Wiring up a bathroom mirror light

[JamesHughes]

Hello all,

Notwithstanding any Part P certification...

Bathroom being redone - I have put in an timed extractor fan, wired up to an isolator and RCD, fed from the ceiling rose. I now need to splice in the wiring for a lighted mirror, 12v 20AMP. I intend to take the feed to the fan (3 core+E - N,L, and Switched L)), and spur off L&N to a transformer in the loft (its on the way to the fan anyway), wired down to the mirror. OR, should I just take the feed from the RCD and bypass the isolator and have a completely separate cable to the transformer, or doesn't it really matter?

J

 

[Chri5]

12v 20AMP.

Are you sure, cause that seems a tad high to put on either a 6a or 10 a lighting circuit

 

[JamesHughes]

That's at 12v - although I am simply going by what the specs on the mirror say - hasn't arrived yet! (12v 20A == 240v 1A). Its only 2 halogen bulbs, so not going to be a great drain I would not have thought.

The transformers for 12v all seem to supply over 100A so I guess that's par for the course.

 

[ban-all-sheds]

Notwithstanding any Part P certification...

Do you mean that you don't care about notifying or you don't care about P1?

I have put in an timed extractor fan, wired up to an isolator and RCD

Where's the RCD, and what supplementary bonding do you have?

I intend to take the feed to the fan (3 core+E - N,L, and Switched L)), and spur off L&N to a transformer in the loft (its on the way to the fan anyway), wired down to the mirror. OR, should I just take the feed from the RCD and bypass the isolator and have a completely separate cable to the transformer, or doesn't it really matter?

Depends on whether you mind the mirror light not working when the fan is isolated.

The transformers for 12v all seem to supply over 100A.

 

[JamesHughes]

Notwithstanding any Part P certification...

Do you mean that you don't care about notifying or you don't care about P1?

The former. I want the system to conform to all the required part P regulations.

I have put in an timed extractor fan, wired up to an isolator and RCD

Where's the RCD, and what supplementary bonding do you have?

In fact, been a change. Double checking the layout, the fan is not over the bath at any point, so in fact an RCD not required. So the RCD is now being used for the shower that has been fitted (not by me). RCD is on the wall outside the bathroom. Supply that feeds the RCD (from ceiling rose - shower is a low wattage power shower) will also go to the isolator, and the cables go up in to the loft space from there, across the room, then down to the fan. What would be the appropriate bonding to put in? The guy doing the rest of the bathroom did the wiring for the shower, so not sure what he has done there. Will check and get to fix up as necessary.

I intend to take the feed to the fan (3 core+E - N,L, and Switched L)), and spur off L&N to a transformer in the loft (its on the way to the fan anyway), wired down to the mirror. OR, should I just take the feed from the RCD and bypass the isolator and have a completely separate cable to the transformer, or doesn't it really matter?

Depends on whether you mind the mirror light not working when the fan is isolated.

Aware of that, but not particularly worried about it. Wanted to know whether it was appropriate.

The transformers for 12v all seem to supply over 100A.

Sorry, should have written watts not amps! Thought it sounded a bit bonkers. I am wondering whether the spec on the mirror has made the same mistake. Here is the spec list as cut from the website....

• Height: 750mm
• Width: 1050mm
• Depth: 170mm
• Supplied fully assembled
• Hi-gloss white finish
• Includes storage shelf & compartment
• Includes 3 x light fittings (12 volt and 20 AMP) <<<<<<<< Ooops
• Includes pull cord

 

[JamesHughes]

Supplementary bonding addition....

The bathroom is plumbed in a combination of copper and plastic. Shower and sink feeds are all plastic. Bath/dunny are copper. Shower uses the standard earth in the cable and is fed from the ceiling rose, as is the fan. So, both fan and shower are on the same earth as the ceiling rose.

There is a radiator in the room which is copper connected.

Not sure about the equipotential bonding on the house as a whole. Think I need to test that/have that tested to be sure.

Given what I do and don't know, I think supplementary bonding from the rad and bath/dunny pipes to the lighting circuit would be the correct approach, although testing the resistance between the rose earth and those pipes would and finding it less than 1667ohms would mean no bonding is required.

 

[ban-all-sheds]

The former. I want the system to conform to all the required part P regulations.

OK - that's fine.

In fact, been a change. Double checking the layout, the fan is not over the bath at any point, so in fact an RCD not required.

No new concealed wiring?

Plus there's the thorny issue of an RCD being required for all circuits of the location. (Supplementary bonding is not an alternative to that, it's the other way around). Whether you're required to add an RCD to any circuits you alter, or only to new circuits you introduce is a point of contention, but most people say the former, so if push ever came to shove about whether you'd complied with P1....


Edited to remove OP's text left by mistake, and unquoted.

Aware of that, but not particularly worried about it. Wanted to know whether it was appropriate.

It's fine.

• Includes 3 x light fittings (12 volt and 20 AMP) <<<<<<<< Ooops

I was going to say if that's true you'll need sunglasses, but I'm not sure you wouldn't actually need welding goggles

 

[JamesHughes]

My recent readings on the subject do make me think that the 17th Ed of Part P means that the the fan, which has wiring sunk in the wall, does need an RCD, because the wiring is concealed, even though it is outside of the bath zone.

In general, I think from my reading that all new circuit should be fused RCD'd. It would seem to be sensible anyway, whatever the regs allow you to do. If I can run both fan and shower off the same RCD this would be simple to implement. After all, both are on the same spur from the ceiling rose, so I would think that would be OK.

So, what I would end up with - spur from ceiling rose to RCD. Cable from RCD to shower, cable from RCD to fan and transformer->mirror lights.

One fly in the ointment might be the switched live from the rose to the fan. IIRC The RCD only takes L&N, no SL, so there would be no RCD rotection on the switched circuit. Would I need to add another RCD just to handle the switched side of the circuit?

 

[ban-all-sheds]

It might be simpler to put an RCD FCU before everything in the bathroom, or to replace the lighting circuit MCB in the CU with an RCBO....

 

[JamesHughes]

Cannot do it at the CU by switching components - it's rather old....

As you say, wondering whether to run a spur from the lighting circuit that powers the whole of the bathroom including the lights, and running it by the FCU RCD. That's shouldn't be too bad a mod - got the required junctions boxes somewhere in garage. That would cover all leccy equipment under the same RCD and cope with the SL.

Long term I need to replace the CU I think. (When I say I, I mean a leccy.. not doing that myself!)