Many thanks for the reply to my recent post regarding kitchen electrical
wriring regs. All issues are now sorted except for the oven and hob - which
has now arrives and 'er indoors is nagging to try it out :-)
To summarise - the oven is rated at 5.18kW (22A @ 230v) (double under
counter oven) and the hob at 5.4kW (23A @ 230v).
The boss wants to have only one DP isolator above the worktop, which I agree
with, so the question is how to wire this lot in.
The oven and hob manuals state that a 30A fuse should be used with a minimum
of 4mm T&E cable.
My idea was 10mm T&E cable from the CU to the DP isolator then 6mm T&E to
two cooker outlet plates then to the appliances. The CU currently has a 32A
MCB on the cooker radial circuit, but I could change this to a 45A MCB.
However my concern is that the overall fuse is 45A, but the individual
appliances should really be 30A. Can you get fused cooker outlet plates or
something similar, or must I go for two completely separate circuits from
the CU with 32A MCBs.
This cannot be an uncommon application, but what is the best way to proceed,
with the regulations.
BTW the CU is only a few metres from the oven hob, max 4m or cable from CU
to DP switch then 1m to the appliances.
Any help greatly appreciated so I can get it done this weekend
I personally would wire two 30A supplies to two isolators.
If you wanted to hide the switches you could put them in the cupboard
at the side of the oven housing, but not right at the back where you
cant get to them. You won't need to switch them off other than
maintenance so you're not going to need to access them that often.
Also keeps both switches from being above the worktop, double happy
for the boss!
Your original way wasn't really up to scratch, the 6mm wouldn't be
protected other than by the 45A fuse, not good.
You could get 32A fuses to go by the hob & oven but you'd take up more
room than the 2 switches so would have to put them in the cupboard
next to the oven anyway, so if you have to put something in there you
may as well make it the switches, cheaper, easier and more convenient.
If the hob goes You've still got the oven, without having to take it
all out to isolate it.
The Regs allow you, sensibly, to treat the hob + oven as comprising a
single cooking appliance, to which you apply standard Domestick diversity
rules for a cooker. The combined Hamperage from your ratings is 45A.
The Diversity allowance for domestic use is "10A + 30% of the rest
+5A if you've a socket on the control panel". In your case, ass-U-ming no
such socket, the diversity-corrected load is (10 + 35*0.3) = 20.5A.
Thus it's well within Regs to use a single circuit, with a 32A MCB,
wired in 6mmsq as far as the switch (4mmsq kind-of OK on short run without
thermal insulation, but don't arse about, use 6mmsq). From the DP switch
make the connections to the hob and oven in 2.5mmsq if you already have it,
or 4mmsq if you're buying new (might as well allow for the odd Christmas
lunch peak load).
I do hope your hobnoven come with thermally resistant oversleeving for the
L and N conductors where they're connecting in to the hob, though - it
does get warm in these appliances (no s**t Sherlock ;-) and plain PVC
gets soft and less useful at higher temps (70C upwards is the usual guide)...
HTH - Stefek
On 14 Jan 2004 21:25:14 GMT, email@example.com wrote:
Have you ever wired anything up before. Stating regulations willy
nilly does not constitute a safe and constructive answer to help the
OP along. If I came across a cooker and hob wired the way you suggest
I would probably condemn the whole installation on the grounds anyone
who plays around with one part of the electrical installation
regardless of knowledge or competence will probably have had a go at
the rest of the wiring.
It's generally not good practice to run multiple supplies from one
isolator, especially without providing any protection in the way of
"Advise" is over-strong, but it's both within the letter of the regs and
consistent with safe procedure. The two partloads are - if you read the
poster's original query - each going to draw no more than 24A (10 of your
best British keelowatts); and that's only under peak conditions when all
4 themostats/simmerstats on the hob cut in at once, or when both ovens
have their element thermostats switched on (e.g. when warming up) and
their fancy-dancy side-mounted auxilliary browning elements switched on
(or whatever else is in there to get the full load up to 10kW). Like I
said, if buying new anyway then 4mmsq would give a bigger margin for
safe operation; full paranoia would be 6mmsq all the way through, Just
In Case some other occupant at a later date decided to attach an all-in-one
older-style freestanding cooker to just one arm of the circuit beyond
the DP switch; but 2.5mmsq for the short (earth loop impedance considerations)
downstream sections to separate hob & oven *is* permissible, safe, and
practised, where the loads are as the original poster described.
