Can you use two three cores for three circuits?

[pedr0]

I've got two three core cables (i.e. three core and earth) running to a junction box in my garden from the switch panel inside.

Can I have three different lighting circuts using those two cables? I.e. treating the six cores as if they were like three pairs, rather than two triples?

 

[sxturbo]

How are you getting the earth?

Im not sure you can rely on the swa sheathing for the earth, I may be wrong though I'm sure someone be along soon to advise in greater detail

 

[Harry Bloomfield]

I've got two three core cables (i.e. three core and earth) running to a junction box in my garden from the switch panel inside.

Can I have three different lighting circuts using those two cables? I.e. treating the six cores as if they were like three pairs, rather than two triples?

Not a good idea, current flow and return really need to be in the same cable.

 

[pedr0]

Damn. When we embarked on our full house renovation, I hadn't thought about the garden in detail, so I just told the contractor to allow for three circuits outside. Now a couple of years later, we're finally getting round to the garden. Opened up the junction box in the garden, expecting to see the three twin and earth cables, and instead only two three core cables. Those two cables run to the patress box indoors where there is also a twin and earth feed coming from the consumer unit. The feed is live ready to be connected for the garden lighting.

Contractor has since gone bust, so couldn't follow up with him. Hence asking you helpful lot.

@Harry Bloomfield . When you say not a good idea, where does that fall on the scale of "OK, but not graceful" through "contravenes the regs" to "downright dangerous"?

 

[333rocky333]

How are you getting the earth?

Im not sure you can rely on the swa sheathing for the earth, I may be wrong though I'm sure someone be along soon to advise in greater detail

That is a myth, It is fine to use the Earth of Swa as most sizes due to the amount of steel, the resistance equates to be as good as using one of the copper cores.
In the Ops case i believe he has Pvc cable rather than Swa, so the Earth is a bare copper wire, which can only be used as Earth.
Live and Neutral should be combined in the same sheath really, so I do not agree with the Ops proposal.

 

[JohnW2]

Im not sure you can rely on the swa sheathing for the earth, ....

SWA?? ...

.... (i.e. three core and earth) ....

Kind Regards, John

 

[JohnW2]

Not a good idea, current flow and return really need to be in the same cable.

Theoretically true, but for lighting circuits, and outdoors, I don't think I would be too concerned about that.

Kind Regards, John

 

[333rocky333]

521.8.1, though ther is an exception for parralel conducters depends how you interpret the regs

 

[JohnW2]

521.8.1, ...

I wasn't aware of that - although it remains the case that I would not personally be concerned!

... though ther is an exception for parralel conducters depends how you interpret the regs

There is - but that's not really relevant to the OP's situation.

Kind Regards, John

 

[333rocky333]

I wasn't aware of that - although it remains the case that I would not personally be concerned!
There is - but that's not really relevant to the OP's situation.

Kind Regards, John

I did not get that parralel
bit can you elaborate please what they mean please

 

[Harry Bloomfield]

@Harry Bloomfield . When you say not a good idea, where does that fall on the scale of "OK, but not graceful" through "contravenes the regs" to "downright dangerous"?

My concern is eddy currents, if flow and return/ live and neutral do not follow exactly the same route. Low current garden lights will likely not produce much of a problem, none the less it is very bad practice to separate the two.

Basically - any current flowing through a conductor creates a magnetic field around it, which in the case of ac current is how a transformer operates. Any steel through which such a lone cable passes can have an eddy current induced into it, generating heat. Having both flow and return adjacent to each other, cancels this out completely. Basically, you never drill a hole in iron or steel and pass just the flow or return through it, always both through the same hole. You never run either flow or return by itself in a conduit, always both flow and return to avoid this issue.

 

[pedr0]

In the Ops case i believe he has Pvc cable rather than Swa, so the Earth is a bare copper wire, which can only be used as Earth.

Yes, the cable is the three core stuff with grey pvc on the outside, then black, brown & grey inside and a bare copper for earth.

 

[sxturbo]

SWA?? ...


Kind Regards, John

Misread what he said, I saw external cable mentioned and assumed the cable type and speed read the rest of the post

 

[AdrianUK]

I've got two three core cables (i.e. three core and earth) running to a junction box in my garden from the switch panel inside.

Can I have three different lighting circuts using those two cables? I.e. treating the six cores as if they were like three pairs, rather than two triples?

Providing that the O/P doesn't exceed the rating of a core and arranges things so that each cable has two switched lives (ie from the same OCPD), a neutral & an earth - I don't see why he cant have 4 different switched circuits using the two lots of three core plus earth cables that he has.

 

[pedr0]

two switched lives (ie from the same OCPD), a neutral & an earth

Ooh! this is interesting. There is just one feed into the patress box, so all power for the garden lighting comes from one MCB on the consumer unit (I believe there are other interior lights on that MCB too but would have to double check to be sure).

I haven't chosen the fittings yet so don't know total power yet, but I'm just doing a handful of leds for my small garden - not lighting up any giant oak trees.

What sort of ampage/power could I have on any one live core if I arranged it like you say @AdrianUK with two lives and a single neutral?

Edit: I can see one of the cables is 3x1mm+1, so I assume the other is 1mm too.