I've just bought a house. it originally had a double garage and the CU
was in the garage. One half of the double garage has been converted by a
previous owner to a family room and the CU boxed in.
What I would like to do is move the CU to the unconverted part of the
garage which means extending the meter to CU tails by a good 3m or so
and extending the two ring mains, two lighting circuits, immersion and
the cooker circuit to the new CU location.
I would be quite happy to replace the CU completely as the old one is of
metal clad construction.
I would also like to separate out the kitchen & utility room sockets so
that they are on their own ring main,
run a separate MCB for new interlinked smoke/CO detectors,
run a separate MCB for the new boiler,
run a separate MCB for outside sockets
run a separate MCB for outside lights
run a separate MCB for loft sockets, a separate MCB for loft lighting.
(this is because I plan to convert the loft at a later date)
I am capable of doing the job, but want to cover my backside legally
Has anyone here who is not Part P registered, moved a CU, added
additional circuits and got it signed off OK by a Part P electrician or
by Building control or was there too much hassle?
Or did you end up getting a sparky in to do the whole job?
The county of interest is Northamptonshire
Our BCO is friendly and agreed to letting me produce an EIC provided I
obtained some sort of qualification (to cover his arse) and had a test
As I was rewiring the whole house, I would have had a test meter anyway -
got a Megger 1552 off ebay and paid for fresh calibration cert.
 EAL Level 2 Domestic Electrical Installation (or something like that) -
2 weekends including exam. Course was actually useful as it covered testing
in detail. In theory I should have the C&G for the regs too, but no-one's
The EDF bloke who pulled the cutout fuse started off thinking I was a sparky
and asked for my NICEIC number. I said it was a Part P job and had left my
Megger conspicuously hanging on the stair rail near the CU.
I'd also made an effort to present nicely laid tails in the meter box and
used IP65 Kopex pliable conduit to run a waterproof cable route from the
external box up through the soffit. In the end, after he'd done a cursory
polarity and earthing check, I got a compliment of having done a more decent
job than he normally sees :)
Anyway, back to your point...
That approach, whilst watertight, might be too much effort for a CU swap
without a full rewire - although it is highly advisable to actually run a
full test on a CU change.
So, 3 other choices:
1) Get a sparky;
2) Do it and tell no-one. However, if you have to get the cutout fuse
pulled, you might get grief from the supply company if you can't produce a
decent story. Some modern meters have an isolator switch built in, it's not
obvious, some sort of rotating arrangement in teh meter body rather than a
big obvious switch. Some areas will fit a 100A isolator free or for a
nominal charge. Siemens (meter operator) in London area I believe will, EDF,
now sold to UK Power in the SEEBOARD area will not, ever. If you have a
henley block to work too, it's not too bad live working, but I baulked at
handling heavy wires with live ends as opposed to fixed down live terminals
and wires with dead ends. Will be a hell of a big bang in your face if the
live one touches anything earthy, no to mention electrocution risk.
Be aware that 3m is the typical max length for meter cables with most
companies and some stipulate shorter. If you are exceeding that by much, the
technical standard answer is to run the short tails into a 60/80/100A
switchfuse (box with fuse and DP isolator). Make the fuse the same as or
lower than the cutout fuse rating. The reason for this is that the supply
company have strict limitations on what they deem their cutout fuse to be
sufficient to protect and it normally protects the wiring from the cutout
right up to the breaks in your CU.
In theory, it is possible to run such long or undersized meter tails that a
dead short in the CU may not blow the cutout fuse before the tails suffer
3) Find a friendly sparky who will let you sort the CU out, then he will
come and test it all and handle the final connection.
3b) Part P it with 3rd party doing a test - more or less the same as 3).
If you aren't planning on selling the house anytime soon and don't really
care about paperwork and think you are upto it, there will not be much
likelhood of comeback for a DIY technical breach of building regs (failure
to notify) as long as the job is sound. But if you choose to do this, I
really would get a tester and read up on the testing procedure (I can
outline the basics here if asked) - it's not difficult, but you'd need a
copy of the Electricians Guide to the Wiring Regs to have the basic domestic
tables of conforming numbers to work to, otherwise your test readings
will mean nothing.
 Things like max permitted Live-Earth loop impedances by cable size and
breaker type being the main one that requires any degree of thinking. The
rest of the tests are things like RCD trip times being in spec and a 500V
insulation test of everything (without blowing up dimmers and stuff). Loop
testing ring mains are a bit of an art as you do a figure of 8 test that
makes perfect sense from a physics POV but is by no means obvious.
You'll need to understand concepts of earthing and bonding, but the uk-d-i-y
wiki expalins that stuff well.
That's not complete, just to give you some idea of what you should do to do
a by-the-book DIY job.
