Dedicated RCD for boiler

[ldoodle]

Hiya

We've just had our boiler replaced which means we can now do away with our electric show (and use mixer taps instead). The shower was on it's own RCD which is now no longer required.

Is it OK to use this RCD for the boiler; I was planning to run from this to a fused spur, then into the boiler. As it's a single feed, should I use something bigger than 2.5mm, like they do for cookers?

The RCDs all have the same model number so I guess they're all the same power wise.

Thanks

 

[ban-all-sheds]

The cable going to the FCU will need to be the right size given the rating of the MCB protecting it - probably the existing shower cable is OK.

From the FCU to the boiler only needs to be 1mm².

• http://www.diynot.com/forums/viewtopic.php?p=81696#81696
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
  

If the boiler is in the kitchen this work will be notifiable.

 

[ldoodle]

Thanks for the quick reply!

The cable going to the shower is 4mm I believe (off the top of my head), but its been buried in the walls with no trunking/conduit up to the loft then back down into the bathroom, so is not easily pulled back!

I have a reel of 2.5mm T&E so was going to run a new length from MCB to FCU then onto boiler using flex.

As said above, the part numbers, voltages etc on all the MCBs (except lighting) are the same so should be OK to use 2.5 T&E from what was the shower MCB, as that (2.5mm) is what is currently feeding the boiler as part of the 'downstairs' ring main.

 

[bongos]

The cable going to the shower is 4mm I believe (off the top of my head), but its been buried in the walls with no trunking/conduit up to the loft then back down into the bathroom, so is not easily pulled back!

I would have thought a 4mm² t+e supply would be possible but unlikely. What was the rating of the old shower and MCB that protects the cable?

I have a reel of 2.5mm T&E so was going to run a new length from MCB to FCU then onto boiler using flex.

If you run a new length of 2.5mm² from the MCB onto the RCD and then to the boiler (is this what you mean) then you must make sure that the MCB is appropriate for the 2.5mm² cable. Replace it with a 16A one. This work then becomes notifibale though (if it wasn't already)!

As said above, the part numbers, voltages etc on all the MCBs (except lighting) are the same so should be OK to use 2.5 T&E from what was the shower MCB, as that (2.5mm) is what is currently feeding the boiler as part of the 'downstairs' ring main.

I'm a bit confused by what you're saying. If the boiler is already being fed by the downstairs ring via a FCU then I think i'd be tempted to leave it as it is...

 

[ldoodle]

The old shower is currently still installed so will check it's rating. The cable is definately bigger than 2.5mm, i'm just not sure if it's 4mm or 6mm off the top of my head. If the MCB can drive bigger than 2.5mm and I want to run 2.5mm all should be OK?

There's no point in having an empty MCB so may as well utilise it for something.

I'll post the values printed on the MCB's when I get home.

Thanks again.

 

[nozspark]

Why not keep the existing shower..

then if you have a problem with the boiler at any time you'll at least be able to have some hot water for washing!

 

[ban-all-sheds]

If the MCB can drive bigger than 2.5mm and I want to run 2.5mm all should be OK?

Absolutely not - you've got that completely the wrong way round.

Did you actually follow the links in my first reply, and read the material?

 

[wingcoax]

What BAS is trying to tell you is that the mcb is there to protect the cable, and a mcb of that size on 2.5mm will allow the cable to burn out before tripping.

 

[ldoodle]

Sorry, we're misunderstanding each other here (my fault). What I meant to say is that if the shower MCB is driving a 4-6mm cable, and all the other MCBs are driving 2.5mm cable and they have the same specs, then I should be OK to run 2.5mm cable off the current shower MCB?

The MCB's all have a code of LN5932s B32 240V~ 6000_3. This is the one http://cgi.ebay.co.uk/MK-SENTRY-LN5...temQQimsxq20100321?IMSfp=TL100321127002r21087

Thanks again

 

[1john]

you cannot have a single 2.5mm cable on a 32A MCB. You seem to be making work for yourself that isn't really required.

 

[ban-all-sheds]

Sorry, we're misunderstanding each other here (my fault). What I meant to say is that if the shower MCB is driving a 4-6mm cable, and all the other MCBs are driving 2.5mm cable and they have the same specs, then I should be OK to run 2.5mm cable off the current shower MCB?

Absolutely not - you've got that completely the wrong way round.

Did you actually follow the links in my first reply, and read the material?

 

[ldoodle]

Sorry guys! Trying to get to grips with the whole thing; wiring in general i'm comfortable with but the theoretical side is where my knowledge dips!

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?

However 4mm is rated at 25a so the shower cable must be at least 6mm. The cable is in trunking for the first metre or so, then under the floorboards and up to the loft in the wall.

The boiler I have is 155W and 230v @ 50Hz (manufacturers PDF);

155w/230v = 0.68a

So in this case the 32a MCB will be way to much for a 0.68a appliance. Although the PDF does say to go for the next highest value if you can't get an exact match, it doesn't state the maximum 'difference' you can go?

Thanks

 

[ColJack]

define "buried in walls"?

plastered in is the same as clipped direct as far as the tables go, so 2.5mm² is 27A and 4mm² is 37A

if the boiler is really that low then you could go for a 16A breaker and a 3A fuse in the FCU...

 

[ban-all-sheds]

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.

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.

 

[ldoodle]

define "buried in walls"?

Hmmm that's a tough one as it's an inherited installation. I won't know for sure until I either rip it out when we redecorate the hallway or try and pull it back for use to the boiler.

if the boiler is really that low then you could go for a 16A breaker and a 3A fuse in the FCU...

Well the manufacturers Installation and Servicing PDF definately says 155w/230v for the 24Kw boiler we have (Potterton Heatmax Combi HE 24)