There is a flaw. If you size the cable in step 2, you can't apply the rating factors. The steps are as follows
1 calculate the load (Ib).
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
/ product of rating factors") you talk about (for which there seems to be no recognised symbol) is not
"known as 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
) 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
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
. 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