Firstly, please forgive me if this post is not very clear as it is my
I am planning to add a spur to my kitchen circuit so I can have access
to power outdoors. I have identified all the sockets that are on the
circuit I want to add my outdoor spur. I have checked the sockets at
the end of the circuit to determine whether the circuit was a radial
My problem is that I found two sockets with only one set of wires, all
the others had 2 sets. I thought this would be classed as a radial
circuit but then discovered that the MCB on the consumer unit (rated
20 amps) had two wires connected to it.
Does this sound like a radial circuit to you or should I buy a
continuity tester to be 100% sure (if so any recommendations?)
It's a radial, one T&E from the CU goes to one socket, the other T&E
from the CU goes to the other socket. A radial can be branched
anywhere, even at the CU. A 20 amp MCB is correct for this type of
Could be a ring with those two as (unfused) spurs, could be a radial.
Depends on what the builder decided would use least cable... You'd think
if none of the sockets had *3* wires in 'em, it must be a radial, since
when you spur off a ring the take-off point has three cables (two for
the ring, one for the take-off); but someone may have tapped into the
ring with a junction box you haven't found to feed the spurs!
From the visual examination you've done so far (and despite what the
usually-reliable Chris Green writes), you simply can't tell whether
you've a ring or a radial without more work, with a strong preference
for using a continuity tester/multimeter.
But first of all, why do you feel you need to know? Your outside socket
*definitely* needs RCD protection, and it would be a whole lot better for
it to have its very own RCD, not to share one with the kitchen circuit.
Also, you say the kitchen circuit is currently protected by a 20A MCB,
which is a relatively low value: kitchens tend to have a number of
relatively high-power (therefore high-current) appliances in 'em, to
get the cooking and maybe washing done - kettle, toaster, microwave,
dishwasher, washing machine - so there's relatively little headroom on
a 20A (5kW) circuit if you're going to supply something meaty in the
garden, such as a 2kW shredder. I'm not suggesting it's strongly unsafe
- a significant overload will trip the MCB - but it's not smart to run
so close to the limits. Although the kitchen circuit might be physically
convenient, a dedicated new circuit from the consumer unit would be better
to supply the occasional higher powered appliance outside than tapping
in to the kitchen ring. (I note, in passing, that the proposed make-the-
trade-bodies-fat-and-happy regulations make doing the Right Thing (running
a new final circuit) a Get-A-Nominal-Professional-In-Or-Pay-For-An-Inspection
job, while leaving a tap-into-an-existing-circuit solution unregulated.
Thanks, Mr Prescott...)
All that said, if you want to use the existing circuit - maybe you can
convince yourself that the existing circuit loading is low, and/or have
a small enough household to make simultaneous heavy use of kitchen and
garden appliances unlikely - then wiring in a *fused* RCD spur as the
take-off point for an outside socket will be OK, regardless of whether
you have a ring or a radial circuit, since the fuse in the spur connector
will provide overload protection. If your existing consumer unit already
provides RCD protection for the kitchen circuit - for example by being a
split-load affair with the kitchen circuit on the RCD-protected side -
there's little point even using a fused RCD: rather just use a switched
fused connection unit to provide isolation and overload protection for the
outside socket. Me, I'd prefer a separate RCD-protected radial for the
outside socket(s) - you might want such sockets at more than one position
on your outside walls, e.g. at front of house for vacuming/power-washing
a car/bike, and at back for gardening kit. Wire it on the RCD-protected
side of a split-load CU, or best of all (but pricier) give it its own
RCBO (combined circuit-breaker and RCD).
If you really want to know whether your kitchen circuit is ring or radial,
you'll need to properly and totally isolate the live AND NEUTRAL connectors
to the circuit at the CU. A very good first indication of ring-or-radial
will then be to check continuity (or better, to measure the resistance:
cheap digital multimeters start under a tenner at Maplin and similar)
between the two red conductors you say you have at the CU for this circuit,
and (for completeness) between the two black ones too. For a radial circuit
where the first branch happens to be at the CU, there will be a very high
resistance (no continuity) between these conductors; for a ring, there'll
be a very low resistance (continuity). Full circuit mapping will mean
disconnecting at each socket in turn, and plugging in extension leads
between the socket-under-test and the previously-found 'end' point, to
work out for sure where each wire goes. In a newish property you're likely
to find a simple, straightforward answer - either a radial wired in a
pretty obvious way, or a ring with a couple of spurs. (The ring would be
more usual for a kitchen circuit, but it's made a bit less likely given you
have a 20A rather than a 30A MCB on this circuit - unless the CU installer
ran short of 30A MCBs, or swapped the kitchen ring finals over with the
immersion heater wot wuz supposed to go into the 20A MCB ;-) The older the
property, the more chance of, umm, creative wiring having crept in over
successive occupiers (both their own d-i-y efforts and bodge-it
sparkies/GeneralBuilders) - not that it's unknown for brand-new builds to
have horrendous short-cuts where some subcontractor is shaving a few more
quid or a few more minutes off the job!
