Breaking ring main

[xnetdude]

Hi,

We are having a bathroom refit and are adding underfloor heating (just 150w version) but there is no easily reachable spurs to connect the heating system to.

There is, however, a 32A ring cable passing below the floorboards. Is it acceptable to cut this ring cable and connect the underfloor heating directly to the ring using WAGO clips inside a WAGOBOX (all rated at 32A)?

Due to the limited space for the run of the underfloor heating cable we can only get 1.5mm T/E to connect to the ring, is that also acceptable?

Whilst I appreciate this is not the "ideal" practice the paramount question is would this installation be unsafe?

The ring is already protected by an RCD at the CU.

Many thanks.

Pete.

 

[Taylortwocities]

It is unsafe.

You will be connecting a spur to the ring. This must be in the same sized cable as the ring (2.5mm)

Don't forget that the ring (and your new addition) will be protected by a 32amp CPD back at the consumer unit . 32A is way outside the spec of 1,5mm cable.

Also, your spur will need to go to a fused connection unit with a fuse in it sized for the underfloorheating.

 

[xnetdude]

Hi,

Thanks for the advice.

Is a 2.5mm t/e still required if the only thing wired to this is the underfloor heating as this does not draw very much current and will not be accessible to plug anything else in? My understanding is the 1.5mm is rated around 20A which is more than sufficient for the heating mat.

The problem we have is where the new cable will need to be run does not allow for the extra size of a 2.5mm t/e.

Thanks in advance.

 

[bernardgreen]

Is a 2.5mm t/e still required if the only thing wired to this is the underfloor heating

Think FAULT if there is fault that shorts out your 1.5 mm then the fault current will be the maximum the MCB will pass. That current will be enough to over heat, possible ignite the 1.5 mm cable.

The problem we have is where the new cable will need to be run does not allow for the extra size of a 2.5mm t/e.

Then you cannot install the heating. 150 watts of heating is not going to give much heat, you probably wouldn't notice it.

 

[Bluehair]

Hi,

Thanks for the advice.

Is a 2.5mm t/e still required if the only thing wired to this is the underfloor heating

Yes.

 

[xnetdude]

Yes, the fault issue was my concern and the then load on the cable.

What about, as an alternative, replacing the 32A MCB with a 20A which I read is done fairly routinely when rings are broken.

What is your view on that?

Many thanks again for your time.

 

[bernardgreen]

Put a FCU somewhere convenient on the ring and fuse it at 5 amp and then you can run 1.5 mm to the heating. Easiest would be to put the FCU next to an existing socket on the ring. That is the least work on the 2.5 mm ring even if it means a longer run of 1.5 mm

 

[xnetdude]

I hear and take your advice but just to further help with my understanding, what is the difference between what we are discussing here and the following scenario :-

A 2.5mm spur is run which in turn connects to the cable connected to (and provided by) the underfloor heating mat. This underfloor heating mat cable is 3m long and is only rated at 20A (according to their literature).

Connecting this manufacturer supplied cable to the spur is the same principle ie. a 20A cable run being connected to the 32A ring.

Is that different to connecting diretly to the ring or is the primary difference here the fact there is no inline fuse?

Pete.

 

[bernardgreen]

3 metre of heating cable rated at 20 amps ?

At 230 volts that is 4600 of heating spread along the 3 metres of heating cable.

If you do not fuse the supply to the heating mat at 5 amps or less and there is a fault then you could have 32 amps ( more as the MCB will not trip instantly on 32 amps ) heating under your floor, thats 7360 watts ( more than 7 electric fires ) which will very likely set the foor on fire.

For this reason there are strict rules about how and where under floor electric heating is installed.

If you are happy to risk setting the house on fire then go ahead, the best advice has been given. It won't change no matter how often you ask.

 

[xnetdude]

I wasnt trying to get a different answer, I was merely trying to further my knowledge.

Thank you for your advice, which will be taken.

 

[xnetdude]

ps for clarity.

The 3m cable I mentioned was the mains cable leading TO the heating mat, NOT the actual heating cable itself.

 

[Taylortwocities]

ps for clarity.

The 3m cable I mentioned was the mains cable leading TO the heating mat, NOT the actual heating cable itself.

Yes, and that too has a limit on how much current it can carry.

Also, the thermostat/controller has a limited current-carrying capacity. A short onn the heating mat will mean the maximum current (32amp in your proposal) will fry the tstat.
You maunufacturer's instructions will mandate a fused supply is essential. 5amp is usual ror a small mat.

But hey, what's the point.? You come on here, asking for advice. You have been given it in the interests of your safety and wiring best practise.
Sorry that you just don't like the answers.

 

[Lectrician]

I can't believe you can squeeze a 1.5mm in and not a 2.5mm - this sounds suspect.

Where exactly are you trying to run the cable than prevents you from using THE CORRECT sized cable that is only marginally larger than your bodge?

I assume you intend to fit an FCU and your timeclock/stat on a wall somewhere?

 

[xnetdude]

I'm a little surprised at the vitriol!

I came on asking for advice which was gratefully received and will be followed.

Once I had my answer I simply wanted to further my knowledge and understanding of the subject matter, not cast doubt on the answers given.

This forum is called "diynot" which suggests it is for "amateurs" who hope to better their knowledge and not get shot down for asking, what you may consider as, dumb questions.

I greatly appreciate the advice proferred but I am not sure the sarcasm is helpful to both me and any prospective posters.

Thanks again for the constructive advice.

 

[Lectrician]

Mine was a valid question - I cannot understand why you can run a 1.5 and not a 2.5mm cable - are you trying to squeeze it in somewhere you possibly should not be?

If you explain where you intend to run the cable and the reasons why you cannot use the correct size cable on that route, we may be able to offer advice...