I suspect, based on what you've said, 3 or 3a will probably be the most
suitable courses to consider and are perfectly practical, but in the case of
3, find the bloke before you start. Mate of a mate is the best bet -
"foreign" sparkies will likely be scared by such a request out of the blue.
With tails that long you will probably need a switchfuse close to the meter.
I have done minor works and had BC accept my minor works cert (part of a
larger job that need LABC oversight for other reasons). There are others
here, who have also done notifiable stuff, with varying degrees of
hassle depending on the BCO in question.
For notifiable stuff the procedure should in theory be the same, submit
building notice, and let BCO signoff on the job when done. Make the BN
woolly enough (like "rewire as required") and get it all covered by the
one. However the reality is that many BCOs don't have the skills or
confidence, or possibly LBA authority to sign off electrical work
themselves, and may insist on you paying for an inspection - which
starts to get silly. Others may have the authority, but actually lack
the knowledge to sign off on correct work, and insist on it being done
incorrectly instead (see recent thread on here regarding cooker circuits
Personally I would probably phone the BC department and ask them how
they do it in your area. Then you can elect to go the BN route if it
sounds like they have a reasonable way of doing things, or make
alternative arrangements if not.
(the alternative arrangement selected by 99% or more of technically
competent folks being to simply ignore part P and get on with it - after
all, what is going to happen?)
Thank you Tim & John, you both have been very helpful.
I can give more detail on the work involved. basically the meter tails
are currently 2m long from the white external meter cupboard to the old CU.
The old CU is the corner of what was the old double garage but is now a
single garage and family room (one of the garage doors was bricked up
and a window put in, and a dividing wall put in between the two garages)
The old CU and meter cupboard are essentially on the the same wall, the
CU being on the internal surface and the meter cupboard on the external
surface. The CU is 1m higher than the meter cupboard.
The new CU would go into the surviving garage, on the dividing wall so I
am looking at 5 metres of meter-tail cable from old meter cabinet to new
CU. This would be run via the dormer void (see later) and would run from
the meter cabinet, over the family room ceiling and then down into the
surviving single garage.
I am more than happy to uprate the meter tails cables to 35mm2 to deal
with voltage drop due to the increased distance between meter and new CU.
I have no problem with replacing the 1.5mm2 cable from the nearest
lighting roses to the new CU, ditto replacing the 2.5mm2 from the
nearest sockets back to the new CU (avoiding junction boxes etc)
The cooker cable would be extended using those round suitably rated
brown junction boxes. They would be accessible via a wall door in a
dormer bedroom (which happens to be above the family room converted part
of half of the double garage).
The immersion circuit will become redundant due to a impending
combi-condensing boiler replacement. I was actually going to use this
circuit as the new boiler circuit and down-rate the immmersion MCB to suit.
Can I get away with putting the switch fuse in the meter cupboard or is
that against the regulations? SWMBO would not like having a switch-fuse
in the family room as that's no better than leaving the the old CU
There is an alarm panel as well that needs moving but that's another
story and obviously much easier to deal with from a part P perspective.
I would definately put a switchfuse in for that, near the meter, eg
That's where the elctricians guide to the 17th will be useful. That may not
be necessary for another 3m.
No harm in well made junctions, eg crimp (wiht proper crimpers), DIN
terminals or WAGO connectors (cable size permitting).
Your extended tails will not be RCD protected at 30mA so you cannot bury
them, without resorting to armoured cable or metal trunking or conduit.
Visible runs are fine - I would use a bit of box trunking.
Here's what a highly respected poster here did (Andrew G):
Technically, no. But it is often done. Depends if you need the supply
company to be present. The meter reader (if you don't have remote readings)
I wondered about that. Some galvanised metal trunking earthed back to
Well the new meter tail run would go from the meter cupboard upwards
through the cavity (in the cavity wall) over the ceiling and joists for
2.5m As mentioned earlier, this is the eaves part of a dormer bedroom
and I can make an access door in the plasterboard walls in dormer
bedroom for access to this dormer eaves. Then the meter tails would go
through a hole in the garage dividing wall and drop down a metre or so
to new CU which will be at elbow hieght. (old CU requires a step ladder
I believe that in the meter cupboard, there is the DNO's fuse, then the
Meter, then a double pole isolation switch. So that looks like I do
not need to call in the DNO to disconnect old CU and connect up new CU.
OK, dissecting this:
Does the cavity or could the cavity contain insulation? Running unprotected
PVC in cavities is frowned upon (though used to be done) as if someone fills
that cavity with polystyrene insulation granules, you get a bad reaction
with PVC cable insulation (specifically the plasticiser).
There's also a matter if derating cables. However, the cable is likely to be
Overall, I would not do this. Can you not keep this on the surface, or is
your meter cupboard flush mounted?
Failing that, can you get uPVC conduit down the cavity - this does not have
a problem with polystyrene - no plasticiser - and will protect the cables).
