How does my downlight circuit work?

session2 · 30 Jan 2026

I am looking at adding some lights to a kitchen lighting circuit. Iv had a poke around to establish the initial circuit and it's going to be a "leave it to the professionals" job, however, I would like to satisfy my own curiosity and frustration. If anyone would be kind enough to explain I would be extremely grateful.

I have drawn a diagram of what I know so far.

Where would the "unknown destination" cable be going and why can't I find a switch cable - could it be wired from a terminal to the switch directly?

In which case, how would I extend the circuit?

I only have one MCB feedback all my downstairs lights.

The switch arrangement is two gang. Two cables into one switch and 3 in the other for another set of lights.

EFLImpudence · 31 Jan 2026

session2 said:
Where would the "unknown destination" cable be going and why can't I find a switch cable -

As it is drawn, presumably the unknown destination is the switch and it is a switch cable.

session2 said:
could it be wired from a terminal to the switch directly?

I don't think you are using the word terminal correctly but - yes.

session2 said:
In which case, how would I extend the circuit?

Run a cable from the switch to a new switch and then the lights.

session2 said:
I only have one MCB feedback all my downstairs lights.

So what?

session2 said:
The switch arrangement is two gang. Two cables into one switch and 3 in the other for another set of lights.

Ok. You will have to use the switch with the electric supply.

ericmark · 31 Jan 2026

The standard British wiring uses the ceiling rose as a junction box, so when it is replaced with down lights, we should have a junction box

to replace the ceiling rose, the other method is to use the switch back box as a junction box, as done in many other countries. But where the change has been done DIY we often find a mass of tapped up cables.

If there are three wires in a switch it is likely two-way, i.e. connects to another switch somewhere else.

Often wiring of lights is dictated by the access we have to wiring.

session2 · 31 Jan 2026

EFLImpudence said:
As it is drawn, presumably the unknown destination is the switch and it is a switch cable.

I don't think you are using the word terminal correctly but - yes.

Run a cable from the switch to a new switch and then the lights.

So what?

Ok. You will have to use the switch with the electric supply.

Thanks alot EFL.

I think I meant - the wire from the fuse board goes to a junction box directly to the switch and from the switch feeds the light circuit, correct?

I want the new lights to be on the same switch, in which case, could I just continue off the last downlight, it's connecting to a 3 way terminal block for the new lights fitting.

Cheers

session2 · 31 Jan 2026

ericmark said:
The standard British wiring uses the ceiling rose as a junction box, so when it is replaced with down lights, we should have a junction box View attachment 406239 to replace the ceiling rose, the other method is to use the switch back box as a junction box, as done in many other countries. But where the change has been done DIY we often find a mass of tapped up cables.

If there are three wires in a switch it is likely two-way, i.e. connects to another switch somewhere else.

Often wiring of lights is dictated by the access we have to wiring.

Thanks Eric.

Yes, that's right. The 3 cable is feeding a two way. So does that mean that the 2 cable is one power cable going into the switch and one cable going out to the first LED i.e. The unknown destination wire.

I have no idea where the junction boxes are hidden

pete01 · 31 Jan 2026

Feed in, feed out, maybe a 3 core for the 2 way going to the next switch, feed to downlights

No junction box needed.

session2 · 31 Jan 2026

pete01 said:
Feed in, feed out, maybe a 3 core for the 2 way going to the next switch, feed to downlights

No junction box needed.

Thank Pete. I see, so the live cable would go to one switch and then just feed all other additional switches?

pete01 · 31 Jan 2026

session2 said:
Thank Pete. I see, so the live cable would go to one switch and then just feed all other additional switches?

Yes thats one way of doing it.

EFLImpudence · 31 Jan 2026

session2 said:
I want the new lights to be on the same switch, in which case, could I just continue off the last downlight, it's connecting to a 3 way terminal block for the new lights fitting.

Ok, yes.

ericmark · 31 Jan 2026

Switch back box used as junction box in your case.

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How does my downlight circuit work?

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