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.

 

[Pete999]

You don't seem to have a Neutral anywhere, without a Neutral you will be hard pressed to form a new circuit.

 

[hillwalker01]

The neutral connections are all in the junction box - there is no neutral in the existing switch wiring, only live and switched live (and common for the two-way circuit). And earth of course.

 

[Pete999]

The neutral connections are all in the junction box - there is no neutral in the existing switch wiring, only live and switched live (and common for the two-way circuit). And earth of course.

Sorry what junction box are you talking about do you have any pictures?

 

[DPG]

Yes, photos and a diagram of existing wiring would definitely help here.

 

[hillwalker01]

The junction box is essentially the same as the following, with cable C being one of the existing cables to the switch (for the single-way light circuit):

The second cable for the existing two-way circuit is essentially the same as cable D in this diagram, going to the lower of the two switches shown (but is actually wired to a junction box rather the other switch).

 

[Pete999]

Yes all very straight forward,
Questions
1 Can you get access to: the JB the switch drop to the 2 gang switch, as you will need to get a twin and cpc from the switch and the JB to the new lighting point, or a Twin and cpc from the JBto the existing switch and fro the existing switch to the new lighting position.

 

[Wilko]

If I understand correctly - the light switch has 2 L in it now and one of them is part of a 2 way? I may have missed it - but have you confirmed its 1 mcb or 2 that control the existing?

 

[Lucien Nunes]

Yes, this is important. You mention two or three circuits - you do not have enough cores if there is more than one actual circuit, because no parts can be shared. If it is all one circuit and you mean two or three lamp loads on that circuit, then you do have enough cores.

I infer from your post that the SL and PL of the 2-way are the strappers L1 & L2, and the commons of the two switches are simply linked. In this case I would use the 3C+E as L1, L2 and Com of the 2-way, pick the feed for the other two switches up from whichever of L1 or L2 is the PL, and make the 2C+E the other two switched lives from those. This makes the 2-way wiring conventional and easier to follow.

If instead L1 and L2 are linked, and com at one end is PL and the other end SL, then you would need to ensure that the PL is at the end with the additional switches and the SL the other end, or reconnect as the drawing above.

If the cables from the JB to the switch follow the same route, there is no harm in current from one cable returning via the other - it will not generate interference. If they follow different routes, then the unbalance of current can create mild radio interference if a dimmer or certain kinds of LED driver are in use. To avoid confusion, it is absolutely unacceptable for current from one *circuit* to return via another *circuit*.

 

[Deleted member 9648]

To the OP, a circuit is a single circuit provided it is supplied by a single protective device (fuse or mcb) at it's origin. Regardless of loops/spurs/add-ons etc it will remain one circuit as long as the above applies. Inter-connecting two separate circuits,(supplied by two separate fuses/mcb's) either lives or neutrals is unacceptable and dangerous.

 

[Toneyz]

To the OP, a circuit is a single circuit provided it is supplied by a single protective device (fuse or mcb) at it's origin. Regardless of loops/spurs/add-ons etc it will remain one circuit as long as the above applies. Inter-connecting two separate circuits,(supplied by two separate fuses/mcb's) either lives or neutrals is unacceptable and dangerous.

borrowed neutral !!!!!.

 

[hillwalker01]

Many thanks for the replies, and apologies for my confusing use of the term 'circuit' - rest assured it is all a single MCB circuit, just three lamp loads. (No borrowed neutral!!!)

I think Lucien has reassured me that it's fine, since the cables are taking the same route. And yes, the two-way is wired L1-L1 (PL) and L2-L2 (SL), so I'll do as you suggest.

Thanks!

 

[Pete999]

Many thanks for the replies, and apologies for my confusing use of the term 'circuit' - rest assured it is all a single MCB circuit, just three lamp loads. (No borrowed neutral!!!)

I think Lucien has reassured me that it's fine, since the cables are taking the same route. And yes, the two-way is wired L1-L1 (PL) and L2-L2 (SL), so I'll do as you suggest.

Thanks!

Sorry, I think I may have missed something, can someone explain how the OP is getting the Neutral to his new light and turning the 2g2w into a 3g2w

 

[hillwalker01]

One last thought. Is what I'm doing relatively common?

I've thought of a simpler scenario that doesn't have the added complexity of the two-way switch.

Suppose I have a pair of 2C&E cables currently running to a two gang switch, powering two lamps (one way). No neutral involved in the switches, since there's a JB.

I want to add a third switch and lamp without a new cable to the switch, so I simply take PL for all three from one of the existing PL cores; convert the other old PL to my new SL; and now have one core for PL x 3, and three cores for three SLs - split across two cables.

Is that fairly common practice?

Thanks again!

 

[hillwalker01]

Sorry, I think I may have missed something, can someone explain how the OP is getting the Neutral to his new light and turning the 2g2w into a 3g2w

All neutral connections go via a junction box.

Does my follow up question (simplified scenario) explain it better?

Thank you for you time!