In uk.d-i-y, Paul wrote:

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
or ring.

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.

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!

Does this sound like a radial circuit to you or should I buy a
continuity tester to be 100% sure (if so any recommendations?)

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,
that is).

Hope that helps - Stefek