# Is it a radial or ring circuit?

• posted on September 2, 2003, 11:34 am
Hi all,
Firstly, please forgive me if this post is not very clear as it is my first time!
The scenario: 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.
Does this sound like a radial circuit to you or should I buy a continuity tester to be 100% sure (if so any recommendations?)
Many thanks,
Paul
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<%-name%>
• posted on September 2, 2003, 11:41 am

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 circuit.
--
Chris Green ( snipped-for-privacy@x-1.net)

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<%-name%>
• posted on September 2, 2003, 2:20 pm

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...)
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
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<%-name%>
• posted on September 2, 2003, 2:35 pm
snipped-for-privacy@hp.com wrote:

I did wonder whether to qualify my rather definite comment but decided not to.
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.
--
Chris Green ( snipped-for-privacy@x-1.net)

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<%-name%>
• posted on September 2, 2003, 2:48 pm

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 Dishwasher 2kW Kettle 2kW Fan oven 3kW Toaster 2kW Microwave 1.5kW
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, though.
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.
Christian.
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<%-name%>
• posted on September 2, 2003, 9:01 pm

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 circuit.
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?
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<%-name%>
• posted on September 3, 2003, 10:01 am

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 outside loads.

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)
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<%-name%>
• posted on September 3, 2003, 12:14 pm

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.
--
Chris Green ( snipped-for-privacy@x-1.net)

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<%-name%>
• posted on September 3, 2003, 1:01 pm

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).
Stefek
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<%-name%>
• posted on September 3, 2003, 1:18 pm

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.
Christian.
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<%-name%>
• posted on September 3, 2003, 2:11 pm
Stefek wrote | 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,[1] on one 15A rewirable fuse in my parents' house.
Owain
[1] And possibly the socket in the 3rd bedroom too, because that was built without any sockets and one was added later.
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<%-name%>
• posted on September 3, 2003, 3:03 pm

I have to say, the socket in the upstairs hallway suspiciously stopped working after I removed the old immersion heater circuit.
Christian.
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<%-name%>
• posted on September 3, 2003, 7:41 pm
snipped-for-privacy@hp.com wrote in message

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 future....

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 connector blocks?

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 better!)

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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<%-name%>
• posted on September 4, 2003, 3:48 pm

I did the same when I replaced our consumer unit as well.
--
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<%-name%>
• posted on September 3, 2003, 10:29 am

Ensure you use SWA armoured cable for this. It isn't that expensive and is much more suitable for burial than T&E.

Obviously, a new consumer unit would be best. However, a 13A/30mA RCD fused spur on the 30A ring would suffice for now.
Christian.