Jim Ingram wrote:

Could someone please expain to me what the requirements are for
ensuring a balanced road on a ring main.

The essential requirements are in Reg. 433-02-04 and can be summarised thus:

- min. conductor size is 2.5 mm^2 (with one exception: 1.5 mm^2 is
permitted for 2-core MI cables);

- the as-installed cable rating (Iz), taking into account the
installation method and grouping factors, etc., must not be less than 20
amps;

- under the intended conditions of use, the load current in any part of
the ring should be unlikely to exceed, for long periods, the actual
as-installed value of Iz.

The last few words are significant. Short term loads such as kettles
and toasters (assuming normal domestic use) can be disregarded. The
actual Iz can vary between 20 A (the minimum allowed) and 27 A (clipped
direct, no grouping).

As part of some minor building work I'm planning to install a new
ring main for the kitchen. As is often the case dishwasher and
washing machine are next to each other and would be relatively close
to the start of the ring. I would estimate about 4 m in from the cu
with a total length of circuit of around 17m. If the two appliances
were on together this might be around 6.5 kW giving a current of 27A
of which I estimate 76% would travel down the shorter leg. This would
be getting fairly close to the cables capacity. An additional load
near this point could take the cable to above its rating (~24A for
2.5mm2) and not trip the 32A mcb. I would have thought this situation
to be just about acceptable, as at least some of the loads will be
fairly short lived (minutes rather than hours).

In this example there's probably nothing to worry about, unless you have
other long-term loads. Even dishwashers and washing machines aren't
particularly long term loads: thermostats and diversity will conspire to
reduce the 6.5 kW quite quickly.

Alternatively I could lose some cable under the floorboards to
lengthen the tail or put the wm/dw on a separate radial circuit.

If you are worried, other design options might be to take off the DW &
WM feeds from nearer the mid-point of the ring, running unfused spurs
back to the kitchen (but watch the grouping factors and circuit
impedances), or to wire the whole ring in 4 mm^2.

HTH
--
Andy