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Common live wire for three circuits

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hillwalker01

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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.
 
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hillwalker01 said:
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.
Click to expand...
Sorry what junction box are you talking about do you have any pictures?
 
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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):

[ElectriciansForums.net] Common live wire for three circuits


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).

[ElectriciansForums.net] Common live wire for three circuits
 
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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.
 
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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?
 
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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*.
 
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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.
 
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wirepuller said:
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.
Click to expand...
borrowed neutral !!!!!.
 
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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!
 
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hillwalker01 said:
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!
Click to expand...
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
 
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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!
 
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hillwalker01 said:
All neutral connections go via a junction box.

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

Thank you for you time!
Click to expand...
As I see it, you have 2 switches 2 cables 1 twin and 1 3 core, the 2 core is a live and a switched live controlling a light, the 3 core is part of a 2 way lighting system controlling another light.
You want to add a third switch to control an new set of lights, is my assumption correct?
 
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Pete999 said:
As I see it, you have 2 switches 2 cables 1 twin and 1 3 core, the 2 core is a live and a switched live controlling a light, the 3 core is part of a 2 way lighting system controlling another light.
You want to add a third switch to control an new set of lights, is my assumption correct?
Click to expand...
Yes that's correct - in my original scenario. In my follow-up, I gave a simpler version of the same principle, where everything is one way.

So two cables, both twin core, each currently live and switched live.

Convert to:

1 core = live, common to all three lights.
3 remaining cores = 3 switched lives.

You now have a three gang switch with just two twin-core cables.

(My main concern was induction interference, since the cables are carrying imbalanced currents - e.g. 3A out in one core, only 1A return in that cable, and 2 x 1A return in the second cable.)
 
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hillwalker01 said:
Yes that's correct - in my original scenario. In my follow-up, I gave a simpler version of the same principle, where everything is one way.

So two cables, both twin core, each currently live and switched live.

Convert to:

1 core = live, common to all three lights.
3 remaining cores = 3 switched lives.

You now have a three gang switch with just two twin-core cables.

(My main concern was induction interference, since the cables are carrying imbalanced currents - e.g. 3A out in one core, only 1A return in that cable, and 2 x 1A return in the second cable.)
Click to expand...
Your description says a twin and a 3 core, the three core will go from Switch 1 to Switch 2 which I assume Switch 1 is for example a hall light on the one way, and the 2 way controlling the landing light.
 
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Yes, but the 3 core runs right past the junction box on its way to switch 2, so I can easily break it and re-purpose the run of 3 core that goes to switch 1.

But don't get too hung up on my original description - the basic principle I was querying is better described by my follow-up, simpler scenario. It's really just about feeding live for three switches from a single core in one cable, while the three switched live cores are split across two cables.

Lucien has indicated this is fine, as long as the two cables take the same route.
 
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hillwalker01 said:
Yes, but the 3 core runs right past the junction box on its way to switch 2, so I can easily break it and re-purpose the run of 3 core that goes to switch 1.

But don't get too hung up on my original description - the basic principle I was querying is better described by my follow-up, simpler scenario. It's really just about feeding live for three switches from a single core in one cable, while the three switched live cores are split across two cables.

Lucien has indicated this is fine, as long as the two cables take the same route.
Click to expand...
You haven't explained how you are getting a N to your new light
 
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Pete999 said:
You haven't explained how you are getting a N to your new light
Click to expand...
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! :)
 
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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.
 
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