Is it possible to power a single oven and a combination microwave/oven off
an existing single radial (I presume) circuit (currently powering a double
oven)? Both appliances have a max power rating of 3.6 kW (or 3.6kW and
3.7kW). I make it that each would draw 15A max, so both together would draw
30A max; would this work using the existing radial circuit? Bearing in mind
also that the single oven will be moved to the other side of the kitchen
(approx 2.5m) from the existing double oven, so a feed will need to be taken
across there, with the combi oven on the side of the existing double oven.
There will also be a fridge on the combi oven side, and we would need a
double socket outlet for general purpose. How would this be wired up?
Would the ovens be individually fused?
An electrician I had around yesterday said he could get away without using
the existing double oven electricl circuit for the new single oven on the
other side of the kitchen, using the ring circuit on that side in some
way... would this be possible / safe? From my research, this is not
Many many thanks!!
I did find that useful, but first it says "As a general rule an electric
cooker will require its own radial circuit. This means it requires a direct
connection to the consumer unit with its own fuse way there". Then it says
"A 30amp fuseway can support an appliance of up to 7.2kw providing that the
control unit does not also have a socket outlet". What I need to know is
whether I can treat my two cookers as a single appliance and feed them both
off the existing radial circuit; the total load is 7.2kW, which places it
within this stated limit. I'm a complete novice, but I can't see the
difference between powering a double oven (which is just two ovens in a
single unit) and powering two separate ovens from a single radial circuit,
providing the max loads are equivalent. If it is possible, how would the
two ovens be wired to the radial circuit? Just like any other multi-point
radial? Does it make a difference that the two ovens are separated by
Many thanks for your help!!
My guess is that your microwave "combi oven" comes with a standard 3-pin
plug. If this is the case then the best choice is probably to just plug
it it to a socket - that's they way they are normally designed to be
http://www.mxf.info - information for developers of mxf systems
That would certainly simplify things; although the spec suggests it is just
as hefty as, if not more hefty than, the single oven (3.7kW max for the
combi oven; 3.6kW max for the single oven). By my reckoning that is 15A.
Can you not have a separate switch for each; one on one side of the kitchen
near the mwave/oven, the other on the other side near the oven, both from
the same radial circuit? Forgive my complete ignorance if this is the most
idiotic suggestion ever made on this newsgroup !
Coming back to a related aspect of my original post, is it at all possible
to power the single oven (3.6kW; 15A) off the ring circuit? I'm guessing
the "diversity" calculation I've read about only applies to estimating the
likely max load for a number of separate ring loads, but for the oven "max
15A" really does mean that it's going to draw 15A when you have it on the
most power-intensive mode?
If you try news.google.com's archive, you'll see a fine old ding-dong on
more or less this exact subject about three weeks ago. Roughly summarising
and applying to your enquiry: first, it would not be best practice to run
your single oven off the kitchen ring. Even if you have a ring dedicated
to the kitchen, it's prolly got two or three heavy loads on it (dishwash,
washmosh, tumblebumble) and a few more intermittent 1-2kW loads (toaster,
kettle). "Preloading" that ring with an oven (when it's on and its
thermostat is calling for heat, it's pulling the 15A specified) is not
the best of ideas. (I put it no more strongly than that: depending on how
your particular household runs, it could be anything from definitely
stupid (say there's 6 of you, serious amounts of cooking and washing
going on espepercially at particular times of day) to entirely fine
(one or two members, appliances used lightly and rarely at the same time,
etc.) But on balance, and allowing for future occupants, it's distinctly
better to use a dedicated higher-current circuit (or two) for your
electric cooking needs. Diversity, as the Regs, On-Site Guide, decent
books and so on all stress, is a "what's sensible in practice" notion which
needs to be applied with relevant knowledge of how the circuit's going
to be used in practice, and in the absence of detailed usage knowledge
one should err on the side of caution.
