Post
by ericmark »
Tue Jan 03, 2012 10:07 am
Either horizontal or vertical is permitted from an item which shows there is likely a cable feeding it.
Cables buried less than 50mm in a wall need to be either a special cable (Ali-tube for example) or protected by a RCD.
All new sockets 20A or under also need RCD protection except where for a single item and marked as for that item only like "Fridge".
There are two ways to run cables. As radials or as a final ring the former may have 2.5mm sq, 4mm sq, or 6mm sq cable. The final ring is normally wired in 2.5mm sq. To extend there are two options. Either expand the system or take a spur from the system. When expending the system you need to use the same cable as already used. When taking a spur you can use a lighter cable. 2.5mm is the normal for an unfused spur but that is limited to 3 meters and one single socket. Using a fused connection unit (FCU) the cable size can be reduced to 1.5mm if the loop impedance and volt drop permit.
So step one is to find out what the orignal socket is connected to? What size fuse feeds it? If feed with a 30/32A fuse/MCB then likely part of a ring. Once power is off you need to check the wiring. If either 4mm or two x 2.5mm then likely you can extend. What you should do is measure the loop impedance this needs a special meter and the results will tell you if you can extend. With RCD protection on a final ring we are limited to 106 meters of cable. Seems a lot but by time it goes down and up walls and back and to across ceiling to comply with 1/3 rule for drilling timbers it is easy to exceed.
Radial systems are rare in UK. But are becoming more popular because of the problems in having too many sockets on same RCD. Without knowing what system you have can't advice on cable size.
As to current carrying capacity this depends on both type and route of cable. A 2.5mm csa cable can be anywhere between 33A and 13.5A according to insulation used and route taken. Ali-tube cable has thermosetting insulation so can run hotter and take more power than thermoplastic which is more normal. Mineral insulated can carry even more current. In general standard twin and earth 2.5mm Reference Method 100# (above a plasterboard ceiling covered by thermal insulation not exceeding 100 mm in thickness) is considered good for up to 21A. So we use two cables in a ring formation giving 42A at centre of ring but we are permitted to protect to just 32A as 1/4 way around the ring the current in both cables will not be even.
The problem with DIY is people doing it rarely have the test instruments required to know if it's safe. So all they can do is look at the last electrical condition report and judge from that if there is any lee way. (It was called a PIR) The problem again is often the DIY guy has no idea as to size of house which may present a problem. As electricians we plug in our Earth loop impedance tester on every job so we can normally tell by just looking at a house if we are likely to have problems or not.
There are some cheap plug in testers which will test the loop impedance at around £50. The Martindale has 4 lamps each giving a range of loop impedance not sure with the socket&test type on how many lights. But really you do need to test. (There are also very cheap types without ELI tests these are no good).
DIY work which disconnects the ring and makes it into two radials is very dangerous and special care must be taken to ensure it remains a ring circuit. You need to read the PROJECTS section on this forum. Can't normally post pictures on here but there are pictures in PROJECTS. Once you have read then come back and ask less general questions. By then you hopefully will know what your system is?
