protection?

Nothing wrong with a radial if its properly designed, and it might be argued that a 32A radial is better for a kitchen than a ring, a radial can't get unbalenced due to large loads clustered together like a ring can