We've just had our house rewired and the electrician faced a
difficulty establishing a route from the understairs consumer unit to
the first floor. Eventually he settled on running the cables through
the cavity wall, which looked the easiest option to my untrained eyes,
but which the NICEIC have now told us we shouldn't accept.
As an alternative he's offered to cut a 50mm deep channel from the
consumer unit to the first floor (which would be a diagonal line up
our hallway wall) and run the cable in a metal conduit, but he's
concerned that this will weaken the exterior wall and has asked us to
sign a disclaimer if we ask him to do this. Does anyone have any
experience of this? Has a 50mm channel ever weakened an exterior
An alternative we've been wondering about is to keep the metal
conduits on the surface and run them down the corner of the hallway
and along the floor to the consumer unit, covering them over with a
wooden box / skirting board. Would this cause any problems from a
regulations point of view?
Any help would be much appreciated,
The NICEIC are correct re not accepting cables in a cavity,
for reasons of derating re insulation (now/future) & damage.
Perfectly possible for a CWI surveyor to drill through one,
standing on wet ground, cigarette in mouth, no RCD.
Ok, required as a diagonal line.
No, that is the alternative.
You can a) run it 50mm deep or b) run in steel earthed conduit.
The problem is the electrician is miss-reading BS7671,
and that that may extend to what he means by a 50mm
channel particularly adding a metal conduit to that figure!
Your electrician may be suggesting...
o 50mm depth from the brick surface not finished surface
---- that would be half way into a 4" brick
o 50mm depth to the surface of 20mm metal conduit
---- thus 70mm deep deep into a 4" brick
BS7671 526-06-06 requires for a diagonal run...
Cable concealed in a wall at a depth of less than 50mm
from any surface to be enclosed in earthed metal conduit.
Galvanised conduit (re earthing) not black enameled, via
proper conduit earth clamps, with all galvanised surface
removed during fitting repaired by cold galvanising paint.
Thus you can either...
o Chase 50mm+ from a plastered surface & run bare cables
---- 50mm+ re cable surface 50mm below plastered surface
o Chase little to allow 3mm skim over 20mm galv conduit
---- ensure screws galvanised/stainless re plaster staining
Some 1950s plastered surfaces are 23mm deep BTW,
being a skim over a sand-shown-the-cement-bag render.
Most people use steel rather than trying to chase.
It is not ideal to chase nearly half way through a brick inner
leaf on a Semi outer wall with no natural bracing. Some lack
sufficient wall ties & sufficiently corrosion protected wall ties.
I have a problem with the suggest solution anyway.
o CU runs involve a LOT of cables
---- ring legs require derating re N=2 M3 in the same conduit
---- similar derating apply to other cables re M3 install
---- conduit does not hold many cables - at all
o It would be far better to move the CU
---- either to somewhere on the 1st-floor wall
---- or just below the ceiling on the ground floor wall
---- thus fewer cables needs extending - that from the CU
o To extend the cable from the CU may need a Switchfuse
---- DNO/REC limit meter tails to 2m or 3m length (phone)
---- longer runs require 100A Switchfuse (DP + HBC fuse)
Electricians is not reading BS7671 correctly re routing outside
zones (16th). There is a chance to zoning under 17th regs which
design can conform to 1-Jan-08, but I do not think that is of any
use to you re CU design options re non-RCD protected circuits.
You could, but aesthetically see above.
If you later add a case pipe you need 25mm separation from any
cables unless separated by electrically insulating material and
that rises to 6-inches for a consumer-unit/switch/socket etc.
If you later add central heating pipes spacing is required and in
addition temperature derating may be necessary (cable upgrade).
So plan accordingly re (potential) future usage of the area.
When the CU is fitted...
NICEIC will tell you not to accept a CU with 2.5mm holes at
cable entry points, causing £600 CU replacement cost at a
subsequent date. The rod test involves more detail than that,
but illustrates how you need to check general workmanship.
There is an NICEIC PDF showing zones - anyone got the link?
Any pointer please to what are acceptable methods of sealing?
(Presumably tougher now new-builds may have CUs are at toddler height.)
Not a direct link but is it (like) the diagram within this article
(retrieved from my much used - or abused - folder of idiot's guides)?
