I didn't look close enough at the electrics before I bought this
house. I have found that none of the sockets are on a ring: they are
all wired as radials, some with spurs. I know you can have a radial
with a 20A MCB but they are not wired like that (32a MCB in CU making
it look like rings)! I found at least one socket that did not have an
The lights are wired curiously. Some of it is red and black T&E. Some
of it is T&E that has stranded conductors. And some of it is (if such
a thing exists) "single and earth": grey outer insulation, with a red
insulated wire inside and an un insulated earth wire. Though earth
wires may be present in the cable, they are not always connected;
sometimes they are cut level with the grey insulation.
What is frustrating is that a professional firm of electricians fitted
a new consumer unit a few months before I bought the house. How did
they let the vendor get away with this? Shouldn't they have tested for
earths at the sockets, etc? It seems to me fishy that someone should
buy a new CU just before moving house. I wonder whether someone else
made an offer on the house, had a survey, and withdrew their offer
because of the electrics, so the vendor fitted a new CU to give the
appearance that everything was ok?
I thought I would begin by rewiring the lights. My house is like this:
ground floor: lounge and kitchen
first floor: front bedroom, rear bedroom, bathroom
second floor (loft conversion): bedroom and en suite
I thought I would start on the first floor as I am already doing some
decorating on that level.
The loft conversion above is making it very difficult to follow wires.
What do the professionals do when rewiring houses: remove carpet and
cut holes in the floor above or cut holes in the ceiling below?
Rather than move the bed and roll back the carpet in the loft
conversion, I went with cut a hole in the ceiling on the first floor.
It seems that the builders have run chicken wire between the old
ceiling joists and placed rock wool insulation slabs on top of this.
This has interfered with me using rods to push cables along. Why would
they put insulation between the first and second floors? Surely it
would be better to put it above the loft conversion, not under it?
It is very filthy up there, lots of black dirt has fallen onto me.
To make matters worse, the loft conversion is on new joists that run
between the old ones (which are still used to hold up the ceilings of
the rooms below).
The light at the top of the stairs is on the ground floor light
circuit, yet the light in the rear bedroom is connected to that lamp
for its live and neutral. It seems the loop connections on the light
at the top of the stairs are used for the upstairs light circuit but
the switched live for that light comes from the ground floor circuit!
I think this is dangerous as you would not expect one fitting to have
conductors from two circuits. Do you agree?
What is the best way for me to rectify this? Should I put the light at
the top of the stairs onto the upstairs circuit? Or should I leave
that light on the ground floor circuit and just remove the first floor
live connection to that fitting? What is the common practice with
stair lights; what circuit do you put them on, or do you give the
stairs a circuit all of their own (perhaps shared with fire alarms?)
On Friday, 22 January 2016 20:01:43 UTC, Stephen wrote:
Radials are permitted & common. 32A with 4mm^2 is OK. If it's 2.5mm^2 you s
hould use 20A MCBs. However these figures change if wire is buried in insul
ation. See Wiki 'cable' for full detail.
easily fixed. I'd expect the occasional fault in a building with oldish wir
presumably historic cable. Nothing wrong with that as long as its condition
is ok, which with pvc it normally is.
not good, but easily fixed hopefully
It doesn't sound like the work was legal
probably. It's normal.
why? From what you say it sounds like you only need connect the earths & me
asure to check things are ok.
as Phil said that would be from before the loft conversion
I'm not clear how that makes things worse
It's normal practice, millions of houses are wired that way.
Does it need rectifying for some reason? I can't think of one offhand. I ca
n recommend getting a bit more familiar with house electrics before setting
off on a good deal of work that by the sound of it is 99% pointless. Meanw
hile I'd go round reconnecting all the earths - just check they are earths
first, not a wire repurposed as a switched live.
And you might want to consider meditation.
On Fri, 22 Jan 2016 12:58:40 -0800 (PST), firstname.lastname@example.org wrote:
It is 2.5mm^2 cable
It wasn't that the earth wire had snapped, it didn't run to the socket
at all! There was a junction box beneath the floorboards where the
radial split in two and they forgot to take the earth from the
Only that it means there is less space to poke a rod before you hit
one set of joists. The bigger problem is that the rod gets caught in
the chicken wire that is supporting the insulation.
On Monday, 25 January 2016 09:58:03 UTC, Stephen wrote:
Then if you change the MCB to 20A it should be compliant
normally easily fixed
I can't help thinking you're making an awful lot of unneecssary work for yourself by considering a rewire. It sounds like a fairly easy repair job. Testing circuit resistances would also be best practice.
Technically you can have a 32A radial, but it is impractical due to
trying to get 2 large conductors into standard terminals.
Sounds more like someone forgot to "join the ends".
A new CU would have required full circuit testing afterwards, so none of
those faults should exist. So much for Part P!!
Have you got any documentation on the CU fitting - because I would refer
it straight to the firm's professional body if there was one!
It's recommended that the 20A radial (usually 2.5 mm^2 cable) should not
serve more than 50 m^2 floor area, and the 32 A radial (4 mm^2) no more
than 75 m^2. Spurs and branches are allowed, provided the requirements
for voltage drop and earth-fault-loop-impedance are met at all points.
This is subject to consideration of the likely loading (after
diversity)- e.g. the 20A circuit will often not be appropriate for a
That was commonly used in the late 60s - early 70s period, especially on
low-cost housing. The stranded wire is probably 3/0.029
If you're saying that the supply to the lamp is between the line
('live') conductor of one circuit and the neutral of a different circuit
then, yes, that's potentially dangerous - although it can still be found
in the form of the 'borrowed neutral' in older installations.
