Separate lives and neutrals through conduit

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And the earth should be thicker in conduit. Or maybe that is just habit
I think it's probably just habit.

In fact, if (in terms of the regs) there is any difference at all (I doubt that there is) it would have the effect of allowing a CPC which was a 'single' to have a smaller CSA than would be the case if it were in a multi-core cable (like T+E). If singles in conduit were NOT regarded as 'bunched' (although they probably are - but the regs don't define the meaning of that word), then the k-value used for the adiabatic calculations to establish the minimum permissible CSA of the CPC would be appreciably greater (143, per Table 54.2, as compared 115 for, say, a CPC in T+E, Table 54.3) - leading to the minimum CSA being nearly 20% less than would be required for a CPC in, say, T+E.

However, as above, I suspect that the intention is that singles within conduit are to be regarded as 'bunched', in which case the calculation, hence required CSA, would be exactly the same for singles as for the CPC of a multi-core cable (such as T+E).

Kind Regards, John
 
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If the CPC is the same size as the live conductors, it isn't necessary to work out if it is big enough.

It's only T&E that uses a smaller CPC.
 
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I hope you all had a great Christmas Day - as explained above, I still have a couple of days to wait for mine :)
Yes, in that you seem to support the ignoring or the wiring Regs, based upon a purely personal view of risk assessment :)
In a literal sense, you are correct, although I think that "ignore" is a fairly hash word to use in relation to the result of an intelligent risk assessment (which one might call 'electrical common sense'). Don't forget that the legal requirement is to comply with Part P of the Building Regs, which does not necessarily require compliance with the letter of BS7671.

In any event, as I've said, the example we are discussing is not really 'as bad' as you are making out. As I've said (even though I regard even this a probably being 'excessively cautious'), I probably would not (and certainly would not advocate) having cables within the same 'containment' carrying LV and ELV if the ELV ones were not sufficiently well insulated give adequate protection at the LV voltage concerned. However, it is my belief that PVC insulated+sheathed signal/comms/whatever cables are more than adequately insulated for 230V, the only problem (at least in some cases) being that (presumably because they have not been tested as such voltages, they may not be officially 'rated' to carry 230V.

For what it's worth, in my opinion an over-obsession with the regs can lead to inconsistences and anomalies - which, at least in some people's minds, could be dangerous.

I respect the fact that you believe that what we are talking about could represent a risk to life/limb to 'a telephone engineer up a pole'. However, as I've said, if you believe that, then I would expect you to at least equally believe that to allow a telephone engineer within your house to touch the 'mains leads' of your cordless phone, answering machine, modem, computer or whatever would represent an even greater risk (only half as many 'layers of insulation' between live conductors and the engineer) of electric shock (which could kill, even when the victim was not 'up pole')? Despite that, you seem to feel obliged to take precautions to minimise the former risk (because 'the regs say you must') but not to take precautions to minimises the latter (probably greater) risk (because 'the regs do not require it').

Kind Regards, John
 
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If the CPC is the same size as the live conductors, it isn't necessary to work out if it is big enough.
That's true, but only because of 434.5.2 (which we rarely hear people mentioning).

We hear a lot about 543.1.3, which includes the adiabatic calculation of minimum CSA for a CPC. However, 434.5.2 says exactly the same, and presents exactly the same equation (and the same tabulated values of 'k') in relation to live conductors.

Hence, if the live conductors have an adequate CSA (per 434.5.2), then a CPC of the same CSA will obviously also be adequate (using the same equation, the same values of PFC & 'k' and the same OPD characteristics).

However, the point is/was that, when using singles, a CPC with as small a CSA as would exist in T+E is perfectly adequate - although it seems conventional/traditional for a 'single' CPC to have the same CSA as the associated live conductors - as Andy and myself has said. presumably just 'habit'.

Kind Regards, John
 
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although it seems conventional/traditional for a 'single' CPC to have the same CSA as the associated live conductors - as Andy and myself has said. presumably just 'habit'.
Yes, because then it isn't necessary to work out if it is big enough.
 
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Yes, because then it isn't necessary to work out if it is big enough.
Maybe, but it seems that people do not even do enough (and/or are not capable of doing enough) thinking to realise that 'working out' (calculation) is not necessary.

From T+E, we know (without questioning it) that, for example, a 1.5mm² CPC is regarded as adequate in relation to a circuit for which 2.5mm² live conductors are appropriate - so, without doing any 'working out', anyone capable of thought should realise that a 1.5mm² 'single CPC' would be adequate for a circuit whose live conductors were 2.5mm² singles.

Kind Regards, John
 
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From T+E, we know (without questioning it) that, for example, a 1.5mm² CPC is regarded as adequate in relation to a circuit for which 2.5mm² live conductors are appropriate -
...and 4mm².

so, without doing any 'working out', anyone capable of thought should realise that a 1.5mm² 'single CPC' would be adequate for a circuit whose live conductors were 2.5mm² singles.
Yes - and 4mm² - I suppose it is adequate in all circumstances, is it?
Would you know without working it out?

Perhaps the wrong questions are being asked.
Why does T&E have smaller CPCs?
Why does 3 core (and more) not have smaller CPCs?
 
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I thought it was for mechanical reasons, so it didn’t get damaged.

or as said, so you don’t accidentally use too small cpc
 
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...and 4mm².
Yep, that too.
Yes - and 4mm² - I suppose it is adequate in all circumstances, is it? Would you know without working it out?
Are you asking whether a 1.5mm² CPC is always adequate for a circuit whose live conductors were 2.5mm² (or 4mm²)?

If so, then that's an interesting question, which I doubt that any of us ever really consider - if the CSA of the live conductors is adequate for the design load, then we simply 'assume' that the CPC that comes in T+E of appropriate size will always be adequate.

I don't know whether it ever happens in practice but I suppose that, in theory, the CPC of T+E is not always (adiabatically) adequate. The required CSA depends upon I²t and, as far as I am aware, I²t continues to rise ('indefinitely') as PFC rises. Since there is 'almost no limit' to how high PFC can be (one sometimes sees figures of 'thousands of amps mentioned), I presume that it is 'not impossible' (with a very short circuit in an installation next to a substation) that it could be so high that the CSA of the standard CPC was inadequate. However, I cannot recall having even seen/heard anyone even considering that possibility.
Perhaps the wrong questions are being asked. Why does T&E have smaller CPCs?
That's easy, but you're perhaps asking the question the wrong way around. The point is that, in adiabatic terms, the live conductors are 'oversized' - but, for those live conductors, there is also the consideration of the ('continuous') current-carrying-capacity (the antithesis of 'adiabatic'). In other words, considering 2.5/1.5mm² T+E, 1.5mm² would be adiabatically adequate for the live conductors, but the cable would then only have the CCC of 1.5mm² cable, not that of "2.5mm²" cable.
Why does 3 core (and more) not have smaller CPCs?
You mean flex? If so, that's another 'good question' - since theory would indicate that the same 'conventional relationship' between CPC and live conductor CSAs would be just the same as in T+E.

Kind Regards, John
 
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Thanks all. Very helpful. I have bought some Brown and Blue multistrand for the rest of the job.

Merry Christmas to you all.
 
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Well for when the cpc gets used for something else
Indeed - but I didn't even dare mention that one :) ....

... I somehow doubt that the manufacturers of flexible cables will have based their design on the possibility of such a controversial practice!

Kind Regards, John
 

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