No pointers, just follow the instructions.
The wording on MK for example is...
"To be installed by a competent person (e.g., a qualified electrician)
in accordance with the current edition of the IEE wiring regulations"
"3. Remove only the minimum amount of relevant cut outs for the
incoming/outgoing cables. Ensure that precautions are taken to maintain
the IP rating eg, correct use of glands, cut outs and knock outs."
"IMPORTANT Insulated Units: All cut outs are to be scored prior
Simply ensure cut outs are not gaping big holes.
o There are many cut outs moulded for a knife to cut along
o Cut outs vary from small rectangles which combine to create big holes
o It is incorrect to cut out all the big holes for 1 little cable :-)
"IMPORTANT: All cables are are to be aligned properly and not
stressed. All illustrations relate to a typical split load unit. Exact
configuration will depend on unit purchased and installation required".
Simply ensure cables are distributed across multiple cutouts across
the CU rather than all trying to enter at one point. This makes wiring
hard and later fitment of (say) tall RCBOs physically frustrating.
Glands refer to 20/25/32mm knock-outs in metal CU backboxes.
Grommets are not sufficient, glands should be used unless sufficient
cables can be positioned so as to effectively achieve the IP rating.
Glands should not be fitted to insulated unit cut outs as cable flexing
causes moulded knock out rebates (knife cut lines) to break. It is
possible to fix glands elsewhere, but not an issue here (a thumping
great big SWA CW25 gland must be fun to do on most smaller CU).
Hager offer a line of CU with some benefits...
o Piece of foam to cover the cable cut-out areas - better than nothing
o More wiring space re RCBO fitment - RCBO are taller than MCB
o DIN rail fuse holders for sub-main (shed) circuit fault discrimination
The electrician may have his own preference - it varies because
MEM were particularly easy to work with in the 1990s for example.
story there, which will repeat as a lot of installers are not
using grommets on 20mm-hole-sawed holes in metal C-Stud. It
will be fun because some cables used are variations of LSH LSOH
which differ in their fire retardancy - ignoring electrocution risk.
That is re partitions - there is one re all zones (diagonal) meant
for end consumers in terms of "knowing where cables might be".
In that vein...
o If your house was built prior to c.1970? cable runs *will vary*
---- zones were not in regulations prior to around that time
---- diagonally around a corner to horizontally 2" above a door
o If your house was built since, zones may have been ignored
---- qualified electricians who should know better do this
When Manweb did domestic electrical call outs they used a
brace-n-bit to penetrate a 10mm cooker cable 1m from CU.
The low cable resistance (R1+R2) & easy path to earth (Ze)
aided in the route identification of the cable: two layers of tiles,
plaster & brickwork were blown off as water in the solid brick
exterior wall turned to steam with cable combustion byproducts.
Cable/cartridge-fuse/CU were completely destroyed, main fuse
was blown and sub-station took the street out until it reset.
When another Man from the Web arrived he found the fault
remained further up the mostly vapourised cable. So had the
first just replaced the main fuse (& CU/fuse) it would go with
a potentially higher fault current the second time. Incredulously
I recently found the electrician (who effected the then repair)
re-used the remaining cable by just crimping & taping new on.
Perhaps I am glad the 17th reg is dropping "zones" (AFAIK).
As people move around renting houses or rooms etc, they may
assume cables are in "safe zones" when it is just unreliable.
Zones are thus more about "where to put cables".
They are not a guide to be relied upon as to where cables are.
Either buy a detector, and do not trust it fully, or turn the power
off and scrape away plaster with a VDE screwdriver to confirm
you did just pick the one place on the wall where there is a cable!
You only need a few millimetre, widen it a little, fill it in and add
the desired nail, screw, flat panel, picture of the mother-in-law.
On Thu, 29 Nov 2007 23:52:34 -0800 (PST), a particular chimpanzee,
email@example.com randomly hit the keyboard and produced:
The Building Regulations permit only a vertical chase of 1/3rd of the
wall or leaf's thickness (whichever less), and a horizontal chase of
1/6th. Anything not vertical should also be limited to 1/6th. Assuming
a 100mm or 102mm inner leaf, that's a maximum of 17mm.