If these comments seem too telegraphic and require further decoding, I'd
suggest proceeding no further yourself and getting in a non-cowboy electrician
to fit you a nice new outside circuit. Similarly, if it's not obvious to
you that you should test, test, and test again that the sockets you
*think* you've disconnected *really* are dead, don't make yourself (dead,
Hope that helps - Stefek
I did wonder whether to qualify my rather definite comment but decided
It would be easy enough to see if the two sockets are on the 20A MCB
by themselves simply by switching the MCB off and seeeing if only
those two sockets are de-energised. Then you *know* that it's a
radial circuit with two sockets on it. Even if the wire goes round
and round the house, is wired in a ring and the two sockets are
spurred off the ring, treating it as a radial will be quite OK.
It is a radial. If it is the only circuit in the kitchen, it is grossly
underpowered. It will just run a couple of appliances. For a large kitchen,
it is a definite no. You may have:
Washing machine 3kW
Tumble Dryer 3kW
Fan oven 3kW
on the circuit, amongst other things. If all going at once, it will draw
close on 70A. OK, this is unlikely to happen to practice, but 20A is nowhere
near enough. I certainly wouldn't want to draw more off it.
If you have a utility room with the washing machine and tumble dryer off a
separate circuit and the oven is off the cooker circuit, it may be enough,
My guess is that the circuit is a radial, as rings are usually "fused" at
30/32A. It is obviously only a guess. You can get a better idea by measuring
continuity between the two end conductors, which will be low for a ring main
(hopefully) and infinite for a radial (hopefully).
So, in summary, I wouldn't dream of taking more spurs off this circuit. Your
kitchen may need a rewire if it is of any size and only has this circuit
feeding it. Your outside electrics should really go back to its own RCBO on
the consumer unit (non RCD side if you have a split). Alternatively, you can
use an MCB on the RCD side, but this will probably lead to nuisance trips.
Thanks to all for your input! Sorry I forgot to mention that I have
the following unusual kitchen setup:-
- 30 amp ring circuit powering a microwave, fridge/freezer, boiler and
2 x under-unit lights
- 30 amp radial powering a cooker, toaster and kettle
- 20 amp circuit as mentioned before powering a washine machine,
tumble dryer and computer (the circuit goes up into my back bedroom to
power the computer!)
I would like to spur of the 20 amp circuit to my outdoor socket simply
because it is more convenient, although now it seems to be better to
use the 30 amp ring as it has less wattage on it.
I have discovered that the 2 sockets at the end of the 20 amp circuit
I mentioned before are joined via plastic connector blocks which has 3
sets of wires (1 from the CU, 1 each for the sockets) - This doesn't
seem right to me (even more so because they are behind a blanking unit
right next to my sink). This still leaves me puzzled to why the CU has
2 wires on the 20 amp MCB.
Unfortunately I have no spare spaces in my consumer unit to fit a new
BTW - I will be protecting the outdoor socket by using a RCD adapter
and burying the cable in PVC conduit 45cm+ under ground.
Do you think my best bet would be to spur of the 30 amp ring or
install a new consumer unit either with more MCB's or in addition to
my current CU?
Like I opined earlier, any of them will be "safe enough" if you use a
*fused* RCD connection unit. The 30A ring seems like the least loaded
circuit of the three, though I'm not quite sure what your "boiler" is
(a rice cooker? a Burco?); even if it's a 3kW jobbie, your 30A ring
will deliver 7.2kW, with the other appliances pulling under 2kW. So
that's the circuit I'd tap into. From the further details you give,
I'd firmly *not* use the 20A radial, as the w/mach and t/drier (which
could well both be on at the same time) will be eating maybe 4kW at
peak (though to be fair, w-machines spend relatively little of their
cycles heating water) leaving only 1 kW or so for your monster 4-way
Athlon-with-liquid-cooling computer setup ;-) and thus naff all for the
It's possible that the sink has been moved at some point, or just that
someone got iffy about having a socket right next to the sink, as the
blanking plate+choc-box suggests that there used to be a socket in that
position. The two wires at the CU are indeed puzzling, and it'd be worth
tracing the Other one (since you know that just one of them feeds the
kitchen-and-back-bedroom run, right?) - initially by simply disconnecting
one of those two wires and seeing what, if anything, goes dead. Remember
it's still possible this 20A circuit is actually a ring, not a radial -
so the second wire could be the other part of the ring (maybe returning
from your back bedroom). If this is the case, you'd find that disconnecting
either wire alone would still leave all the sockets on the circuit live...
