I have now done a lot of research on this including other forums and asking two local electricians and they all tell me its up to 20 amps. I even found this quote regarding MK double sockets....
Research by ourselves and third party organisations has shown that all MK double sockets can safely withstand a continuous load of 19.5A for an indefinite period. Double 13 amp sockets are required to handle 20 amps in total, they are tested with 14 amps on one side and 6 amps on the other, and must not overheat when tested thus. The combination can be changed to for example 10 amps per socket or 11amps on one side and 9 amps on the other.
It is 13A per twin socket. Each socket is (or should be) tested up to 20A but the official line is you should not draw more than 13A from each twin socket. If you look at the back of a socket most clearly state 13A maximum
Looking at MK web site it is not clear, it says 13A per socket, but does not define if socket refers to each outlet or the combined outlets, however in the same listing it says how with filtered sockets and other specials it is 13A for the pair, so it would seem your allowed 13A for each outlet so 26A for the pair.
However it does seem it depends on manufacturer and the spec says 14A from one socket and 6A from other as minimum, but MK don't seem to comply with this with filtered sockets, and sockets with RCD built in.
I tried to find info from other manufacturers but they don't seem to publish much.
However in the main it is down as much to time as amps, so using a welding set drawing 16A for 3 minutes at a time with 2 minutes off is unlikely to over heat the socket or plug, but a 4 bar heater unregulated plugged in drawing just on the 13A will likely will overheat the plug at least.
So a washing machine or smoothing iron no problem, but a tumble drier may be a problem as it runs for so much longer at full output.
As said that is the British standard, however your unlikely to find a designer when on the phone, your talking to some one who is reading the BS standard, and playing safe.
It is not the socket that has the heat problem, it is the plug, a fuse works by the link melting, and so they must get hot, that's how they work, and the heat has to be dissipated, to the air and to the socket, the solid line pin could clearly transmit more heat into the socket than the new safety line pins as plastic does not transfer heat as good as metal, and also the tighter the grip on the pins the more heat is transferred.
This means the socket gets hot, if it gets too hot the spring tension can be lost, and then you get a poor connection and even more heat builds up.
So on average over an hour or so the load per pair needs to be below 20 amp to give time for heat to escape, however that's an average and if the time it is at 26 amp is same as time at 14 amp over the hour, the average is still 20 amp.
Also remember all plugs should be in free air, if behind an appliance unless the appliance has a fan was with most ovens, then it's not in free air, even colour matters, black will get rid of heat better than white. And some plugs are bigger than others and the bigger they are the better they can get rid of heat.
Using a 15A fuse and a radial circuit the 15A un-fused plug of yeastier year ran much cooler, having an immersion heater on a 15A plug was no problem as nothing was producing heat, but with 13A plug the fuse produces heat over an extended time and inside an airing cupboard there is not much air flow so common for a 13A plug to over heat. Putting the fuse in a fused connection unit it can transfer heat into the wall, so they work a lot better.
You can get grid plates that take two 13A sockets, and you can get twin back boxes so two plugs can be right next to each other, technically these would allow you to draw 13A each, however the same problem arises one plug will heat up the socket so there is less differential so using full 26A over an extended time can cause over heating.
So even with a single socket in free air I would not plug in an immersion heater, but with a washing machine and dish washer, although both rated at 13A would be happy both plugged into a double socket as the time it is at 13A is so short. And it takes time to heat up.
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