Your existing radial would be a good and proper circuit to feed both
appliances. I'm rather assuming the single oven is a built-in job
which really truly will be used as a conventional oven quite often, pulling
its rated 3.6kW: this makes powering it off a 13A plug and socket less
than a brilliant idea (13A plugs aren't that great for continual loads
at their full rating). Depending on the physical layout of your kitchen,
it's OK to have either a single place where separate isolators (switches)
for each appliance are located, or to have the isolators closer to each
appliance. There's a requirement that these isolators be within 2m of
the appliance they control.
In your case, you can provide the isolation for each appliance using
any of: 20A double-pole switches (with neon indicators if you like),
cooker-control switches (usually rated at 45A, and possibly physically
bigger than you want), or fused-connection-units with a 13A fuse: but
these last are kinda marginal, with the 13A fuse rating telling you
they're specced for a nominal 3.12kW. If you use a decent brand (MK,
Crabtree) they should be just fine, especially as your appliances won't
be pulling their full rated power most of the time (as their thermostats
cut in and out, etc.), but I wouldn't use that no-brand cheapie you found
at the bottom of the bargain bin ;-)
The argument we had a few weeks back were about the guage of cable you
could use for the final leg of the feed once you've split the radial
into two for the two appliances. After some marginally ill-tempered
discussion, we more or less agreed that 2.5mmsq is OK in most cases,
4mmsq would avoid doubt for any split-level use, and 6mmsq preserves
maximum flexibility for either of the exit positions being usable in
future for a single all-in hefty-cooker. (Balanced against the thicker
sizes is the awkwardness of working with them: you want nice deep back
boxes (47mm) if possible, though 4mmsq isn't too much of a pain). It
also depends *critically* on how your radial's laid out: if the two
isolators are daisy-chained, i.e. a single cable runs first to one
of them, with the first appliance connected to the load terminals, while
a second cable runs from the incoming-mains side of the first to
the incoming-mains terminals of the second isolator, *only* that second
cable can be reduced in size from the radial feed, since the first
one is supplying both appliances.
Hope that helps - Stefek
The argument we had was all about the thickness of
On Sun, 25 Jan 2004 21:21:07 +0000, stefek.zab wrote:
Also ISTR that for fixed cooking appliances then diversity calcs can be
applied when considering the load for the circuit.
A 'standard' 6 knob cooker can certainly pull more than the 32A circuit
they are usually supplied from but in fact a 32A radial circuit is deemed to
be able to supply a 6-knob cooker and ONE 13A socket.
IIRC the first cooking appliance is rated at 100% and the second at 30% [A]
so there should be enough capacity for a 32A radial to run both appiances
and a socket and the fridge on the one circut.
[A] I the domestic environment (in a hotel it would be different IIRC.)
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
If the 3.6kW is a maximum load, then the switch won't be asked to break more
than 15A. I suppose you could argue that "nature of the load" arguments
don't apply to microwaves.
OTOH, I'd probably just install a 45A DP switch anyway.
Sigh - though I'd've rather written "OK in many cases" rather than "OK
in most cases", on reflection.
Which part of the reasoning did you have a concrete counterargument
to? You seem to insist on applying the safe rule-of-thumb that every
circuit element ought to be rated at or above the nominal rating of the
circuit protection device. It's a *safe* rule of thumb, agreed: but one
which can lead sometimes to excessive cost, inconvenience, or ugliness.
In this case 20A-rated switches *are* adequately rated: the downstream
loads each draw (well) under 20A, and can't cause an overload; while their
short-circuit rating is considerably higher than 20A or 30A or 45A.
