I thought it was a general rule that the CCC of T&E when surrounded by insulation was simply half of the CCC when the cable was clipped direct.
Any ideas why this does not apply to 4mm T&E in table 4D5 of the regs?
Yes, although 523.7 (3rd paragraph) makes it clear that the factor of
0.5 only applies "in the absence of more precise information" - so, for T&E, Table 4D5 takes precedence.Dunno - interesting observation though: the ratio of the ref. method 103 rating to the clipped-direct figure is exactly 0.5 for all sizes except
4 mm^2. For '4-mil' it's 17.5/37 or 0.47 - a 'loss' of one amp of CCC. 4-mil has the smallest CPC, in relation to the live conductors, but I can't see how that would affect the removal of heat from the cable significantly. Perhaps it's a typo in the Good Book - oh, surely not! 4D5 was added in an amendment to the 16th ed., and the new reference installation methods arrived with the 17th. The figures were based on new tests commissioned at ERA Technology (IIRC) - there was at least one article in /Wiring Matters/ about this, and the clipped-direct ratings went up an amp or two. Checking back through older data might find a clue as to what's happened here.Anyone else care to speculate...
Other than the CPC anomaly, I can't think of any obvious explanation...
Oft copied typo would certainly be a possible explanation (we even have it faithfully transcribed into the wiki ;-)
If it were CPC related, would you not expect to see a similar "non linearity" between 1.0 and 1.5mm^2 T&E which both share the same 1mm CPCs?
Or indeed that 16mm T&E (that has an identical CPC/line conductor ratio as
4mm T&E) is derated by 50% when installed in insulation according to table 4D5!HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.