John Rumm wrote:
Christian McArdle wrote:

Forget about any "more spurs than sockets" rules of thumb. They are just
guidelines for mentally challenged electricians. The important things to
determine a

Yup, ta for the confirmation... I was pretty sure it was only a
guidline, I just wanted to make sure it did not have some obscure regs
"heritage" that I had not spotted!

2 aspects come to mind:

Firstly, heritage is what it is.

15th ed IEE & (IIRC 14th & maybe earlier) had explicit rules for ring
circuits. These were moved to OSG after 16th ed published, so they are
now guidelines. The rule is now in OSG appendix 8 - Spurs page 149 in
my vintage 1998 yellow cover copy.

IIUIC you can't easily be fingered (part pee aside) if you follow OSG
to the letter, but you can depart if you are prepared to justify the
design.

Secondly, the original post said the circuit dated from the 50s. If so
it would most likely have been cabled in imperial 7/029 (earth
conductor (CPC)is 3/029 IIRC) - that is approx 3mmsq with 1.25mmsq
earth. 7/029 has a rating of about 30A on the same basis that modern
2.5mmsq has 27A. The whole installation would also have run off a
conventional (probably wired) fuse box.

In very broad terms, and leaving aside any derating for the cable being
run down the wall in conduit, each socket would have had the full
nominal 30A ring current available. Nowadays spur ratings are fiddled.
After maximum derating a 2.5mmsq spur cable is held to have a current
rating of 20A and the socket at the end, whether single or double 13A
is held to have a maximum demand of 13A.

Now that RCDs are in wide use and we have precise instruments to
measure fault currents and we are very aware of things like earth loop
impedances and so on, there would be a major quibble with a 3/029 (ie
1.27mmsq) CPC. It is a tad too small - in the 'standard' fault
condition this could leave a fault greater than the 50V maximum -
limiting the maximum cable run length. (Standard 2.5mmsq cable uses
1.5mmsq CPC & avoids the problem. Modern 4mmsq, with a 1.5mm CPC,
suffers from the same fault - which limits its popularity.

One cynical way of complying with OSG is simply to insert ghost 13A
sockets between each drop in the attic, though that might be queried by
pundits pointing to the requirement to evenly spread load around a
ring.

HTH