by ericmark »
Wed May 11, 2016 5:24 am
Normally the shaver point is taken from the lights, if that is the case then no you can't take a supply from the shaver socket.
When adding sockets job one is to find out how the place where your getting the supply is its self supplied.
We have a number of ways to supply a socket.
1) Ring final circuit.
2) Radial circuit.
With the latter we have two basic types, from 16 ~ 25 amp fuse/MCB using 2.5mm sq cable and from a 32 amp fuse/MCB using 4 or 6 mm sq cable.
From 1 and 2 above we can fit fused or unfused spurs, with a fused spur you can fit as many sockets as you like, with unfused spur your limited to one device only.
Once we have found how supplied we need to measure what we already have. Easy way is with a loop impedance meter, although it is permitted to calculate. In the main because all new sockets must be RCD protected the limiting factor is volt drop. With a 32A ring final we are limited to 106 meters of 2.5 mm sq cable, all well and good with a new build or re-wire we ensure only one reel of cable is used. With an existing system we know the 5% or 11.5 volt equals a drop in the loop impedance of around 0.6 ohms. So with an incomer of 0.35 ohms we want 0.94 ohms or less at the furthest socket.
However using a FCU we are limited to 13A so we can have an impedance of 1.2 ohms over the incomer value.
As an electrician it takes seconds to plug in my loop impedance tester and find if I have spare ohms to work with. As a DIY person however it is really a bit of hit and miss. As a result I tell people best option is to use a FCU it is unlikely you will exceed the limits.
As to if volt drop is that important is another question, but also with the MCB there is a cut off point with the prospective short circuit current. A B32 MCB will in time trip at 32A but to trip in the 0.04 seconds using the magnetic part of the trip it needs five times the rated current (That's what the B tells us) so it needs 160A PSCC for it to trip which works out at 1.44 ohms. A "C" MCB is 10 times and a "D" is 20 times the rated value.
The EZ150 plug in tester is cheap at £50 against the £300 for proper meter but does show if the ELI is over 1.50 ohms, clearly designed when we used 240 volt as over the 1.44 limit but it does highlight any massive errors. However it is rather pointless testing after a socket is fitted really want to know before it is fitted. But plug the EZ150 into an extension lead and you can work it out.
