Using earth wire in 3-core cable (for Nest install)

[daveaspy]

I have a Nest thermostat to wire up - the heatlink which goes near the boiler requires 240v and a low voltage switching pair. I have a load of 3-core+earth cable from a lighting project and was wondering if it is a reasonable to use two cores for the supply, and the remaining core plus the earth (suitably sleeved) for the switching circuit?

The alternative, probably more correct way, is 2 runs of twin+earth but it would be nicer with 1 cable If I have to go with Twin+earth, the heatlink has no earth connection as far as I know, would you connect earth at the boiler end but just snip off at the heatlink end, or snip off at both ends?

 

[ban-all-sheds]

was wondering if it is a reasonable to use two cores for the supply, and the remaining core plus the earth (suitably sleeved) for the switching circuit?

No.

Never.

Under no circumstances.

Absolutely not.

Don't even think about it.

The alternative, probably more correct way, is 2 runs of twin+earth but it would be nicer with 1 cable

Never mind.

If I have to go with Twin+earth, the heatlink has no earth connection as far as I know, would you connect earth at the boiler end but just snip off at the heatlink end, or snip off at both ends?

Not the latter:

411.3.1.1 A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed-conductive-parts and suspended from such a point.

And preferably terminate the cpc in a bit of choc-block rather than cut it back in case it is ever needed.

 

[daveaspy]

ha - that is what I expected, but wondered about "the real world" and all that It isn't a problem using two cables, just have to buy some two core!

 

[ban-all-sheds]

ha - that is what I expected, but wondered about "the real world" and all that

In the real world people do all sorts of dangerous, stupid, illegal things, and sometimes people die as a result.

It isn't a problem using two cables, just have to buy some two core!

Or 5-core.

Or use 2 x 3C+E.

 

[daveaspy]

I suppose 2 x 3 core might be ok, it is only small stuff iirc. thanks

 

[ban-all-sheds]

It's only 2-3mm wider than T/E.

 

[ericmark]

I would agree triple and earth is the way to go however one of the major scheme operators has pointed out the regulations states:-

514.4.2 Protective conductor
The bi-colour combination green-and-yellow shall be used exclusively for identification of a protective conductor and this combination shall not be used for any, other Purpose.
Single-core cables that are Coloured green-and-yellow throughout their length shall only be used as a protective conductor and shall not be over-marked at their terminations, except as permitted by Regulation

It stipulates single-core cables which could be read as meaning multi-core cables can be over-marked.

Personally I would never over-mark and earth cable but it does seem some people do.

 

[AndyPRK]

I think the OP was considering using the bare conductor.

Anyway 5 core flex is the stuff for the job

 

[daveaspy]

yes, my question was over using the bare earth conductor as the switching pair in my boiler is not at mains level - but I wont be doing that now

Any reason why flex is a better option - just because it has sufficient cores? For fixed applications I always thought flat cable was better, although maybe white flex blends into the wall colour nicer...

 

[AndyPRK]

As you say it has the correct number of cores.

It's not unusual for heating controls to use flex.

 

[bernardgreen]

If the switched pair are ELV ( 12 or 24 volt etc ) then I would run twin AND earth to supply the 230 volts to the state and then run a separate twin cable for the ELV circuit. And that twin need not be rated at 230 volt.

My reasoning is to separate the LV from the ELV to avoid cable damage put 230 volts in the ELV circuits and thus damaging / destroying the PCB and other ELV components.

 

[ban-all-sheds]

If the switched pair are ELV ( 12 or 24 volt etc ) then I would run twin AND earth to supply the 230 volts to the state and then run a separate twin cable for the ELV circuit. And that twin need not be rated at 230 volt.

But if he already has 3C+E cable, and it will fit, why contribute to the consumption of more raw materials and the production and transportation of more finished goods if he doesn't have to?

My reasoning is to separate the LV from the ELV to avoid cable damage put 230 volts in the ELV circuits and thus damaging / destroying the PCB and other ELV components.

There are millions of multi-core cables out there carrying a mix of LV & ELV - I think your concerns are exaggerated.

 

[JohnW2]

If the switched pair are ELV ( 12 or 24 volt etc ) then I would run twin AND earth to supply the 230 volts to the state and then run a separate twin cable for the ELV circuit. And that twin need not be rated at 230 volt.

But if he already has 3C+E cable, and it will fit, why contribute to the consumption of more raw materials and the production and transportation of more finished goods if he doesn't have to?

I slightly misread what bernard wrote. Given that the OP already has lots of 3C+E cable available, and will have to use two of them to get enough usable cores, I thought bernard was simply advocating using one for the LV and the other for the ELV (I hadn't noticed the reference to a 'separate twin cable'). Whilst I agree with you that concerns about mixed LV/ELV cables are probably unnecessary (provided all cores are rated for LV), if one has to use two cables, then it seems to make total sense to have one for each voltage.

Kind Regards, John

 

[ban-all-sheds]

Well yes - surely nobody would consider splitting LV between the 2 cables?

On a wider, digression sort of thing, if the application required two ELV pairs, then given that 411.3.1.1 has been satisfied by the cpc in the LV cable, what would the panel think about using the bare core in the other cable for ELV?

 

[Jackrae]

I would agree triple and earth is the way to go however one of the major scheme operators has pointed out the regulations states:-

514.4.2 Protective conductor
The bi-colour combination green-and-yellow shall be used exclusively for identification of a protective conductor and this combination shall not be used for any, other Purpose.
Single-core cables that are Coloured green-and-yellow throughout their length shall only be used as a protective conductor and shall not be over-marked at their terminations, except as permitted by Regulation

It stipulates single-core cables which could be read as meaning multi-core cables can be over-marked.

Personally I would never over-mark and earth cable but it does seem some people do.

Eric

Sorry but your logic is somewhat screwed.

A regulation relating specifically to single core cables does not imply you can do what you like with multicore