Multiple FCUs

[Bannor]

Having had a combi boiler fitted, I no longer have an immersion heater and this supply (2.5mm T&E direct from CU) is therefore now available.

I'm toying with the idea of taking this up into the loft space and using it to run a series of FCUs. One of these would supply an aerial amplifier in the loft, the others (all RCD) would feed back down to 2 sets of underfloor heating + shaver socket (bathroom + en-suite).

I estimate the total load to be less than 800W so I can't see any problem there, and due to the location of the normal ring circuit it would be far more convenient to wire this way.

Is there any reason (particularly regs!) why I shouldn't do this? Obviously I'll need to get a qualified sparky when it comes to the actual bathroom wiring, but at the moment I'm just concerned with getting the cable routes sorted. I don't want to go to all the hassle of putting in the conduit etc. only to find I can't run this many FCUs off a single feed.

Many thanks

 

[Steve]

RCD protect at the origin of the circuit. It'll be cheaper.

In fact, given its 2.5mm², it'll probably be on a 16A breaker, theres no harm having a 13A RCD FCU at the source, then do what the hell you like in the loft.

To make the bathrooms fully 17th ed, you' need to feed the lights from RCDs too.

I'm not sure on the technicalities of this though, WRT bonding. Await professional advice.

 

[ericmark]

As far a fused part of FCU you are unlikely to require it. But the problem is with feeding lights and sockets off same supply. It is not cut and dried and if should the RCD trip you could still see to exit the bathroom it is very likely you would have no problems.

But some guy will need to sign your installation certificate and if he does not like what you have done then no certificate.

So decide who you are going to get to test and sign off. And agree with him what you can do and what he will do before you start. Then do as agreed.

It really does not matter what we say on here we are not signing form. You may however want to look at RCD's as according to how installed the price can change a lot. There are special bathroom and garage mini consumer units which may help to isolate circuits i.e. loft amp. Some sink into walls. Some look ugly. There are also RCBO's which if they will fit in existing CU may be best option.

 

[Bannor]

Many thanks for the replies - running everything off a single "master" RCD seems like a good idea.

I was hoping to do all the preliminary work before calling in an electrician, so he'd have the minimum amount of work to do (thus saving the readies!). But I guess it makes sense to get someone in first and make sure they're OK with the basic idea.

Must admit I wasn't aware of the requirement for RCDs on bathroom lighting. I have 12v downlighters - does the requirement still stand, given that the mains is all in the loft and therefore can't be touched by someone in the bathroom?

Thanks again

 

[flameport]

Must admit I wasn't aware of the requirement for RCDs on bathroom lighting. I have 12v downlighters - does the requirement still stand, given that the mains is all in the loft and therefore can't be touched by someone in the bathroom?

Yes, as the 'transformers' for the lights will actually be electronic devices which won't isolate the output from the input, so if they failed it is not impossible for the mains voltage to appear at the downlighters.

If a proper isolating transformer was fitted outside of the bathroom, then fitting an RCD would not be required in theory (and would not work even if it was fitted) - however the regs do not state anything about that situation.

 

[securespark]

A 2.5 radial on a 20A breaker does not need fusing down further, providing the EFLI is acceptable at the far end of the circuit.

 

[Taylortwocities]

If a proper isolating transformer was fitted outside of the bathroom, then fitting an RCD would not be required in theory (and would not work even if it was fitted) - however the regs do not state anything about that situation.

It would not be required with respect to the bathroom if the SELV lighting was the only electrical output there

HOWEVER

there may be other stuff - shaver point, dual feed rad, shower pump and these all require RCD. Also an RCD is required if any of the wiring is concealed and less than 50mm from the surface.

 

[RF Lighting]

It would make much more sense to use the old immersion supply to power your TV booster and FCUs for the underfloor heating, and take the shaver supplies of the local lighting circuit, ensuring both are RCD protected.

 

[Bannor]

Thanks to all for the advice.

I really don't know how you guys get your heads around all these regs - I'm starting to think it's actually all about cauldrons and eye of newt...

So now we have to fit RCD to any concealed wiring that's less than 50mm from the surface? In my (admittedly very limited) experience, that must mean 99% of all household wiring is no longer up to regs! Surely almost every light switch in the country will have the cables coming straight down the wall with no more than 6-10mm of plaster over it? Or have I misread TaylorTwoCities comment?

 

[Taylortwocities]

For concealed wiring that is true. Less than 50mm and you must fit RCD. So that is all switch drops, runs to sockets, everything. There are some exceptions but lets not dig too deep.

BUT

Remember that wiring regulations are not retrospective so you only have to apply them for new installations and any new work (including alterations).

So no RCD is needed if you extend your immersion circuit on the surface, inside the airing cupboard and into the loft.
If you were to route the cable to the wall on the landing, into a socket there (say) then up the wall to the loft then its RCD time for that circuit.

Its not easy being an electrician..

 

[ban-all-sheds]

Remember that wiring regulations are not retrospective so you only have to apply them for new installations and any new work (including alterations).

So no RCD is needed if you extend your immersion circuit on the surface, inside the airing cupboard and into the loft.

And for exactly the same reason no RCD protection needs to be added to his lighting circuit, as he's not touching that, dunno what Steve and eric read...

Yes, as the 'transformers' for the lights will actually be electronic devices which won't isolate the output from the input, so if they failed it is not impossible for the mains voltage to appear at the downlighters.

And the definition of Separated Extra-Low Voltage is.....?

If a proper isolating transformer was fitted outside of the bathroom, then fitting an RCD would not be required in theory (and would not work even if it was fitted) - however the regs do not state anything about that situation.

The loft is outside the bathroom.

 

[bernardgreen]

And the definition of Separated Extra-Low Voltage is.....?

derived from a source via an isolation barrier that will fail safe under worst case fault conditions.

Dual bobbin transformers are unlikely to connect mains to the output in a burn out situation but an "isolation" transformer with the primary coil wound on top of the secondary coil can put mains onto the output when the thing burns out.

Electronic transformers often have a 3 or 6 mm separation bridged by a transformer and while working normally do provide isolation. But when damaged or severly scorched with components failing and PCB turning to carbon the isolation is severely compromised or removed completely.

 

[ban-all-sheds]

derived from a source via an isolation barrier that will fail safe under worst case fault conditions.

Not according to my copy of BS 7671.

Electronic transformers often have a 3 or 6 mm separation bridged by a transformer and while working normally do provide isolation. But when damaged or severly scorched with components failing and PCB turning to carbon the isolation is severely compromised or removed completely.

If they are CE marked, and say "SELV" then they are SELV.

 

[JonoPashley]

Generally having lighting and other power outlets on the same rcd for that room/floor isnt recommended. The best way is with dual rcd boards and puting lights and sockets on opposing rcds so if say your shower tripped the rcd then it would take downstairs lights out instead of leaving you in the dark trying to navigate the bathroom.

 

[bernardgreen]

derived from a source via an isolation barrier that will fail safe under worst case fault conditions.

Not according to my copy of BS 7671.

My opinion is based more on common sense and a bit of experience of disaster recovery.

Electronic transformers often have a 3 or 6 mm separation bridged by a transformer and while working normally do provide isolation. But when damaged or severly scorched with components failing and PCB turning to carbon the isolation is severely compromised or removed completely.

If they are CE marked, and say "SELV" then they are SELV.

More often they are designed to meet the requirements that have been foreseen as to be expected. It is when the un-expected happens that they can fail in a way that makes they no longer SELV compliant.