cable and mcb for central heating

[amstel]

Hello everybody,
I have a modern central heating system, protected at a FCU with a 3 amp fuse, this in turn is supplied from the CU on its own mcb radial circuit,.....but the mcb is 20 amp and cable is 2.5 T&E, there are no problems, but should it be a 6amp mcb with 1 or 1.5 T&E ?
Thanks.

 

[PrenticeBoyofDerry]

I assume that the radial circuit supplies more than one heater?
There is no problem with 2.5mm T&E on a 20A MCB. As 2.5T&Emm can carry a maximum of 27A.
If your heaters only require 3A independent supply on full load( which seems low) and you have 5 heaters on at the same time, you will require a minimum of a 18A supply.
So the question would be how many heaters have you?
But it sounds like its a FCU spur to a combi gas boiler, which is normal, there is likely to be sockets also on this circuit that require a higher current.

 

[riveralt]

The purpose of the MCB/fuse is to protect the cable.
Consequently the size of the MCB should always be less than the current carrying capacity of the cable.
When clipped direct 2.5mm2 T&E has a ccc of 27Amps - so the 20Amp MCB is fine.
The FCU also acts in the same way. I would expect that the cable from the FCU to the boiler is 1mm2 T&E or something similar which has a ccc of around 13Amps. So again the criteria outlined above is satisfied and therefore it is okay.
Assuming you have gas central heating leave it alone - you shouldn't really be looking to change MCB's yourself because it can be very dangerous.

 

[TicklyT]

The MCB is selected to protect the cable to the FCU, and a 20 Amp MCB is usually appropriate for a 2.5mm spur, so there's nothing wrong with your present arrangement.

As you say, a lower rated MCB would be appropriate for a smaller cable, but there's nothing to gain from changing anything.

 

[amstel]

Thank you all,
there are no spurs, no extra heaters, no sockets, it is a straight supply to the fcu from the cu.
From the fcu, the load side is 1.5 flex to a junction box which supplies the boiler,pump, 2 motorised valves etc.
The boiler manufacturers handbook say a 3amp fuse for this fcu, which it is.
I just thought for a 3 amp maximum load a 6 or less mcb with 1/1.5 cable would be ample.
Thanks

 

[EFLImpudence]

I just thought for a 3 amp maximum load a 6 or less mcb with 1/1.5 cable would be ample.

It would.

It may have been for immersion heater or something else before and could be again.