On Wednesday, 27 May 2015 22:31:56 UTC+1, GB wrote:
On 27/05/2015 21:03, ARW wrote:
"GB" wrote in message
...


I did wonder whether the cable is overspecified to allow for this.
There must be loads of ring mains with breaks or dodgy connections,
but the number of electrical fires is fairly small.


Most ring circuits are underloaded. If there is a bad connection at a
socket then the socket usually packs up before the wiring (there may be
some small localised wire damage).
The minimum Iz of the cable for a ring is 20A. That's the maximum a
double socket can supply without burning out.

Let me ask a question to make sure I have understood correctly. A ring
main takes 30A. Any more and the circuit breaker trips. If there are no
glitches in the wiring that is carried over two cables with a minimum
current carrying capacity of 20A each, ie 40A total? So, there is a bit
of a safety margin.

20A rating, not 20A carrying capacity. We frequently had over 45A load on a 30A ring without any issue, as lots of kitchens do.


I leant an extension lead to a very experienced builder working in my
house stripping some wallpaper with a steamer. I just left him to it. 30
minutes later the power circuit failed. The steamer was well within the
power rating of the extension lead, but only if it was uncoiled, which
the builder had not done. The whole thing had melted, not just the cable
reel but the insulation off the wires. The insulation was the
consistency of custard. It's a bit frightening, actually.

So he had what, maybe 3x rated load for 30 mins for that to happen. I don't know anyone that runs 90A on a 30A ring for 30 mins.


NT