Rules for multiple lives in one conduit

crj10 · 3 May 2011

As mentioned in my previous posting I've just re-wired my garage.

Whereas before there were two mains sockets, a light switch and two pendants all wired directly to a spur from my ring main, I've now got a 13A switched FCU protecting the garage electrics, followed by sockets and an unswitched 3A FCU protecting the light switch and lights. I believe that conforms to regulations and is safe. (I'm very sure it's safer and more conformant than it was before!)

Doing this meant that for much of the length of the garage I was running two twin-and-earths: one for the 13A supply and the other for the switched 3A lighting sub-circuit. It's just occurred to me to wonder: could I have used singles in conduit instead, with unswitched live, switched live, neutral and earth conductors? By my understanding, that's done for maintained lighting, but is it more generally permissible? Does it matter that the two lives have different fuse ratings (and one might also be thinner than the other)?

My own hunch is that this is probably strictly speaking permitted, but in poor taste and likely to confuse anyone working on the wiring in future. I'd be interested to hear views from the professionals, though!

Adam_151 · 3 May 2011

conduit and trunking systems often carry many different circuits to no problem there, make sure you observe the conduit factors in the OSG though, or your conduit will be overfull/or you'll struggle to get them in.

Also I might be telling you the obvious?, but make sure you have two neutrals as well as two lives and keep them electrically separate (and preferably 2x earths as well)

Taylortwocities · 3 May 2011

Indeed. You cannot, and must not share a single neutral for two separate circuits.

There are safety and conductor loading reasons for circuit separation to include live and neutral conductors.

It would also cause awful problems if the installation were later extended and the socket and lighting circuits were on different RCD or RCBOs.

JohnW2 · 3 May 2011

Adam_151 said:
Also I might be telling you the obvious?, but make sure you have two neutrals as well as two lives and keep them electrically separate (and preferably 2x earths as well)

It may be 'the obvious', but (although I may be wrong) I have a horrible suspicion, from the wording used, that the op was talking about not doing that - i.e. replacing the two T&E cables (6 conductors, including CPCs, in total) with just four single conductors. I must say that, even if it were acceptable (which it isn't), I wouldn't really see any worthwhile benefit.

Kind Regards, John.

crj10 · 3 May 2011

JohnW2";p="1976836 said:
Adam_151 said:

It may be 'the obvious', but (although I may be wrong) I have a horrible suspicion, from the wording used, that the op was talking about not doing that - i.e. replacing the two T&E cables (6 conductors, including CPCs, in total) with just four single conductors.

Click to expand...

Mmm. As I said, that is what happens in maintained emergency lighting systems: switched live, unswitched live, neutral and earth, often even in the same flex. I was just wondering whether this was permitted more generally.

crj10 · 3 May 2011

Also, I can't help wondering why, if there cannot be multiple lives for one neutral (other than as part of a multi-phase supply, obviously), IEC 60446 specify so many possible live colours and only one neutral?

Taylortwocities · 3 May 2011

Mmm. As I said, that is what happens in maintained emergency lighting systems: switched live, unswitched live, neutral and earth, often even in the same flex. I was just wondering whether this was permitted more generally.[/quote]

In the EM lighting situation the lighting circuit is supplied from just one circuit breaker.

In the garage example there are more than one CB. Ring final @ 32amp, lighting @ 6amp.
Don't forget the neutral has to take the full load of the circuit so you would have to size the neutral to take the full load of BOTH circuits.

JohnW2 · 3 May 2011

crj10 said:
Mmm. As I said, that is what happens in maintained emergency lighting systems: switched live, unswitched live, neutral and earth, often even in the same flex. I was just wondering whether this was permitted more generally.

There is no problem in having just those four conductors if the live and switched live relate to the same circuit (in other words, when the switched live becomes live, it is functionally connected to the 'permanently live'). That's quite a common situation. However, as I understand it, that is not what you are talking about.

Kind Regards, John

Spark123 · 3 May 2011

You can have more than one line conductor in a conduit for the same neutral providing the lines are all from the same protective device, you'd normally use brown singles in a single phase conduit system.

crj10 · 4 May 2011

JohnW2 said:
There is no problem in having just those four conductors if the live and switched live relate to the same circuit (in other words, when the switched live becomes live, it is functionally connected to the 'permanently live'). That's quite a common situation. However, as I understand it, that is not what you are talking about.

