jim_in_sussex wrote:

Is it correct to use IEE table 4D1A/ref method 1? (the illustrations
for method 1 don't refer specifically to underground use).

No, for XLPE insulated SWA (to BS 5467) use Table 4E4A. Current rating
is rarely an issue for SWA uses such as this; voltage drop is usually
the limiting factor.

2. Ducting: Instead of burying it in bare soil it looks more sensible
to run it inside a duct, but attempts to source underground ducting
have failed. Does anyone know of a source, or failing that, could
semi-flexible 40mm waste pipe be used? If SWA is ducted, how does it
affect current rating?

SWA is _intended_ for direct burial, as others have said. If you want
to use duct you can get 6m lengths of push-fit service ducting from your
local builder's merchant.

5. Cable size: nominally 6mmsq cable is adequate for the garage load
(ring + freezer radial + lights), but am wondering about using a
larger cable - eg 16mmsq. There'd be 2 benefits - much lower volt
drop, plus less light flicker when a machine starts. Relative to the
whole project the extra cost is small, but are there any practical
snags? Does anyone know the outside diameter of these cables?

For 3-core the o/d for 6mm^2 is 17.0mm and for 16mm^2 it's 22.4mm (and
19.9mm for 10mm^2). [figures from Pirelli data sheet]

6. Finally looking up the ratings for these cables in the IEE regs &
comparing with the TLC catalog, why the differences? eg TLC show swa
16mmsq 2 core (6942x) as 91amps, IEE table 4D4A shows 89amps. In fact
most cable amp values given by TLC are lower than the IEE tables
give.

The TLC figures might be out-of-date info for the older BS 6346 PVC
insulated SWA cables. Table 4E4A gives 110 A (method 1) for 16mm^2. I
advise ignoring any cable current ratings given in re-sellers'
literature as they're frequently wrong. The Pirelli data sheet gives
ratings of 141 A for direct-in-ground, and 115 A in duct, the latter
agrees with 4E4A method 12.

--
Andy