Rules for multiple lives in one conduit

[crj10]

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).

Mainly, noticing that the neutral travelled from the double-pole isolator, half-way along the garage, through an unswitched FCU, past a light switch and then back the other direction in another piece of twin-and-earth. Effectively, it's a continuous, unswitched, unfused conductor that's gone six metres to end up mere millimetres below where it started out.

My general instinct to optimise says that ought to be unnecessary, though I do understand the arguments against going four-wire instead. (-8

 

[JohnW2]

Mainly, noticing that the neutral travelled from the double-pole isolator, half-way along the garage, through an unswitched FCU, past a light switch and then back the other direction in another piece of twin-and-earth. Effectively, it's a continuous, unswitched, unfused conductor that's gone six metres to end up mere millimetres below where it started out.
My general instinct to optimise says that ought to be unnecessary, though I do understand the arguments against going four-wire instead. (-8

Fair enough, but one needs to consider what one is optimising (you can't please all of the considerations all of the time ). Quite apart from the technical arguments, I suspect that time, effort, 'conventionality' and quite possibly cost would be optimised by using the ('conventional') 2xT&E approach - so that would have to be balanced against the considerations which could be optimised by going the four-wire approach.

Kind Regards, John.

 

[crj10]

I suspect that time, effort, 'conventionality' and quite possibly cost would be optimised by using the ('conventional') 2xT&E approach - so that would have to be balanced against the considerations which could be optimised by going the four-wire approach.

Mmm. That does make sense, and is roughly what I expected people to say, but it's interesting (to me, at least) to explore the edge cases.

So suppose one did want to wire in conduit and, for the sake of conventionality, send neutral along with the live and then back. How is that achieved? Two neutral conductors in different colours? Numbered neutrals? Two pieces of T&E in the conduit so neutrals get grouped with lives? Or does one have to use two conduits one above the other?

 

[JohnW2]

I suspect that time, effort, 'conventionality' and quite possibly cost would be optimised by using the ('conventional') 2xT&E approach - so that would have to be balanced against the considerations which could be optimised by going the four-wire approach.

Mmm. That does make sense, and is roughly what I expected people to say, but it's interesting (to me, at least) to explore the edge cases.
So suppose one did want to wire in conduit and, for the sake of conventionality, send neutral along with the live and then back. How is that achieved? Two neutral conductors in different colours? Numbered neutrals? Two pieces of T&E in the conduit so neutrals get grouped with lives? Or does one have to use two conduits one above the other?

Your getting me confused now. As I said, the conventional approach would be two pieces of T&E - and that would go into conduit if you considered that necessary for mechanical protection or cosmetic reasons - but you seem to be talking about singles. Even with 2xT&E, you'd still need to identify which was which.

Kind Regards, John.

 

[crj10]

Your getting me confused now.

Sorry. I was just wondering, if sharing a neutral single-conductor in conduit is regarded as a bad idea, what one should do instead if using conduit. And I was considering several options. I'll try to list them more clearly.

One option would be two lives in different colours and two neutrals in different colours - but neutrals must always be blue, by my understanding?

Another would be to label the singles in some way other than colour. (Conductor size? Numbering?)

Another would be to just stuff two browns and two blues in the conduit and leave people to work it out for themselves.

Alternatively, could you put two twin-and-earths in a single conduit? That would at least group each live with its associated neutral. Maybe the T&Es could then be labelled or distinguished by size?

The solution that would obviously be acceptable, but seems like complete overkill, is two separate conduits.

 

[JohnW2]

Sorry. I was just wondering, if sharing a neutral single-conductor in conduit is regarded as a bad idea, what one should do instead if using conduit. And I was considering several options. I'll try to list them more clearly.

I'm afraid I'm still a bit confused, since you syaing your interest was in "what one should do instead of conduit", yet most of the options you go on to list actually involve conduit!

(with my numbering added)

(1)..One option would be two lives in different colours and two neutrals in different colours - but neutrals must always be blue, by my understanding?
(2)..Another would be to label the singles in some way other than colour. (Conductor size? Numbering?)
(3..)Another would be to just stuff two browns and two blues in the conduit and leave people to work it out for themselves.
(4)..Alternatively, could you put two twin-and-earths in a single conduit? That would at least group each live with its associated neutral. Maybe the T&Es could then be labelled or distinguished by size?
(5)..The solution that would obviously be acceptable, but seems like complete overkill, is two separate conduits.

As you say, (1) is not possible and I'm sure I don't need to tell you that (3) is a bad idea! You also seem to again be overlooking the fact that with two T&E's, you don't necessarily need conduit at all.

I'll leave the professionals to answer your question properly, in terms of what they usually do. There are various ways of identifying cables. (Single-insulated) singles obviously must be in conduit. As I understand it, provided it does not result in too much de-rating of current carrying capacity, there is no problem in having multiple singles, or multiple T&Es, in the same conduit, provided they are adequately identfied - and, as you surmise, you must stick to the standard colours for L & N in addition to whatever other identification may be added.

Kind Regards, John.