I was asking firstly about the case where only a switch separated the two lives (which people seem to be saying is completely fine).

Secondly, I was asking about the case where the switched live is also fused to a lower current (though taken from the unswitched live, so that if the unswitched live is isolated, so too is the switched) if the switched-live wiring or attached devices couldn't take the full current of the unswitched live. People are saying this second case is prohibited, which doesn't completely surprise me. (Though, to use your terminology, the two neutrals are still "functionally connected".)

JohnW2 · 4 May 2011

crj10 said:
I was asking firstly about the case where only a switch separated the two lives (which people seem to be saying is completely fine).

I didn't actually interpret it as two questions but, yes, that is fine. It's no different conceptually from what happens in '3-core+ earth' cable in some lighting circuits, and as used almost universally for timer fans etc.

crj10 said:
Secondly, I was asking about the case where the switched live is also fused to a lower current (though taken from the unswitched live, so that if the unswitched live is isolated, so too is the switched) if the switched-live wiring or attached devices couldn't take the full current of the unswitched live. People are saying this second case is prohibited, which doesn't completely surprise me. (Though, to use your terminology, the two neutrals are still "functionally connected".)

Wiser people than me will probably be able to pronounce on this but I'm not sure that it is explicitly prohibited by the regulations, but (although I can't really think of any specific problem it would create) I don't think it would be regarded as good practice. Given the total load is to be restricted to 13A, a 'combined neutral' would be OK in 1.5 mm² cable, although I suspect you might well use 2.5 mm², so that's not a problem. However, as I mentioned before, what I find difficult to understand is the incentive for doing this. I doubt whether using four singles would be significantly (if at all) cheaper than two T&E cables - and, if the T&E is not in conduit, then it would presumably win hands down in terms of cost.

Kind Regards, John.

plugwash · 4 May 2011

As I understand it shared neutrals are allowed within a circuit but not between circuits.

As I understand it the main reason the regs don't allow neutrals to be shared between circuits is that if someone were to isiolate one circuit but not the other and then start working on a device fed from the isolated circuit they could get a nasty shock as they disconnected the shared neutral conductor.

What i'm not sure on is whether a FCU or similar creates a new circuit at it's output terminals or not.

crj10 · 4 May 2011

JohnW2 said:
However, as I mentioned before, what I find difficult to understand is the incentive for doing this. I doubt whether using four singles would be significantly (if at all) cheaper than two T&E cables - and, if the T&E is not in conduit, then it would presumably win hands down in terms of cost.

The motivation for a fuse to the switched live might be not trusting the internal wiring of light fittings to be safe without. Or wanting to use a 10A architrave switch, or whatever.

As I say, for my garage rewiring I didn't do it. I'm mainly asking out of idle curiosity, though perhaps it would make sense if the wiring was being distributed over a larger area and/or one had made the decision to use conduit anyway.

JohnW2 · 4 May 2011

crj10 said:
The motivation for a fuse to the switched live might be not trusting the internal wiring of light fittings to be safe without. Or wanting to use a 10A architrave switch, or whatever.

I have no problem with that (I have exactly the same in my garage) but that was not what I was talking about. I was pondering what might be the motivation for wanting to use four singles in conduit rather than two T&E cables (possibly not in conduit).

Kind Regards, John

JohnW2 · 4 May 2011

plugwash said:
As I understand it shared neutrals are allowed within a circuit but not between circuits.
As I understand it the main reason the regs don't allow neutrals to be shared between circuits is that if someone were to isiolate one circuit but not the other and then start working on a device fed from the isolated circuit they could get a nasty shock as they disconnected the shared neutral conductor.
What i'm not sure on is whether a FCU or similar creates a new circuit at it's output terminals or not.

Yes, that makes sense, which I presume is why shared neutrals are not allowed between circuits which have only SP 'isolation' (e.g. SP MCBs). With DP isolation, the problem you are describing would not arise. ....

In the op's scenario, the primary FCU is switched - so if one used that to 'isolate, both circuits would be dead. A problem could only arise if one attempted to 'isolate' the lighting circuit by only remove the fuse from its (unswitched) FCU - which I suppose it not an impossible scenario.

Kind Regards, John

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Rules for multiple lives in one conduit

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