As when this came up last, despite detailed working separating out
the overload protection function from the short-circuit one, and the
factual instances where that rule-of-thumb is not applied (rating of
an appliance's internal wiring and switches, the design of a ring circuit
itself where the 2.5mmsq in an unfused spur can't carry the 32A of the MCB
rating, the Continental practice of using (suitably short) lengths of
0.75mmsq flex on appliances whose nature means they can't cause an overload
plugged unfused into 16A Schucko-socket radials, and similar), you continue
to insist that As A Pro, you have superior insight into what constitutes
Bluster like that just won't cut it. It's more useful to us in uk.d-i-y
to have the *reasoning* and *principles* of safe working in the various
trades examined and argued over, than ex cathedra pronouncements
asserting an opinion, however well-founded that opinion might or might not
be. The previous discussion showed in gory detail that 2.5mmsq could in many
cases safely supply the final feed to separate 20A-25A appliances, while
agreeing that it was marginal, under spec in the case of higher ambient
temps, penny-pinching, and left you with little flexibility or safety
margin. (All in all, just what you'd expect in a mass-market new build ;-)
Stefek and others: many thanks for taking the time to provide very
I have a couple of final (dumb) questions related to my installation:
1) You and others have mentioned that the regs say that the control
switch for an appliance must be within 2m of that appliance. Do the
regs say whether the control switch must be above the counter (i.e.
instantly accessible) or is it OK to have them out of sight under the
counter, but still accessible through a cupboard?
2) A couple of remaining questions with domestic wiring that are
troubling me are as follows. A ring circuit is usually wired with
2.5mmsq cable (rating < 30A), but is apparently able to provide up to
30A... is this because it's a ring not a radial and so current to any
point can go both directions and is therefore split in half, so that a
ring supplying 30A to a particular point would actually only be
carrying 15A in the supply cables? This brings me to the question:
why are cooker circuits provided as 6mmsq radials and not (e.g.)
2.5mmsq rings, if the latter will take just as much loading as the
email@example.com wrote in message wrote:
I'm pretty sure, but don't claim to be authoritative, that the switches
are supposed to be accessible. The full-on answer would depend on the
function the switches are considered to be performing: if emergency
shutoff, then visible-and-accessible; if isolation, less accessible would
be permitted. Let's use common sense here: definitely one of the reasons
you want the switches is to cut power to your cooking appliance if something
catches fire on it or in it. Then you want to be able to cut the power
PDQ alongside doing Other Sensible Things (covering with well-dampened
towel, for example) - and the "you" might just be Auntie Mabel cooking
lunch for all of you. Hence the requirement for the switch to be in plain
view and close by - i.e. I'd think long and hard about pretending a
cooking appliance isolator does *not* have any emergency switching function.
That's the general idea - the load is shared among the two paths to
any place on the ring. But it's not shared half-and-half other than at
the midpoint of the ring - it's in proportion to the resistance of
each path, which is mainly determined by the length of each cable path
back to the CU, along with how well each joint along the way has been
made. That's why you can't just double the current-carrying capacity of
the 2.5mmsq cable in designing the ring: and also why it's poor practice
to design a ring layout in which heavy loads are concentrated at one end
of the ring - e.g. a ring to serve all downstairs which starts off in the
kitchen, where the heavy loads are, and then snakes lazilly all round the
house before coming back to the CU.
Wiring practice for cooker circuits hasn't fully caught up with the fashion
for split-level hob-n-oven setups and similar separate-cooking-appliances
setups: by and large the conventional circuit design assumes there'll be
one hulking great standalone cooker. Such cookers have a *peak* current
draw of say 40A (turn on 4 rings - there's 7kW - and both top and bottom
ovens - another 3kW - look, ma, 10kW peak loading giving a current of
10,000/240 = 41.3A; or if you prefer to work with the nominal-voltage
fiction, 10,000/230 = 43.5A, though if the supply voltage did drop to
230 the power drawn through the cooker elements would also fall). Now, that
peak load won't be sustained for long, even if Auntie Mabel is cooking
Christmas dinner with all the trimmings and using all 4 rings and both
ovens, because the simmerstats/thermostats will be switching each bit of
the cooker on and off. That - and the fact we don't in practice turn on
all our cooker's component loads at once - is the thinking behind the
diversity rule which allows us to treat the worst-case-40A-plus load
of the cooker as less than that (10A + 30% of the rest in a domestic setting)
for purposes of counting up the total demand of the installation. *But*
we still size the wiring and switchgear for this appliance which on
occasion *will* draw its full 40A to suit that peak demand. (And voltage
drop along the length of the cable matters too). Hence, the Right Answer
is almost always 6mmsq for the dedicated cooker circuit feeding either
a single cooker, or split later to feed oven + rings as separate appliances.
