Help: Trying to fit new electric Towel Radiator and underfloor heating in bathroom

[RottieMad]

Hi all, I am modernising our bathroom on a very tight budget (just had our first baby) and am trying to prioritise what needs doing at the moment.

I have replaced the bath, toilet and sink, fitting electric underfloor heating and laid new laminate flooring over it. Its only a small UFH mat at approx 1.5m2 which is approx 275/300 watts

I have also fitted a electric 1200 x 500mm towel radiator with a 350w heating element (non thermostatic)

My flat is a 'coach house' style place with a 2 bedroom flat over 3 garages and the current wiring is as follows:

Consumer unit has 2 rails.......... the left rail is RCD 30mA protected and currently only has the sockets on it.
The right hand rail has everything else electrical within my flat, including what was the mains heat alarms x2 and mains smoke alarm on a 6a MCB!
The heat and smoke alarms are all being replaced with battery operated items which I have already have.

Its going to be a major nightmare trying to feed new cables from the CU to the bathroom and if I can get away without, it would be great because any chasing etc that needs doing and making good would go to the very back of the list of jobs and my walls would look a mess in the mean time until I could afford to have them made good.

So I have taken the heat and smoke alarms circuit, moved this from the the right hand rail to the left hand rail (which is RCD protected) in my CU and fitted a 16a MCB
The smoke alarm that was right outside my bathroom door is where I have terminated this cable and fitted a electrical rose (i think thats the term)
Out of this rose I have 2 cables goto 2 x Switched FCU with 13a fuses and then from these, 2 cables which drop down opposing bathroom stud walls (drilled through wall headers from the loft and fortunately there wasnt any noggins and the casbles both dropped to the dryline box's and out to the faceplates.


TL;DR:

Consumer unit (Left rail 30mA RCD protected rail currently only has sockets)
16a MCB
Rose
13a SFCU
Thermostatic Face plates for UFH and towel Rad
Standby works
Power on towel rad and it trips 30mA RCD in CU

When I turn the electric back on the standby light illuminates on both faceplates and all is fine............. I turn the towel rad thermostat on ( https://www.screwfix.com/p/kudox-electronic-heating-element-controller-800w/64054 ) and it trips the RCD in the CU

What have I done wrong here with my setup?

Will the tripping be down to the small cable used (its either 1 or 1.5mm T&E) originally for the heat and smoke detectors just not man enough now for the towel rad and UFH?
The distance between the CU (downstairs by my front entrance), up the wall (3m approx) across the loft to hallway where the cable terminates in the rose is (7m approx) and then to SFCU is (1m approx) then from SFCU to faceplates (2m approx)

I want to do this as properly as I possibly can without getting a professional in (at the moment)
I will be back in work once I have recovered (8/10 weeks) and will have the funds to get someone in who is qualified to Part 'P' to sign off on what i have done, but being self employed, off of work and with a brand new baby sucks and things are super tight at the moment, so any help or advice is much appreciated

Thank you and I will apologise now for getting terms wrong or if ive not explained myself clearly

RM

 

[freeonboard]

OK.. a qualified electrician will be along shortly who will be far more knowledgeable than me about electrical installation rules, but some points for you immediately:-
1. The mcb, or fuses, in a cable are meant to be the "weak links" in that they will trip out BEFORE damage to a cable can occur...ok?
2. The original 6ampo mcb is there because that's the capacity of that cable... I assume it was doing lights as well as alarms?
3. By re-using that cable as you have done, rating it at 16amp you could allow that cable to be grossly overloaded... e.g. you could set fire to your house if a fault occurred
4. Your towel rail and UFH are only pulling a small amount of current... say 1 1/2 amps each...... a 13amp fuse is massively too high... remember the "weak link" point about cable protection
5. Immediate tripping says, to me, that you have wired up the towel rail incorrectly.

AS A MINIMUM change fuses (load is couple of amps each) & mcb to protect the weakest bit of cable which can only take 6 amps. HOWEVER, re-using this lighting circuit cable in this way is not what I feel you ought to do...... like I say I'm no expert but I feel you ought to be very careful in doing this, particularly if you aren't clear about cable loading etc.

 

[freeonboard]

another couple of thoughts...
1. you "moved the MCB".... I think you are talking about a split load consumer unit... which would have 2 bars for the neutral wires?.....did you also move the neutral wire from the non-rcd side to the rcd bar?
2. are you really sure you understand what you are trying to do??? I'm not being patronising, I am worried you might hurt yourself... electricity can be very dangerous! If you ar not 100% certain what you are doing get someone a little more knowledgeable to at least look at what you are doing... looking is better than trying to understand via a thread like this!!

