20A Underfloor Heating

[Hefferl]

I need to install 3 x 6.5A heating mats.

I intend to use dedicated 20A radial in 2.5 mmT&E.

Thermostat can only switch 16A so I will use a contactor - 2.5mm in 2.5mm out

My question relates to the cable feeding the stat and in turn the contactor coil.

This will not need to be 2.5mm as the load of the coil is very small but as the entire circuit will be protected by a 20A MCB if I use a smaller cable will I have to fuse down this bit of the circuit or does the fact that the coil is a fixed load mean I can omit overload protection?

 

[NozSpark]

So why not just use 2.5 and save the agro?

It's not exactly going to break the bank is it?

 

[M107]

And don't forget the snubber

 

[Steve3948]

Can you not use 2 Thermostats side by side.

One mat company we use suggested this instead of using a contactor.

 

[Chewbacca]

think, given load and long periods of heating, I would feed circuit in 4mm rather than 2.5mm, but that's my personnal preference rather than a requirement, depending on your cable run that is. With regards to wiring stat, you are only looking for a power supply and control circuit to operate contactor. It's never going to pull much load so could probably manage without FCU and using smaller cable. I am assuming you are feeding supply straight to contactor terminals.

 

[Hefferl]

So why not just use 2.5 and save the agro?

It's not exactly going to break the bank is it?

It's not about the cost - I wanted to check my reasoning.

think, given load and long periods of heating, I would feed circuit in 4mm rather than 2.5mm, but that's my personnal preference rather than a requirement, depending on your cable run that is. With regards to wiring stat, you are only looking for a power supply and control circuit to operate contactor. It's never going to pull much load so could probably manage without FCU and using smaller cable. I am assuming you are feeding supply straight to contactor terminals.

Supply via isolator straight to contactor

Yeah I had considered 4mm too and given your advice I will now use it - which makes the question re the use of a smaller cable for the stat even more relevant - suppose I could squeeze 4mm into the stat but that seems over the top when I think I could use something smaller.

Thanks for everyone's input.

 

[Sidewinder]

Distance between stat & contactor would be a consideration.

As would cable run.

IF they were both in the same enclosure then it would be normal to use smaller CSA for the control and not have it fuse protected in any way.

No risk of damage from "outside", as it would be in the same enclosure.

Overload risk minimal, only the contactor coil, and the coil wire csa so small it would "act" as a fuse.

 

[kme]

I used to use a din rail contactor, and 6A mcb adjacent........If you put em in a 4 module enclosure, you`ve plenty of room for the snubber

 

[Chewbacca]

is a snubber an essential requirement?

 

[Sidewinder]

They are also standard on Schneider d.c contactors.

 

[Chewbacca]

but this is AC....

 

[kme]

But the UFH controllers, which are generally the same hardware in a rebadged box, hate the EM pulse - it wipes the time & program settings; when it switches OFF - so it only ever works once, until you reprogram it - then it works ONCE......etc.