Discuss Cable size from ring main to spur for underfloor heating in the Electrical Forum area at ElectriciansForums.net

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M

manny.p

Hi all new on here and looking for advice. I ran wiring for an extension and took a 13a spur from the ring main using 2.5mm2 t&e for the underfloor heating controller. The homeowner has got someone in to lay the the heating mat kit 20sqm of it and when the tiler connected to thermostat he said 1 part of the mat heats up fine but when you put 2 on then it takes time. 2 mats provided as area is over 12sqm. He said the wiring may be wrong nod overheat the cable from ring main to spur. Can anyone help with whether the correct cable was used. Mat total watts is 3kw and thermostat can handle 16a. Thanks manny
 
If you have a 13A fused spur then this is of a suitable size for the load you describe (3kW), there would not be a problem with overloading the cable.
However wiring a permanent 3kW load to the ring is not good practice, but should not cause a problem so long as the ring is balanced.

If you have loose connections in your wiring then they may well overheat and cause resistance and overheating but this is easy to check.
Electrically there should be no problem with the cable or the fusing for that load.
I would check the mats are correctly connected and undamaged.
 
Forgive me for asking this but are you sure its a 3KW rating - seems very high for such an area, and I can't imagine how high his leccy bills are going to be!
 
Thanks for that Richard. As the tiling guy said the wires would overheat and to make the spur part of the ring main. ie he recommended incoming and outgoing to the ring main and the feeding the thermostat
 
Hi thanks for your response Murdoch. The mats are 150w per metre and he has installed 20 sqm. 3kw

Ouch. Love to see his bills as they will be shocking!


Last time someone asked me for that size, I suggested and along with the chap laying the floor's agreement and the clients agreement, greed, the area was spilt the area into 2 "parts".
 
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Heat mats require notification and certification.

This is a reason why tilers should not be doing it.

However, i think your problem may well be a bad connection, or, damaged cable somewhere.

Are both mats the same size?

Have you disconnected the mats at the controller and taken resistance readings?
 
No offence Manny but who is the electrician you or the tiller? This is quite a simple problem by the sounds of it. Check all your connections

I don't think the OP is a spark, especially given what he was asking about the cable sizing. And what self respecting spark would stand back and let a tiler make the connections :hand:
 
All I have yet to check connections and test the mats. The owner just wanted us to do our job, pay us and leave and now saying we have not done our job properly. Certificate has been issued for all the electrics and I did ask if the tiler is qualified. He said the tiler has fitted quite a few of these mats. Hence I then asked the question he should have taken ohm reading if the mat. Gonna go out next week to check. Thanks all
 
Its usually wise to take readings before and after the mat is layed,this will highlight any inadequacies
The certification is puzzling me,did you cover for his installation work,or does your responsibility end at the spur ?

Its best to politely ignore claims by other trades how the electrics should be installed or who should do that installation,it will get you into all sorts of bother
If you had been assertive and insisted on correct procedure,there would be no concern about who did what, who has made the unknown problem, who is now going to have to sort it

"I know a Tiler who can",well he can't on my jobs
 
My job ended at installing the ring main and spurs. When he asked about installation of mats and I gave him a price he said tiler would do it. I'm just going to go in check my spur connection and tell him it's down to the tiler to sort as he has installed and I have installed wiring capable of handling the load. Sometime being too helpfull or kind back fires. Thanks
 
My job ended at installing the ring main and spurs. When he asked about installation of mats and I gave him a price he said tiler would do it. I'm just going to go in check my spur connection and tell him it's down to the tiler to sort as he has installed and I have installed wiring capable of handling the load. Sometime being too helpfull or kind back fires. Thanks

Hum, so can the tiler do the Part P notification??
 
That's for the homeowner to find out. I did my job and gave certificate. Plus have installed quite a few heat mats and never had any problem before.
 
That's for the homeowner to find out. I did my job and gave certificate. Plus have installed quite a few heat mats and never had any problem before.

You are missing out on jobs if you're happy to quote for fused spurs for u/floor heating then not getting the connect and test work.
 
Phone the manufacture and ask the what the resistance is per meter for that model. Take a reading and see if the element is within their guide lines, if not this can now become very expensive. There is a company that specializes in mobile xray machine that they scan the floor to locate a damaged bit of cable on the under floor heating. I think their prices start at £800!!! Some times its worth it, locate damage and only take up one or two tiles and not the entire floor.

In your case I would only accept responsibility for up to the fsu and not beyond if you did not install it.
 
As others have said, check what the manufacturer's resistance readings were (they will have been supplied with each mat) and ensure that's what you are getting on site. If not then it's likely that the one with the reading that's out is damaged. Also, check if a floor sensor has been used and if so, where it has been located.

As for the load on your circuit, just calculate from the resistances in parallel and compare this with the ccc of your cable for the method of installation.

Remember, all this checking should be paid for by the tiler - you appear to have done what you were asked to do (but learn the lesson about fixed loads on a RFC).

Regards,
 

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