0.75mm2 3 core cable for Combi boiler

[danmanchester]

Evening all,

Just extending the cabling from my old immersion heater to my new combi pre-install and wanted to confirm that it's fine to run 0.75mm2 3-core cable from a junction box to power a 3A fused spur next to the boiler?

Thanks in advance

 

[echoes]

No. 2 reasons:

1. You should not mix stranded and solid conductors in a junction box terminal.
2. It is most unlikely that the feed to the old immersion heater provides protection for 0.75 mm² cable.

The immersion supply will likely be fed from a 16A MCB or a 15A fuse, so in this case, 1.5 mm² or 2.5 mm² T & E cable should be used to power the FCU.
Your 0.75 mm² flex should be used from the FCU to the boiler, and the boiler installer will connect this.

EDIT: Mods - could this be moved to Electrics UK?

 

[Agile]

And being strict about it, the 0.75 mm2 to the boiler should be heat resisting!

Few other installers seem to bother though.

Even worse the manufacturers dont use heat resist when they supply the boiler with a cable already fitted ! But they can do that because their design superceeds the Gas Regulations!

Tony


This part should be in the heating forum!

 

[PrenticeBoyofDerry]

I would continue the additional to the circuit in the same core size as is currently in place, providing the protective device at the fuse/consumer unit is rated correctly.
Then from the FCU that is protecting and isolating the boiler I head use heat protected flex this can be 0.75mm or higher if you wanted.
The joint in the cable length you are altering must be easily accessible for maintenance, inspection and testing. So can not be buried in the wall, unless the joints comply with the requirements of BS7671:2008(Amend1) 526.3

 

[ban-all-sheds]

And being strict about it, the 0.75 mm2 to the boiler should be heat resisting!

Can't say I've ever noticed my boiler getting that hot in the areas where there are cables, PCBs etc.

Even worse the manufacturers dont use heat resist when they supply the boiler with a cable already fitted !

Maybe they know that they've designed their boilers properly.

 

[toasty]

Can't say I've ever noticed my boiler getting that hot in the areas where there are cables, PCBs etc.

Fair point, I've never really though of it like that... Have always ran 3core + earth to a flex outlet plate, then a nice loop of heat resist to the boiler pcb.

As you say though, probably not entirely necessary. Flex seems the right choice though, if only because normally you connect to a hinged part of the boiler.... I don't like to see t+e in that situation.

 

[SimonH2]

Can't say I've ever noticed my boiler getting that hot in the areas where there are cables, PCBs etc.

But then the cable is often tucked behind hot pipes etc - or run in the same boxed in section as the pipes. Your's might not be, but many are.

 

[mikely]

Many boiler installation manuals specify the use of heat resistant cable. For example, from a Viessman manual: ‘The mains cable must be at least 0.75 mm2 (24/0.2 mm) PVC insulated to BS 6500 table 16.’