No, that seems OK. You could mention the need to design out any risk of long-term small overload, i.e.anything likely to overheat the cable but not trip the OPD. [Reg. 433.1]
Now, in the box for Example 2 you've got:
As well as the obvious typo, (i) BS 7671 calls that factor Cc, not Cr and (ii) this is not applied in calculating Iz, it's applied *to* Iz to determine It as described earlier in the bit headed "Case 1". Taking the Wiki wording literally would result in it being applied twice, which is OTT, but at least errs on the safe side.
In the last para of the worked example there is a case of flagrant apostrophe abuse. Also, as a second alternative, the example circuit could be wired as a 32 A ring in 2.5 T&E.
Can't immediately think of appropriate wording or an example of a typical long term small overload (other than a design error)!
Will come back to that...
Erm, yup that is a bit conspicuous ;-)
Adam's choice of identifier - so not sure where Cr came from, however it seem different books seem to use different identifiers. Probably a reflection of the fact that 6.2.1 (BS7671 16th Edition) just uses the literal constant 0.725 in the equations, rather than providing a named identifier for it as in the 17th edition.
We have a slight inconsistency here in the style of presentation which is probably not helping. Case 1 includes the check that Iz >= In /
0.725, but the calculation shown later factors the fusing derating into Iz by application of the factors to the tabulated CCC of the selected cable.
Would this be better resolved by simplifying case 1 to simply say:
:In >= Ib (Overload Protective Device rating is equal to or bigger than load) :Iz >= In (cable rating (as-installed) is equal to bigger than the Overload Protective Device rating)
And not mentioning the fusing factor since its incorporated into the sums later?
Erm, move along, nothing to see here... ;-)
Yes, good point actually - kind of shows how getting into a certain mindset (i.e. ring circuits being used for general purpose socket circuits, and dedicated appliance circuits are radials) can blind one to an obvious possibility that they can be used for a dedicated fixed appliance circuit as well.
An example might be the addition of an excessive number of halogen downlighters to an existing 5 or 6 A lighting circuit, such that the circuit is regularly overloaded for hours on end.
Another, common in the past in poorly insulated bedsits etc. (HMOs), arose where each tenant brings in their own electric heater and a common ring cct gets regularly overloaded.
Errm, yes. The flaw really is using Iz in this way at all. The aim for design-from-scratch is to find the required *It*. It's all there on pages 256 and 257 of the big red book. Probably best to follow their methodology more closely (and also to introduce the "not liable to simultaneous overload" formula.
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