Common live wire for three circuits

[hillwalker01]

Hi,

Hoping someone can help me with a lighting circuit challenge.

I have an existing two gang switch, with one 2C&E cable for a simple circuit and one 3C&E for a two-way circuit (using conventional 3 control wiring). It's an older property but pretty conventional loop-in loop-out radial circuit, using junction boxes.

I want to add a third circuit (low voltage under-cupboard lighting), replacing the two-gang switch with a 3-gang. The new circuit will be a simple one-way switching.

I have good access to the junction box, but cannot easily run a new cable to the switch - the existing cables are embedded in plaster and the wall is tiled.

It occurs to me that, in theory, I do have sufficient wires for the three circuits, if I use a single live wire for all three. I.e.:

Cable A
Red - Common Live
Black - Switched Live 1
Cable B
Red - Switched Live 2
Yellow - Common (for the two-way circuit)
Blue - Switch Live 3 (the two-way circuit)

Is this permissible? Obviously I need to ensure the wires are sufficiently rated for the total current. I can think a few possible concerns:

1. Non-standard wiring. Would someone coming to such a wiring be confused, or could they be expected to think "Ah yes, the old 3-circuits-in-2-cables trick... so one of these wires will be common live, etc.."? Obviously I'll mark all the switched live wires with red tape.

2. Maintainability - any future issues likely in terms of fault tracing, testing, extending the circuits etc?

3. Induction loop / interference - I know it's a Very Bad Idea to e.g. run live off one circuit and neutral off another, not least because it turns the house into a giant induction loop. But would that be an issue if the cables are adjacent, going side by side from the same junction box to the same switch?

4. Balance of the currents - e.g. one wire carries 3 amps, and 3 wires (one in the same cable; two in another) carry 1A each return. Any issues with this (again, inductance..?)?

5. Any issues with voltage spikes etc., e.g. for the low voltage transformer?

6. Any other reasons why this would be a big No No?

Many thanks in advance.

 

[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!