These are thermostatically controlled loads, which have no way of causing
an overload beyond the rated power, and even in "full" use draw well under
the full rated power when considered over timescales of minutes: very
different from either a socket circuit, where multiple unknown loads can be
plugged in, or motor circuits where a stall or excess mechanical load can
cause a higher current to flow for a sustained period. And in just such
cases, the Regs foresee and allow for - and qualified penny-penching
circuit design engineers looking to cut another 60p off the materials
costs for the next Barratt box take full advantage of - sizing of conductors
according to the peak load, rather than according to the protective device.
You may not find it pretty; you may argue that it's cutting close to the
edge; and that it's a safe simplification to adopt the simpler principle
"all conductors should be capable of carrying for an indefinite period the
nominal current of the protective device" (In in Regs-speak). But that's
*not* the full story on either what the Regs say or what economical
engineering practice does to balance cost against compliance; and the
tweaks about "nature of the load" allowing sizing by design current rather
than nominal are used internally in equipment wiring (you don't think those
hob elements have 6mmsq conductors leading to them, do you?), and in the
widespread Continental practice of 0.75mmsq appliance flex, unfused,
connecting into 16A radial circuits with no closer fusing, as well as this
particular case of splitting out feeds to Modern Kitchen Hob-n-Oven
Still, don't mind me: I probably don't know what I'm talking about; and
probably never wired nuttin'...
I see no reason why this is not acceptable, assuming no insulation. Indeed,
it is the standard method, as I understand it.
The 2.5mm cable is rated to 27A (IIRC). The nature of the load prevents a
greater load than 22.5A being drawn. Diversity allows both parts to share a
32A circuit. The earth loop impedence and voltage drop on a short run like
this is very unlikely to forbid it, unless the run of 6mm is long.
It is no different in principle from running an unfused spur in 2.5mm on a
32A ring circuit, where the spur cable is also only rated for 27A (20A under
certain installation conditions).
All quite correct, except for one thing no-one's mentioned yet: ambient
temperature correction factors (see Table 6A1 in your On-Site Guide). The
27A 'clipped direct' rating of 2.5mm^2 T&E only applies up to 30 degC, at 40
degrees the derating factor is 0.87, at 50 degrees 0.71. So in this
situation 2.5mm^2 cable is only suitable if the temperature won't exceed
about 40 degrees, and it's quite possible that that could be exceeded where
the cable approaches and enters the appliance.
So with 2.5mm^2 PVC cable Stefek's design is likely to be marginal or
non-compliant. With a 2.5mm^2 heat resistant flexible (3093Y etc.) you'd
probably be OK, provided you've got a bootlace ferrule crimper for making
the ends off in the wiring accessories.
NB (for the pedantic) your statements "nature of the load prevents
[overload]" and "diversity allows [...]" don't sit comfortably together in
the same paragraph without further explanation. Diversity is being applied,
quite validly, to the 'feeder' circuit from the MCB to the point where the
circuit splits after the cooker control switch. Since diversity has been
applied, overload, relative to the design current (Ib), _can_ occur -- but
that's OK since the 32A MCB will provide overload protection for the 6mm^2
cable. After the split point, if we drop down to 2.5mm^2, there's no
overload protection, only s/c fault protection provided by the MCB. Hence
no diversity can be applied after the split and the cables have to be rated
for the FLC of the appliances -- which of course is what we've done.
Thanks for a calm exposition of just how and why the design is marginal.