If there are only two lighting circuits in the house they should be on
seperate RCDs in the new consumer unit, in which case one or both RCDs
should trip whenever that light is switched on.
Golden rule: The wiring of the line & neutral of each final circuit must
be electrically seperate from that of any other circuit, in order to
permit safe isolation.
Much depends on what size cable has been used... also how its been
wired. So for example, 4mm^2 T&E radial (with branches in 4mm^2 and / or
spurs from it in 2.5mm^2) would be fine. Even multiple spurs in 2.5mm^2
taken from the "origin" of the circuit (i.e. the CU).
Yup, that's never good...
Some older imperial sizes of T&E (and modern large sizes) are stranded.
There are even some rare cases of modern 2.5mm^2 T&E that's stranded.
It may be that they actually wanted just a double insulated single...
(or its a bodge - again the devil is in the detail)
Not necessarily. They would have tested for ring continuity, and
probably insulation resistance, but not necessarily tested every socket.
I have even see a place that was "rewired" before sale, where all that
was changed was the CU, the sockets and switches and short lengths of
visible cable, that were then grafted back on to the existing perished
rubber insulated wiring!
Depends on what is easiest and requires least making good. Normally you
would lift a floor if its floor boards, but in cases where it say has a
real wood floor laid over it, it may be less destructive to get through
the ceiling from below.
Yup that is a common building control requirement when adding a second
storey, to ensure you have 30 minute fire protection between the floors.
I had to do it when I built mine. They get very worried about escaping
from second or above storeys since you need to get through lots of
Par for the course alas... all over disposable bunny suit and a good
Again, good practice for several reasons... it saves having to pull down
the existing ceilings and keeps all the new loft work self contained
during the build. When done it also means the new floor is decoupled
from the existing ceiling so you get less noise transmission.
Its probably two way switched, so you may have live wires from both
circuits present at the switches.
Borrowed neutrals are never a good idea - especially when the circuits
are split across multiple RCDs. However it is an unavoidable fact that a
lamp switched from more than one storey is at some point going to have
live wires belonging to the "other" circuit present. Its just one of
those things you need to be aware of.
I would say its best practice to do two way switching in *both*
directions - two switches at the bottom and two at the top. So you can
switch both lamps from both locations.
You may find some useful background in:
Plus plenty more electrical articles:
Yes, exactly as with a 32A ring circuit. The same limitation applies -
one single or one double socket only if the spur is unfused.
(the logic being, as with a ring, the limitation on the number of
sockets will limit the maximum load so you can't overload the cable, and
the 32A MCB will still provide adequate fault protection for the 2.5mm^2
T&E is the shit hits the fan (or the screw hits the cable!))
That seems odd. Either 2.5mm cable is happy with 32A (a double can
supply 26A, even without one of the things being faulty) - in which case
the ring's 4mm is grossly overspecced - or it can't, in which case
obscure circumstances could overload the cable.
A quick google suggests 2.5mm^2 is only good for 20A!
It depends on the installation method.
To be used in a ring circuit then it needs to be 21A "as installed" or
higher. If you can't meet that, with the actual installation (say too
much insulation or circuit grouping) then you will need to use a larger
It is also important to clearly differentiate between overload currents
and fault currents. With a general purpose circuit you need to
provide protection against both types of over current. Normally this
protection is provided by the fuse or MCB at the origin of the circuit.
In the case of fault currents, this is the only place you can do it.
With overloads however you can split the responsibility and provide the
protection via some other means. This is the design principle that a
spur uses for overload protection. The fuse/MCB can't do it, since the
rating is higher than the maximum capacity of the 2.5mm^2 cable even in
the "clipped direct" mode giving the full 27A, but by controlling the
load that can be applied to the end of it, you can achieve the same
result. Normally a double socket is rated at 20A continuous maximum load
(not 26A), and that will be less than the installed capacity of the
single piece of cable in the spur.
(I am sure there will be someone along shortly to come up with a scheme
involving a pair of 4 way extension leads, 6kW of static load, a length
of rubber tubing, and a yak, to highlight why this is not true, and
hence why we so commonly see properties burst into flames daily!)
This is the current that flows when a short circuit fault occurs in a
circuit between either the live and neutral conductors, or the live and
earth conductors. The magnitude of fault currents can be huge (100s or
1000s of Amps) since they are limited only by the resistance of the
wires in the circuit between the consumer unit and the fault, and the
impedance of your power supply and earth connection as delivered to the
This occurs when the total current demand made by the appliances
connected to the circuit exceed its design capacity. This significance
of an overload will depend on its magnitude and its duration. Small
overloads may be tolerable for long durations, while big ones can only
be tolerated for short durations, before any damage to the circuit
 Hence the rule about no more than one single or double socket on an
So there's no danger to the 2.5mm cable as the socket will burst into
Though I suppose that's equally true of any double socket anywhere on
Incidentally I've installed sockets (pre-part P honest!) and noticed the
limit. I'll give odds most people have no idea.
In many cases there will be no danger to the cable at all (since it will
often have a greater installed capacity than the 26A theoretical load of
the socket. Even if if you do exceed that, then the cable does not
immediately fail - it just runs over the 70 degree temperature limit
that allows the cable a virtually unlimited life. If you routinely
overheat it, it will have a shorter life before the insulation becomes
brittle and starts to crack etc.
There is perhaps more risk to the socket, but again it does not seem to
be a problem in real life since its not actually that easy to get 26A
(i.e. over 6kW) of load on a socket for a long duration. Most people
would probably shy of doing it too often once they realise that it would
get 'kin hot running at max capacity for an extended period.
Indeed. Some good quality sockets will carry more current indefinitely
anyway, the 20A is the notional design current.
Part P does not prevent you installing sockets anyway... ;-)
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