Can this chime be powered from a 6amp breaker or a lighting circuit?

back to my confusion… I thought you start at the other end? Firstly, figure out the load requirement of your appliance, etc. A chime in my case. This will then inform the circuit and cable size. Can you clarify this for me please. Thanks
Yes, you're essentially correct - it is a two (sometimes three) stage process.

The design process for a circuit obviously must start with knowledge of the size of the load (or, at least, as estimate of - e.g. in the case of a sockets circuit), and that will dictate the minimum size of cable that would be acceptable (to safely carry the current resulting from the load).

However, that merely determines the minimum cable size which would be adequate - and in the case of a very small load (like a door chime) would result in a 'minimum cable size' that was 'ridiculously small' - so, for a cable supplying a very small load, the real-world cable will almost always be far larger than is theoretically (electrically) needed.

Having chosen a cable size which is at least large enough (maybe even 'much larger than necessary for the load') the second stage is to determine the rating of the over-current device (MCB/RCBO/fuse) that is going to protect the cable. That rating must be no greater than the current-carrying capacity of the cable BUT also no less than the current required by the load.

That 'BUT' may sometimes lead to 'third stage' (changing the decision about cable size), due to the fact that the current-carry capacities of cables and the ratings of available protective devices do not necessarily correspond. Consider a hypothetical situation in which you were designing a radial circuit to supply a 26A load, using cable that would be installed using 'Method C'. With that installation method, 2.5mm² T+E has a CCC of 27A, so would appear to be adequate. However, the lowest rating of an MCB which satisfied that 'BUT' (i.e. a rating of at least 26A) would usually be 32A (a 25A one, even if you could find it, would be <26A, hence theoretically not acceptable), and that would be too high to protect the 2.5mm² cable (since it is above 27A). Hence, in that situation, you would have to move up to the next available cable size (4mm²) in order to satisfy both requirements of the design process.

As I said before, and as others are now saying to you as well, although I greatly admire you desire to learn the skills required to do electrical installation work (particularly within CUs) very properly/correctly/safely, that really must be based on your first gaining a good understanding of underlying electrical principles- so, as you have been advised, some reading is really required (and, as has also been said, I'm sure that people here would be very happy to try to help you with any questions/problems that arose from that reading).

Kind Regards, John

I have previously used flex, running from an existing spur (socket) with a 3 amp fused plug to supply a 24v bell transformer (housed in a 2 gang enclosure). Having spoken to others here, it was absolutely fine to do so.
If it were 0.5mm² flex (CCC =3A) that would be fine, given the 3A fuse in the plug, and similarly would also be fine if supplied via an FCU with a 3A fuse in it.

However, if it were connected directly to the circuit (no fuse in a plug or FCU) that would, at first sight, seem to be theoretically far from acceptable, since you would then have a cable with only 3A CCC protected by the circuit's MCB (which might well be 32A for a sockets circuit).

However, there is a possible let-out. If it could be successfully argued that the transformer was "unlikely" to result in an 'overload' (rather than 'fault') current (i.e. greater than 3A, but less that 160A {for a B32 MCB}) circuit, then ether would be no requirement for the 3A (or whatever) fuse, so that a 'direct connection to the circuit' would be OK.

Kind Regards, John

Yes, you're essentially correct - it is a two (sometimes three) stage process

The design process for a circuit obviously must start with knowledge of the size of the load (or, at least, as estimate of - e.g. in the case of a sockets circuit), and that will dictate the minimum size of cable that would be acceptable (to safely carry the current resulting from the load).

However, that merely determines the minimum cable size which would be adequate - and in the case of a very small load (like a door chime) would result in a 'minimum cable size' that was 'ridiculously small' - so, for a cable supplying a very small load, the real-world cable will almost always be far larger than is theoretically (electrically) needed.

Having chosen a cable size which is at least large enough (maybe even 'much larger than necessary for the load') the second stage is to determine the rating of the over-current device (MCB/RCBO/fuse) that is going to protect the cable. That rating must be no greater than the current-carrying capacity of the cable BUT also no less than the current required by the load.

That 'BUT' may sometimes lead to 'third stage' (changing the decision about cable size), due to the fact that the current-carry capacities of cables and the ratings of available protective devices do not necessarily correspond. Consider a hypothetical situation in which you were designing a radial circuit to supply a 26A load, using cable that would be installed using 'Method C'. With that installation method, 2.5mm² T+E has a CCC of 27A, so would appear to be adequate. However, the lowest rating of an MCB which satisfied that 'BUT' (i.e. a rating of at least 26A) would usually be 32A (a 25A one, even if you could find it, would be <26A, hence theoretically not acceptable), and that would be too high to protect the 2.5mm² cable (since it is above 27A). Hence, in that situation, you would have to move up to the next available cable size (4mm²) in order to satisfy both requirements of the design process.

As I said before, and as others are now saying to you as well, although I greatly admire you desire to learn the skills required to do electrical installation work (particularly within CUs) very properly/correctly/safely, that really must be based on your first gaining a good understanding of underlying electrical principles- so, as you have been advised, some reading is really required (and, as has also been said, I'm sure that people here would be very happy to try to help you with any questions/problems that arose from that reading).

Kind Regards, John

The process is not quite correct. The steps are:
calculate the load (26A in example);
determine the OPD rating (must be at least 26A, so it is 32A); determine the cable size (must be at least 32A).
The example has t&e, installed using Method C. In the relevant table in BS7671 4mm2 cable is rated at 36A. Your values were correct, but the logic not so.