2) The eaves space does not require any mechanical protection by the sounds
of it, but personally I would.
Here's what I did in a similar situation (although my eaves are external):
(Down the bottom)
3) The through-the wall, I would sleeve (even with 32mm PVC drain pipe) for
the reasons mentions vis-a-vis cavity insulation. The final drop could be in
PVC trunking as it is surface.
It's an awkward route and with the new requirements of the 17th, always a
problem with tails as you don't have any RCD protection.
Can you not do a surface run up, in the corner of ceiling/wall then down in
A photo or two at this point might be useful...
The previous owners put in cavity insulation 18 months ago.... But did
not touch the loft insulation which is still at 4 inches(!)
The meter cupboard is flush mounted into the external wall. The CU is on
the internal wall about 1 metre above said meter cupboard.
Is it acceptable to use UPVC flexible conduit that is about 5 metres
long. put the meter tails and the earthing conductor through this
conduit, run this conduit from the meter cupboard, through the cavity,
over the ceiling joists in the eaves (across the family room ceiling)
then through a single skin wall and then down to the new consumer unit?
The single skin wall divides the two garages, the original one and the
converted into a family room one.
Or have I got to use metal trunking?
See my question above about flexible plastic conduit.
Instead of a Switch fuse, could I not put in a 100A rated RCD switch in
the meter cupboard? I will be of course having the standard 30mA RCDs in
the new CU..
so to avoid tripping the whole house, I could make this 100A RCD a 100mA
device so that the CU RCDs trips preferentially so taking out some
circuits but not all of them. This would give some hopefully reasonable
protection to the meter tails......
I am considering the RCBO per circuit approach as I will eventually have
a wood burner which requires its own high integrity supply as if the
pump stops, the WBS will vent steam through the header tank. There is
also the smoke detectors and CO detectors which need its own MCB, and
ditto the house alarm. Also the outside sockets i think should have its
own RCBO so that it does not take out the rest of the house wall sockets
The only times you need mechanical protection are if running in the surface
of a wall less than 50mm down from the surface.
Straight through is OK. If the cables go over the ceiling, but on top of the
4" rafters then there is little risk there. Some risk if it sits on the
I would say that, by the sound of it, none of that route needs mechanical
protection and that uPVC flexi conduit in any form is sufficient
protection for the tails in the cavity against any polystyrene that may be
You shoudl apply cable derating for the cavity run - sorry, I do not have my
tables to hand to check if 25mm2 will be OK.
 I would wait and see if anyone else sees a problem (Adam, John,
others?). Does the bit in the ceiling have a2nd storey floor over it (ie at
risk from nails in floorboards)?
The Kopex pliable conduit I used (32mm) is damn tough (but not drill proof)
and will take 2x 25mm2 and one earth - though you'd want to pre-assemble it
straight, or at least get the bit down the cavity and glanded into the meter
box leaving the rest straight whilst pushing cables. After that, the rest
can be routed pre-stuffed. The Kopex is smooth bore which makes this easier,
but it is not cheap (RS sell it) - about 40-50 quid for the minimum (5m
IIRC) and fittings. It does not count as "mechanical protection" but offers
a good deal of impact and abrasion resistance and even mice would have to be
hard persistent bastards to bother with it.
If it is easy to get trunking over the ceiling, you could run plastic box
trunking for the horizontal and gland pliable/flexible conduit into it at
each end and possibly be cheaper and offer a little more protection.
It also means that the fiddly bit of stuffing heavy wires down bendy tube is
limited to a short bit each end.
No. You need to protect the tails against L-N shorts as well as L-E faults.
In practise the DNO's fuse would almost certainly manage, especicially as
yours would have a similar characterisic (BS88 that most switchfuses have is
not quite the same as their BS1361 (usually) cutout fuse but it's not that
However, you would of course measure the supply impedance - or - assume it
is 0.35 Ohm for a TN-C-S system or 0.8 Ohm max for a TN-S system (by law it
must be and they can and must tell you which system you have),
then add in your caluculated extra resistance from your 5m tails, and if
necessary, use a smaller fuse (eg 80A) or as you've suggested, heavier
cables, to achive the correct disconnection time under a full short circuit
at the far end (CU).
That would be the right device (well, 100mA Type S time delayed) if you had
a dodgey earth (eg TT earth rod) but it does not protect against short
Good idea. I've done that, with a Hager CU.
I've trained mine that function trumps form if function = too difficult to
do in any other way. But I understand...
Cool. I'm gpoing offline tomorrow for 3 days (cannot be tortured by work
If you want me to dig out my regs book, I can tell you the numbers
(resistance of 25/35mm2 tails per m, disconection times, derating for cavity
walls, BS88 fuse characteristics etc). It's kicking around somewhere here -
but I won;t be able to find it until after Xmas too.
But have a lok at TLC's site - they have quite a lot of useful numbers
Have a good one!
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