"Marginal" is just right - i.e. (as I claimed) it's the sort of just-compliant
watch-the-pennies just-about-good-enough no-headroom apporach used in
practice on many mass-market builds; for one's own use at home 4mmsq would be
sounder, but the 2.5mmsq may be justifiable. Though I made passing mention of
the higher temperature of the oven/hob environment, I didn't make explicit the
necessary derating of the cable. Such higher ambient temp is particularly
likely to occur in the space behind the "built-in" oven, where there might
well be a metre or so of wiring slack of your (carefully, gently folded ;-)
final T&E. And I'd also briefly thought to mention the use of a heat-resistant
2.5mmsq flexible, and rejected it on the grounds of the poster being (a)
unlikely to have a bootlace (or ring) crimper to hand, to make the connection
to the terminations reliably, and (b) 2.5mmsq silicone-insulated or similar
flex being harder to source than T&E - the OP might've ended up with 1.5mmsq
"immersion heater" cable from some Saturday-morning trade-counter "sod the
weekend cowboys" type, which would be well under reasonable current-carrying
Again, thanks for the clarification. With the increased popularity of
split hob/cooker arrangements, will (or have) wiring-accessory mfrs
brought out a "dual-control" cooker control switch, with two 45A or
32-A-rated DP switches in the one (probably double-width) accessory?
Obviously you can mount two 45A control switches in a dual box, but
it's more of a pain wiring them up, with the heavier incomer needing to
go both in and out of the first one.
(Oh, and maybe the IEE will issue a guidance note on wiring split hob-n-ovens
from one existing 45A cooker feed, discussing the limits on going down to
smaller cable sizes for the post-control-switch portion, in as concise and
authoritative terms as Andy managed in the text quoted above? ;-)
as I understand it the, temperature rating of the cables applies to
the whole cable being at the higher temperature. When a small local ambient
temperature variation occurs, the derating factors do not, IMO, apply,
provided the temperature at which the insulation is breached does not occur.
The best example I can think of, is the domestic iron, where there is a
major temperature gradient between the sole plate and the cable cores. I do
agree however that at the back of a built in oven, care in cable type is
required. I believe that in the majority of built in oven installations, the
back is actually fan cooled, to alleviate this problem. Anyway, that's why
there is a fuse??!!
The cable size will make zero difference in the majority of
installations, as the I2R losses are too low to be significant against heat
loss from the oven.
I kind of confused now, one answer saying I should use two seperate supplies
and another talking about diversity rules and that I can use one 6mm T&E and
a 32aA MCB.
Even if I apply Stefek's logic and the DP does have a socket I only get
25.5A or roughly 6kW of load. I have had an 8.5kW shower wired into a 32A
MCB via 6mm T&E for years and never had a problem.
Further answers and/or hints / tips / advice urgently and of course
I think you'd struggle to get a single 10mm2 and 2 x 6mm2 into the isolator.
When I did mine a few years ago, I ran 6mm2 to the Isolator and then 2 x
4mm2 to the Hob/Oven - and its been absolutely fine.
My thoughts as well. The 6mm from 32amp MCB in the consumer unit to a 45amp
DP switch on the wall. Then 4mm's to each appliance from the DP switch.
The DP switch will be tight with two 4mm in the connectors, but they are
meant to take 8mm without taking any of the strands off the cables. You're
also better to sink a 32mm deep back box in the wall to make the cabling
easier for yourself. A standard inch deep back box is just a bit too tight.
As was said in another reply, the oven and hob combination can be taken as
one free standing stove, so the diversity of the thermostats working on the
hob will allow the appliances to be run on the 32amp fuse rating. Unless
your instruction manual says different. Anything higher than 32amp and
you'll need to upgrade the supply from the consumer unit to 10mm and use a
60amp rated DP switch on the wall.
What I have done is to pass one of the conductors for the Hob through a current
detection relay, this coupled with a delay off relay is then used to supply
power to the cooker hood fan and light.
That way the hood fan starts each time the hob is used and stays on for a little
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