The process is not quite correct. The steps are: ... calculate the load (26A in example);
determine the OPD rating (must be at least 26A, so it is 32A); determine the cable size (must be at least 32A). ...
Fair enough, but I effectively covered that in the 'third stage' I mentioned - but I agree that it's probably easier to follow when expressed as you have done!
The example has t&e, installed using Method C. In the relevant table in BS7671 4mm2 cable is rated at 36A.
I suppose that depends on which you regard as "the relevant table" - in Table 4D5 it is 37A (36A in Table 4D2A)
Your values were correct, but the logic not so.
I don't think my logic (taking all I wrote into account) was in any way 'incorrect' but, as above, I agree that it was unnecessarily 'contorted' (as compared with your simpler version)

Kind Regards, John

Fair enough, but I effectively covered that in the 'third stage' I mentioned - but I agree that it's probably easier to follow when expressed as you have done!
I suppose that depends on which you regard as "the relevant table" - in Table 4D5 it is 37A (36A in Table 4D2A)
I don't think my logic (taking all I wrote into account) was in any way 'incorrect' but, as above, I agree that it was unnecessarily 'contorted' (as compared with your simpler version)

Kind Regards, John
There is a flaw. If you size the cable in step 2, you can't apply the rating factors. The steps are as follows
2 select the OPD. The rating is (In)
3 calculate the minimum current carrying capacity for the cable.
If no rating factors are in play, this is the value in (In).
If rating factors are in play,
the value = In / product of rating factors
The value is known as (It)
4 referencing the tables in appendix 4,
select the minimum cable which is rated >= (It)

There is a flaw. If you size the cable in step 2, you can't apply the rating factors. The steps are as follows
2 select the OPD. The rating is (In)
3 calculate the minimum current carrying capacity for the cable.
If no rating factors are in play, this is the value in (In).
If rating factors are in play,
the value = In / product of rating factors
The value is known as (It)

4 referencing the tables in appendix 4,
select the minimum cable which is rated >= (It)
In one sense, I can't argue with much of that, since you are largely reporting the process described in Appendix 4 (but I'll come back to the 'flaw' you mention at the end of this post) - but, as below, it seems more than a little odd to me.

As a matter of detail in relation to your statements which I've highlighted in red, I think you have got your terminology a little wrong. As I understand it, the 'adjusted In' ("In / product of rating factors") you talk about (for which there seems to be no recognised symbol) is not "known as It". "It" is, in fact, the CCC tabulated in Appendix 4 (which assumes no rating factors). It is then the 'adjusted In' (for which there is no symbol, and certainly not It) which has to be compared with the tabulated ('unadjusted') CCCs for cables (which IS It) in the Appendix in order (using this method) to select an appropriate cable size.

However, perhaps you can help to educate me, since I've never really understood the logic behind that process, as written in Appendix 4 - and, in particular, don't understand the apparent inconsistency between the process described in that Appendix (which I find strange) and what the actual regulations within BS7671 say (which is totally in keeping with what I would expect).

"What I would have expected" is that the various 'rating factors' (probably rarely for anything other than grouping, and even that probably rarely, in relation to domestic installations) would be applied to the "CCC" figures (which assume no rating factors) tabulated in Appendix 4 (i.e. the It figures), and that the resulting 'adjusted CCC' would then be compared with the actual ('unadjusted') In of the OPD.

The process described in Appendix 4 does essentially the opposite that - applies 'correction factors' to the In of the OPD and then compares that 'adjusted In' with unadjusted tabulated CCCs for cables (i.e. It figures). That seems illogical to me. I admit that, in practice, the two approaches will usually give similar answers, since In will usually not be a lot less than the 'adjusted CCC' (which, as below, is what I presume we mean by Iz) but I don't really understand why the amount of required 'adjustment' (because of rating factors affecting the cable) should depend on the In of the OPD.

It seems (to me) to be particularly illogical/irrational top 'adjust' In (rather than It) for the rating factors, since there is obviously no way in which factors relating to the cable's installation have any effect on the actual In of the OPD. If it's the cable, rather than the OPD, which is affected by these factors, why not (as I would expect) adjust It (to get Iz), rather than In? What am I missing?

However, if one moves from the ('informative') Appendix of BS7671 to the actual regulations in BS7671, then everything seems to be exactly 'as I would have expected' (as above). 433.1.1 is the primary regulation relating to the overload protection of cables, and it simply boils down to "In ≤ Iz ≤ Ib". There is no adjustment of In (for rating factors) and, although I struggle to find a totally clear definition of Iz (Part 2 and Section 3 of Appendix 4 both say "current-carrying capacity of a cable for continuous service under the particular installation conditions concerned") I can but presume (and have always assumed) that it is the tabulated CCC assuming no rating factors (i.e. It) 'adjusted for' any rating factors (i.e. It multiplied by the product of rating factors). In other words, just as I would expect, per 433.1.1 one adjusts the tabulated It for rating factors (to get Iz) and then compares that with the (unadjusted) In of the OPD to ascertain whether 433.1.1 is satisfied.

... so why this difference between Appendix 4 and 433.1.1 (the former seeming strange to me, and the latter being 'what I would expect')?

Furthermore, the "433.1.1 approach" seems to be the way that nearly everyone thinks about this. If there are relevant (de-)rating factors, they invariably talk about 'de-rating the cable', not 'up-rating the In of the OPD' - which seems (to me) to make total sense.

Going back to your initial comment about a 'flaw' in what I wrote, it doesn't really exist if one uses the "433.1.1 approach", since one then adjusts the tabulated It for rating factors (just as one selects the appropriate column of the tabulation, according to installation method) and uses that adjusted figure to compare with (unadjusted) In of the OPD. My only failing was in just saying that I was "assuming Method C", whereas I should have said "assuming Method C with no de-rating factors" - and if there were any de-rating factors, then the figure one would use would be the 'adjusted' version of It (which, as above, I assume is what we call Iz).

Kind Regards, John

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