when looking up current carrying capacities in the 7671 regulations ( IEE ) how would you decide which reference method to use if there where two used in the intallation in question eg if general twin core and earth cable was to run for 10 mtr clipped dirrect and then a further 10 mtr enclosed in a wall?between these two methods the diffrence can be as much as 15 amps

or is it educated guess time?

James,
The ratings are based on the maximum temperature at which the cable should be allowed to run, hence the fact that the "clipped direct" rating is higher because it will lose heat to the surrounding air more quickly than an enclosed cable. (The "clipped direct" ratings are too high in the current Regs., in my not-so-humble opinion, but that's another story. )

So in the situation you describe, you would have to go with the lower current and limit the cable to its enclosed rating.

just to clear it up then no matter what reference method used always go with the smallest rating method!
ie if there was again two core and earth cable to run 20 mtrs cliped direct and 1 mtr to drop down to fuse board in a insulated wall the insulated wall factor would be the factor used?

sorry to drag this one out but i think the regs fall down on scenarios such as this especialy when there is such a diffrence in carrying capacities involved!

James,

The copper leads the heat away along the cable. This means that the current carrying capacity is higher than otherwise if the cable is enclosed or exposed to a higher than normal ambient temperature only for a short distance. I don't know if, and if so how, this is reflected in the IEE regulations, but it is likely that there are special provisions for this case. This doesn't mean you can use the clipped directly/free air value, but you may be able to use a higher value than that of a cable enclosed in an insulated wall for an indefinite length.

Yes, as C-H says, where the length of cable in insulation is only very short, it is permissible to use a higher rating.

Here's an extract from the Electrician's Guide:
http://www.tlc-direct.co.uk/Book/4.3.6.htm