*and* the disconnected end live too - so don't be cavelier, and use a
multimeter to trace connections in preference to the mains supply!!
Or you might discover that the other wire supplies your rarely-used
immersion heater, in stark contravention of the Regs (an imm. heater
wants its *own* final circuit, not sharing with owt else, 'cos when it
switches on it pulls a Serious load for a Long Time, heating as it does
a rather greater volume of water than a kettle or a washing machine ;-)
Fairy Neuf - I see no tearing urgency to fit a new CU.
When you say 'apapter', do you mean a plug-in jobbie made to go on the
end of a flex (not best practice for supplying a permanent circuit!), or
a nice accessory-box-mounted fused-and-RCD'd-spur-connection-unit thing
(the Right answer ;-)?
For occasional use of kit outside, I think tapping into the 30A ring is
quite acceptable, given the constraints on your existing CU. At some
point, depending on time, resources, and other changes you want to make
to the installation, a larger (more ways) CU may figure in your plans, but
it seems like a 'nice to have' rather than a 'must do' on the information
you've given so far.
HTH, Stfeek (or agrnaam thereof)
It's the way things were (and are) wired at our house as well, a
dedicated 16 amp MCB feeds the immersion heater and the CH controls.
(The CH controls are on a FCU with a 3 amp fuse actually). I didn't
do it like this but it seems to me a very reasonable and sensible
arrangement so when I moved some of the controls to make them more
accessible I left the circuit arrangement as it was.
Certainly seems reasonable to me too, especially with halfway decent
labelling - "immersion + CH controls/pump". All I meant to point out
in the original context was that there might be unrelated and mildly
surprising appliances/sockets sharing one MCB in any installation
you've acquired through home ownership (rather than laying out for
yourself from scratch).
Mine's labelled both "Water Heater" and "Central Heating". The stickers came
with the consumer unit. Strangely, there was no pre-printed label for a
kitchen ring main (just upstairs and downstairs). And there was definitely
not one for my kitchen appliance radial circuit, or the outside electrics.
Looks like I need to find a bic.
Yeah. It shouldn't share with a socket circuit as this is confusing and
leads to diversity issues. But I see no problem with an immersion on a
radial shared with other related fixed appliances.
I might see how you could share with a single/double socket, provided that
you applied no diversity. (i.e. 32A MCB to 13A FCU immersion, 13A double
socket and 3A FCU central heating). However, I don't know if this is allowed
and I wouldn't do it myself. I suppose it would allow you to use power tools
safely if it was installed in an otherwise electric free environment, such
as a loft. It would also look less confusing if all the supplied equipment
(including the socket) was in one place and correctly labelled.
| All I meant to point out in the original context was that
| there might be unrelated and mildly surprising appliances/
| sockets sharing one MCB in any installation you've acquired
| through home ownership (rather than laying out for yourself
| from scratch).
Like the bathroom wall heater, one lounge socket, and socket in the
detached-but-joined-with-a-bit-of-flat-roof garage, on one 15A rewirable
fuse in my parents' house.
 And possibly the socket in the 3rd bedroom too, because that was built
without any sockets and one was added later.
The boiler I mentioned is a Baxi central heating + water boiler (Baxi
WM 38 3RS ?). I have tried to look up the wattage for this model but
can't seem to find it - I am assuming it would be between 130-190
watts? Also I've calculated my computer uses around 400w (including a
print and scanner) but who knows what other gadgets I will buy in the
I've found out from a neighbour that the kitchen was entended a few
years ago thus would explain why the electrics are so close to the
sink. Still, I thought junction boxes should be used instead of
Both the socket in the back bedroom powering the computer and the
socket powering the washine machine under the sink have 1 set of wires
connected to them - these are the two sockets that are connected to
the connector block by the sink. I've decided that its best to buy a
multimeter and find out once and for all whether the circuit is a
radial. Would anyone like to recommend a multimeter (the cheaper the
It's a 'Powercut Safety RCD plug', 30mA, 13amp fused. I was planning
to plug it in to a standard socket (located in a ex-coal shed which
joins on to the back of my kitchen).
I agree - I think I will use the 30 amp ring - it should be ok to wire
up too because we just have floorboards in the kitchen!
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