John Rumm wrote:
wrote:

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!

snip

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

It would be a problem on a long spur, but as a short "finger" from a
ring it probably still copes.



or perhaps not, IIRC when I tried out various designs for this place
(sometime ago now), ISTR that I discovered suggested/standard circuits
in OSG are generally limited by voltage drop. Interestingly no 4mmsq
circuits are listed.

When you do the calcs using 4mmsq you find that (depending on open loop
supply impedance and cicuit length) the potential fault voltage at the
point where there is a live to earth fault can be over 50V. This is due
to the small dia of the CPC. SFAIUI 50V is generally considered the
max safe permissable voltage at a fault point, as you might touch an
earthed item near the fault. ie the length of a 4mmsq cct in those
circumstances is limited not by voltage drop, but by fault voltage.

Without doing the calcs my gut feeling is that 7/029 is likely to give
similar results as 4mmsq, as the CPC dia is less than that of 2.5mmsq
cable. IIRC the conductor & CPC sizes in 2.5mmsq FT&E means volt drop
just scrapes home as the premier limiting length factor.

IMHO even if the new ring circuit uses 2.5mmsq, it should be checked by
calculation for 2 or 3 typical spurs drawing 30A - in varying
proportions - between them. The calc should be a doddle with a
spreadsheet.


HTH