Dedicated RCD for boiler

[ldoodle]

OK, so 2.5mm is rated up to 18.5a when buried in walls so can see why it can't be used on a 32a MCB in radial. How does this apply in principle then as my ring mains are all 2.5mm cable off of 32a MCBs? I guess because the power is distributed around the ring the rating is effectively doubled?

It's not actually doubled, but there is recognition that the load is shared. The requirement is that the cable be rated at at least 20A, so if your 2.5mm² is installed in a way which makes it only capable of carrying 18.5A then it doesn't comply.

However 4mm is rated at 25a so the shower cable must be at least 6mm.

No cables are simply "rated at ##A" - their capacity varies according to how they are installed - see the 2nd and 3rd links I posted. 4mm² is rated at 17.5A - 37A, depending on how it is installed.

I've based my amperages on the first PDF. The table says 2.5mm is rated at 18.5a if 'enclosed in wall' and 23a if 'enclosed in conduit' (p.38 ). So far all the rooms i've redecorated, the cables from underneath the floor and into the sockets were just plastered over so i've put them in conduit myself. I took worse case of the former measurement.

So in this case the 32a MCB will be way to much for a 0.68a appliance.

The rating of the MCB compared to the appliance is irrelevant because

I was planning to run from this to a fused spur, then into the boiler.

So as long as I use an adequate cable (ie the existing shower cable) for the MCB all will be OK.

 

[ldoodle]

It's actually the 3rd link i've been reading (the cable selection PDF). For some odd reason I though it was the first link!!!!

 

[bongos]

Afternoon Idoodle.

Just clear this up for me quickly...your existing shower MCB? Is it 32A (or higher)?

If so, and you will be supplying your boiler with 2.5mm² t+e at any point of the circuit between the MCB and your FCU, then you must replace that MCB with a 16A (or possibly 20A) variety.

It has nothing to do with the potential power consumption of the boiler but everything to do with the MCB's ability to protect the cable.

A 32A MCB will not provide overcurrent protection for a 2.5mm² t+e cable.

 

[PrinceofDarkness]

The issue of overcurrent protection is irrelevant here, since the proposed FCU will be supplying a fixed load (boiler) which isn't capable of being overloaded. Therefore, the important consideration is the short circuit protection, in which case the 2.5mm cable would be adequately protected by a 32A MCB.

The Regulations allow for this situation.

The boiler flex is likely to be protected by the local FCU at 3A (or 5A) - so where's the overload going to come from?



Lucia.

 

[ldoodle]

Afternoon Idoodle.

Just clear this up for me quickly...your existing shower MCB? Is it 32A (or higher)?

It is 32A. Well I assume so based on a few searches for MK LN 5932s B32.

 

[ldoodle]

since the proposed FCU will be supplying a fixed load (boiler) which isn't capable of being overloaded.

Just to confirm, I plan to run 1 length of cable (aka radial) from the existing MCB to a FCU with a 3 or 5 amp fuse, then into the boiler using flex.

 

[ldoodle]

Been thinking about this the last couple of days and I may rewire the whole kitchen (excluding cooker) onto the spare MCB, using 2.5mm in a ring.

Out of the 2 proposals, what would you do?

 

[Steve]

What would you do?

With your knowledge? Never touch electrics again. Apart from flicking switches and inserting plugs to sockets to extract electricity for the use of powering appliances.

http://www.diynot.com/wiki/electrics:part p

 

[ldoodle]

What would you do?

With your knowledge? Never touch electrics again. Apart from flicking switches and inserting plugs to sockets to extract electricity for the use of powering appliances.

http://www.diynot.com/wiki/electrics:part p

Come again? As I said above, in terms of the practical side of re-wiring i'm exceptionally good at. In fact, I'll bet you a billion pounds I could do a 'better' job than some sparkys (as can be proved by what i've replaced in my house to date, with 0 problems may I add). Some of the stuff I have had to undo would scare the **** out of you, and it's supposed to be by so-called professionals!

It's just the theoretical side of it I need help with, which is what places like this are for.

Would you tell an unqualified carpenter to 'never touch a saw again'. It's idiots like you that ruin places like this.

 

[Steve]

What would you do?

With your knowledge? Never touch electrics again. Apart from flicking switches and inserting plugs to sockets to extract electricity for the use of powering appliances.

http://www.diynot.com/wiki/electrics:part p

Come again? As I said above, in terms of the practical side of re-wiring i'm exceptionally good at. In fact, I'll bet you a billion pounds I could do a 'better' job than some sparkys (as can be proved by what i've replaced in my house to date, with 0 problems may I add). Some of the stuff I have had to undo would scare the **** out of you, and it's supposed to be by so-called professionals!

It's just the theoretical side of it I need help with, which is what places like this are for.

Would you tell an unqualified carpenter to 'never touch a saw again'. It's idiots like you that ruin places like this.

Calm down dear. In this case, I would rewire the kitchen, but its better to have the boiler OFF any RCD thats present to prevent nuisance tripping.

 

[ldoodle]

Sorry about that last comment! Had a bit of a mad week and am nackered! Should not have taken it out on you though!

Unfortunately I don't have capacity to put the boiler on non-RCD protected circuit, unless I change the CU (that is something I would never do).

This is the whole reason for at least isolating it in some way as our old boiler left us with no heating or hot water over christmas, when it was bloody freezing!

Thanks for your help