Good luck and be ULTRA careful........

 

[winston1]

RCDs don't trip because of a small cable. As already stated the MCB does not need to be 16 amp (although actually 1.00 and 1.5mm cables are rated OK at that). You do not need the FCUs either, especially when you put back the 6 amp MCB.

But back the RCD tripping. Neutral in wrong CU bar is favourite, followed by wiring error elsewhere.

 

[flameport]

What have I done wrong here with my setup?

Pretty much all of it.

You can't just take an existing cable and change the rating of the MCB - it had a 6A MCB for a reasonn.
Why are you using what was a 6A circuit on the non-RCD side for heating?
Why are you replacing mains powered alarms with battery operated ones?
Have you considered appropriate locations for these concealed cables, or the location of the towel rail FCU and thermostats with regards to the bathroom zones?

will have the funds to get someone in who is qualified to Part 'P' to sign off on what i have done,

Not possible.
You either notify building control, pay the fee and find out what they want in regards to testing before you start, or get someone who belongs to a scheme to do the work and the notification.

I want to do this as properly as I possibly can

Put it all back how it was originally.
Devices such as towel rails and UFH would normally be connected to the socket outlet circuit, which in your case already has the RCD and will be wired with cable of a suitable size.

 

[ban-all-sheds]

Plus - too late now, but if you are on a very tight budget, electric UFH and laminate flooring were not a priority.

 

[ban-all-sheds]

I will be back in work once I have recovered (8/10 weeks) and will have the funds to get someone in who is qualified to Part 'P' to sign off on what i have done

What's happened to your lying mate? https://www.diynot.com/diy/posts/2993475/

 

[RottieMad]

Thank you FreeonBoard for the speedy reply, very helpful couple of posts, educating me and for looking out for my welfare. I didnt take your posts as patronizing at all and am grateful for your input.

It was the Neutral I hadnt swapped across to the other bar! I did swap the 16a mcb back to the original 6a and have replaced both 13a fuses in the SFCU with 3a.
All works a perfectly now so thank you all once again for your input
I now have one very happy partner recovering from C Section and a baby whom both can now bathe in the warmth of the bathroom (id rather deal with the consequences of not doing things by the book, then having them both uncomfortable)

Winstone: I choose to keep the SFCU's because id already cut the holes for the dry lining box's and fitted them. Hopefully they will just add another layer of protection and wont cause any issues

FlamePort: I am replacing the mains alarms because they've been switched off for the past 4yrs at the CU. They were all linked, so when one went off they all went off, its a relatively small flat and it was defeaning. Bathroom door open, shower would set the one in hall way off. Cooking bacon/burgers in the grill etc sets the kitchen smoke alarm off, cooking almost ANYTHING set the kitchen heat alarm off. Id deal with the smoke alarm and the heat alarm would start up! Ended up ditching them altogether
Thermostats are beside an existing shaver socket and Towel Rad is outside of all zones i believe (I have the doorway seperating the bath from the Radiator)! Both SFCU's are outside the bathroom, above the bathroom door either side of the extractor fan isolator.
Anythings possible, ill go retrospective permissions with building control in the future if I cannot get a sparky to pop round and sign off on it. Im usually working on sites all the time so am sure if I bung them some cash it will be sorted
I did consider connecting it up to the shaver socket feed, however I read a few posts online statting that a dedicated circuit is best and thats what made me consider utilising the existing cable if at all possible. Turns out its very possible and is now on its own circuit

Shed: UFH mat £26 as I already had a digital stat left over from another job and 1 opened but new pack of laminate flooring £FOC, again I had some underlay already.
3mm ply over a suspended concrete slab isnt very warm! So with winter fast approaching and a newborn.............. Yes it was a priority
He died in a motorbike accident 15months ago unfortunatley, but thanks for caring..............its cute!

Thank you once again, you have all been a great help................. well almost all

 

[winston1]

Winston: I choose to keep the SFCU's because id already cut the holes for the dry lining box's and fitted them. Hopefully they will just add another layer of protection

They won't. A 3 amp fuse will have no discrimination against a 6 amp MCB. (It would have been even worse with your original 13 amp fuse and 16 amp MCB).

Im usually working on sites all the time so am sure if I bung them some cash it will be sorted

Bribery and corruption are both illegal and have no place in a civilised society.

 

[ban-all-sheds]

3mm ply over a suspended concrete slab isnt very warm! So with winter fast approaching and a newborn.............. Yes it was a priority

Amazing how humankind managed to survive all this time when all they had were the options of warm floor surfaces, mats and footwear.

Did you sort out proper insulation, or have you installed a ceiling heating system for the garage?