Common live wire for three circuits

[hillwalker01]

You haven't explained how you are getting a N to your new light

See the first diagram I posted above. There is no neutral going to the switch - only PL and SL (cable C). Neutral runs from cable A to B (the loop), and also D to the lamp.

The whole junction box is effectively duplicated, for the two existing lights - so two x Cable C.

I will be adding a third junction box and light, again essentially the same. BUT, I will be joining the Live cores for all three together (the red core in cable C in the diagram); and taking the three switched lives from the remaining cores I have available.

But there are no shenanigans with the Neutral - that's wired in the normal way in the junction box.

PS Sorry I'm not very good at knocking up diagrams - that would make it all much clearer!

 

[HandySparks]

You haven't explained how you are getting a N to your new light

Read the OP:

...It's an older property but pretty conventional loop-in loop-out radial circuit, using junction boxes.

I have good access to the junction box...

 

[sparksburnout]

You can't help getting the feeling that some poor sod is going to get a call out at the end of this.

 

[Lucien Nunes]

If I've got this right, see attached sketch. The OP has S1 and S2, wired conventionally, but wants to add S3. He proposes to do it by adding the dotted connections, to get the PL of S2 and S3 from core Z of the 3C+E, disconnecting the duplicated PL core X of the 2C+E and re-purposing it as Y, to take the new SL3 back to the JB. From there he can pick up a neutral to go with it to the new light.

His concern was that unlike the existing arrangement, the SL and PL of S2 and S3 won't be in the same cable, so there will be a current loop. This is OK, however, because he confirms that the cables take the same route. So it seems to me, he has it under control.

 

[Lucien Nunes]

Would help if I had attached the sketch.

 

[hillwalker01]

If I've got this right, see attached sketch. The OP has S1 and S2, wired conventionally, but wants to add S3. He proposes to do it by adding the dotted connections, to get the PL of S2 and S3 from core Z of the 3C+E, disconnecting the duplicated PL core X of the 2C+E and re-purposing it as Y, to take the new SL3 back to the JB. From there he can pick up a neutral to go with it to the new light.

His concern was that unlike the existing arrangement, the SL and PL of S2 and S3 won't be in the same cable, so there will be a current loop. This is OK, however, because he confirms that the cables take the same route. So it seems to me, he has it under control.

Exactly!

A picture speaks a thousand words. Many thanks fort taking the time!

 

[Mark42]

Think of it, electrically, as singles in conduit: in which case it's entirely routine wiring practice.
The fact that in this case some cores are bunched with certain others inside a PVC sheath: so what?
But make notes for the next guy. Who may well be you. And you won't, despite understanding everything completely today, remember what it was you did!