Underfloor Heating Electrics

[elvislives]

Hi, I hope someone can help me.

I am having a conservatory being built, and the walls (3 brick walls and one glass front) are about to be plastered. The builders have put in a new ring circuit for plug sockets which will be connected to the main fuse board.

I have bought some underfloor heating cable, which I will get wired up by an electrician. However, since the walls are about to be plastered, I want to run some cables along the wall now ready for the electrician. This is where I get confused.

Because of the size of the conservatory, I have purchased two reels of underfloor cable. Each real is 17W per metre, and 133m long. Therefore, each cable uses 2260W, 4520W in total??

With the cable came a temperature controller. This has 6 connectors, two for the sensor (which goes into the screed), two for the load (the heating cable) and two from the power/ring.

Since there are two cables, it has been suggested that there should be a contactor and a snubber (whatever that is).

The conservatory builders have wired up a ring circuit for plug sockets, and also run a spare 2.5mm twin and earth cable along the wall for the heating. However, I do not think that this is enough.

Am I right in thinking that I should have 4mm cable running from the fuse box into a contactor and then from the contactor into the conservatory, and the underfloor heating cable should be conntected to this? And also, the temperature controller would have power (possibly from the new ring circuit), and a 2.5mm twin and earth cable that goes to the contactor to make it switch the power on.

The temperature controller is an aube TH132-AF. It has a maximum load of 15A (resistive only if that means anything), and a power of 3450 Watts.

The company that I purchased the heating from have suggested that it should be on its own ring, and should have a contactor and a snubber, but they also say that the underfloor heating cable would be connected to the temperature controller, which surely means that all the power is going through it, and the contactor doesn't really seem to come in to it!!

I am really confused as to what to do. My only concern is that I do not want to have the wall plastered, only to have to trace more cables into the wall after.

The cable that the builders have left for me is at plug socket height. The temperature controller will be at light switch height. Should I run some more cable along the wall towards the fuse box, and if so, what sort. Should I run some cable up from the end of the existing cable up to the the temperature cable, and if so what sort?

I hope someone can help me, as I'm really confused.

 

[Spark123]

How big is the conservatory? As the load is in excess of 3kw it will need to be on its own circuit.
New circuits are notifiable under part p of the building regs so better to involve your electrician now.

 

[elvislives]

The conservatory is 24 square metres.

The builders have already added a ring for sockets, although it isn't connected up yet. I am not sure if this is to be wired up to an existing ring, or into a new ring.

They are not dealing with the underfloor heating. I have called a few electricians to ask advice, but I'm not really getting anywhere. I've probably got about a month before it all needs to be connected up, but the wall is being plastered this week.

 

[Spark123]

Ask the builders electrician for advice.

 

[flameport]

Why do these builders not already have an electrician to add the sockets and lighting for the conservatory? It isn't just a case of shoving a few cables in.

As for the heating, it will certainly need its own separate circuit, and as the controller is rated to 15A, you will need a contactor as well.
The 'snubber' they refer to is a device to prevent voltage spikes. Without it, when the contactor was switched off, a large voltage spike would be created and probably damage the controller.

 

[sparkyspike]

2.5mm cable on a 20A breaker should be enough, as long as the run is short and the cable isn't run through insulation. Also, the heating cable shouldn't connect directly through the thermostat, but should go through the contactor, which in turn is switched from the thermostat (using 2.5mmm cable, as you say). This is the whole point of the contactor.

To be honest, I would have thought you could heat the conservatory for a lot less than 20A. 24 sqm could be done with a single 150W/sqm mat, which will be 16A. Unless you're using it as a sole source of heat?

 

[Steve]

Yeah, you basically need two parts to the circuit, a low current control side, and the high current heat side. You conect your control gear up as the instructions say, then where you would connect the heating wire, you connect the contactor's coil, which then controls the heating wire.