Ring-final cable size

[SparkyTris]

I'm trying to work up an estimate for quantities of cable on a forthcoming project:

For a ring socket circuit, I have made a conservative estimate of 73m circuit length. This is basically the perimeter of 4 rooms and a hallway, from the CU back to the CU.

The area of the ground floor is about 70 sq. m.

There will be 15 sockets in total on the circuit

Table 8A of the OSG indicates by floor area that 2.5 sq mm is the minimum CSA of conductor, and that a 30A or 32A MCB is required. Am I correct so far?

The cable will be routed under a suspended floor and occasionally chased through plaster/brick. There will be no runs through thermal insulation.
What correction factors, if any, might reasonably apply in this case? under what circumstances should cable larger than 2.5 sq mm be chosen?

If I was dealing with a fixed appliance I would be considerably more at ease because I would then have more definite design constraints. Is there a "rule of thumb" for ring sockets? something like "allow for 100W per socket worst-case" would be handy.

 

[plugwash]

iirc you are supposed to allow full breaker rating for volt drop on socket cuircuits.

with all load at the furthest point (the worst case) the volt drop is 1/4 that in a radial cuircuit of the same length as the ring

with load evenly spread the voltage drop at the furthest point will iirc be 1/8 of the breaker rating

theese results can be shown with a bit of calculus but i don't plan to repeat that here (typing maths in plain text on a forum is hell)

if you assume the first one the limit would be just over 60m
if you assume the second one the limit would be just over 120m

might be an idea to go for 4mm or split the cuircuit.

 

[SparkyTris]

Thanks, plugwash, but what does iirc stand for?

 

[plugwash]

if i remember correctly