So, 2.5mmsq rings do not 'take just as much loading as a 6mmsq radial' -
the dedicated circuit is much more appropriate to supply a single load,
and can safely meet a higher sustained load than a 2.5mmsq ring.
HTH - Stefek
Just a quick follow-up on the form of the control switch used for each
feed off the radial. Are there any regs about this concerning what
the switch implies about the underlying electrics? In other words, is
it against the regs to use (if you were stupid) a cooker-type switch
when the supply to that is actually off a standard ring circuit? If
the switch also had a three-pin plug outlet then someone might assume
they are safe to power their hefty cooker or hairdryer off that. My
reason for asking is to determine whether the control switches for
both feeds off my cooker radial can/should be cooker-type switches, to
indicate that both are fed from a cooker radial, or would this imply
that both have dedicated radials behind them? Or does it not matter?
I guess any electrician would be able to tell with a little detective
work, but I'm thinking more of Aunt Mable here. Forgive my use of
firstname.lastname@example.org wrote in message wrote:
"Michael Brewer" wrote
| Just a quick follow-up on the form of the control switch used for
| each feed off the radial. Are there any regs about this concerning
| what the switch implies about the underlying electrics? In other
| words, is it against the regs to use (if you were stupid) a cooker-
| type switch when the supply to that is actually off a standard
| ring circuit?
That would be a switched unfused spur and could only be used for supplying
one FCU or one single or twin 13A socket, which would be within the
regulations. (And a twin 13A socket is not assumed to take 26A. The ring
final circuit is designed for general domestic use for a variety of
equipment where diversity will apply. It is the designer of the
installation's responsibility to not use a ring final circuit where it is
inappropriate, eg fixed heavy loads.)
If you're talking about using a cooker control unit to supply a cooker, but
off the ring final circuit, then of course it's against regs.
| If the switch also had a three-pin plug outlet then someone might
| assume they are safe to power their hefty cooker or hairdryer off that.
But that would be limited to 13A by the plug fuse.
| My reason for asking is to determine whether the control switches for
| both feeds off my cooker radial can/should be cooker-type switches, to
| indicate that both are fed from a cooker radial, or would this imply
| that both have dedicated radials behind them? Or does it not matter?
Provided that the circuits are clearly labelled at the CU it should be
obvious that the two outlets share a circuit. The switches should be clearly
labelled so it's obvious (especially to Aunt Mabel) which controls what.
Some switches come with a set of stickers (cooker, hob, oven, water heater,
etc) rather than just being engraved cooker.
I have a feeling I'm breaking all sorts of rules by following up my
own original posting, but it seemed to apply to the whole thread.
I have noticed a footnote in the oven specs (it's a Bosch oven) that
says "we recommend that all models should be connected using a fused
spur outlet". This footnote applies to all the ovens, including the
According to my very limited understanding, a "spur outlet" would
imply to me that it's a spur off a ring circuit. Or could you have a
spur off a radial circuit, e.g. a multi-point radial circuit where you
provide a number of fused outlets tapped off the radial circuit?
How would you interpret this footnote?
It seems to imply that it wants a 13A FCU. However, it could be translated
European stuff. They traditionally run each appliance off a dedicated 16A
circuit, a bit like old UK 15A round pin circuits.
Do the instructions state a value for the fuse?
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