Wiring for 2x 3.6kw ovens, & “Currys Team (don’t) Know How”

[beedj]

Agree with you both MJNewton and BaldOldMan :-)

[MJNewton]

BaldOldMan said:

I don't think anyone questioned the 32A supply - but is it OK to wire 2 x 16A governs directly to that with no additional protection as is OP's existing installation.

By 'protection' I am assuming you mean the MCB? If so, that is to protect the cable - not the appliance. The appliance is required to provide its own protection whether that be through an internal fuse or with materials/construction in accordance with the relevent regulations.

Edited by MJNewton on Monday 21st October 14:52

[Sheepshanks]

MJNewton said:

I am surprised that no one has mentioned 'diversity' yet (apologies if I missed it!) as this is the key starting point when working out the current capacity requirements of cables for ovens. Basically, it reflects the fact that ovens are intermittent heating devices (the heat input far outweighs the heat losses so a thermostat is employed to cycle the power once up to temperature) and hence they do not draw the full rated capacity continuously. The supply cable does not therefore need to be sized to accommodate the full load continuously.

That's a bit of a grey area - is a single oven a single fixed load? If it is then diversity doesn't apply.

[MJNewton]

Grey area? There's nothing grey about cookers and allowance for diversity.

Here's a table summarising all the circuits where diversity is allowed.

Edited by MJNewton on Monday 21st October 15:03

[Sheepshanks]

MJNewton said:

Grey area? There's nothing grey about cookers and allowance for diversity.

It's not a cooker though, it's an oven.

I don't know, I've seen it argued both ways, especially when it's a typical separate oven and hob arrangement - even then some say you can't apply diversity to the oven, only the hob. If it's all one unit (a cooker) there's no issue.

[MJNewton]

MJNewton said:

By 'protection' I am assuming you mean the MCB? If so, that is to protect the cable - not the appliance. The appliance is required to provide its own protection whether that be through an internal fuse or with materials/construction in accordance with the relevent regulations.

To follow this up with an authoritative reference, BS60335 ('Household and similar electrical appliance - Safety') mandates various requirements on dealing with 'abnormal operation' within which, on the subject of overcurrent protection devices, it clearly states:

BS60335 said:

NOTE Fuses, thermal cut-outs, overcurrent protection devices, or similar devices incorporated in the appliance, may be used to provide the necessary protection. The protective device in the fixed wiring does not provide the necessary protection.

Edited by MJNewton on Monday 21st October 15:26

[MJNewton]

Sheepshanks said:

It's not a cooker though, it's an oven.

I don't know, I've seen it argued both ways, especially when it's a typical separate oven and hob arrangement - even then some say you can't apply diversity to the oven, only the hob. If it's all one unit (a cooker) there's no issue.

Don't get hung up on appliance names; BS7671 (on which the wiring regs are based) allows for diversity for any cyclic load. An oven, by virtue of its thermostat, is a cyclic load.

[BaldOldMan]

Still not answering the question......

I don't think the regulations allow you to connect up an oven rated at 16A - with it's own cable - to a 32A MCB as you ceem to be suggesting with the double outlet plate......

In the original post it seems that the manufacturers specify a 16A fuse & as I understand it, that over rides any other regulations ?

beedj said:

Each oven's instructions say 'install on a dedicated circuit protected by a 16a fuse'

[MJNewton]

That's a fair point given the ovens appear to have been supplied with their own cable(s), as whilst the appliance doesn't fall under the protection of the MCB the cable does.

Personally, to err on the side of caution if wanting to make sure all risks are minimised, I would replace the cable with 4mm2 heat-resistant flex (most electricians would likely be content with 2.5mm2 given diversity).

You do need to be careful with manufacturer instructions as if taken literally they can undermine the safety and performance (e.g. in this case the fact it specifies a 'fuse' which has very different performance characteristics as an MCB). As someone else mentioned they are often no re-written for the UK market where our approach to electricity and wiring differs from the continent.

Edited by MJNewton on Monday 21st October 16:27

[Sheepshanks]

MJNewton said:

Don't get hung up on appliance names; BS7671 (on which the wiring regs are based) allows for diversity for any cyclic load. An oven, by virtue of its thermostat, is a cyclic load.

You're using commonsense now, which is allowed under BS7671, as there's no precise definition for cyclic loads - instead it talks about duty cycle. You don't really know what the duty cycle of an oven is though.

[Initforthemoney]

fking hell.

[Teddy Lop]

Regardless of whatever one believes diversity allows (the cooker circuit calc is devised with a multi functional cooker in mind ie 4 rings+oven+grill where its anticipated not everything will be used at the same time on peak power, and apart from a few people with peculiar ideas about cooking Christmas dinner it tends to hold true.) If installing two separate ovens though I'd anticipate them both being switched on and used at the same time thus drawing high load during the 15 min warm up and install circuitry to handle such.

[BaldOldMan]

If they have a pyrolytic cleaning function - that pretty much guarantees full power for an extended period - and I guess there's a reasonable likelihood that both ovens get cleaned at the same time......

[MJNewton]

Indeed but even a constant 2 x 16A is really not going to be pushing the 6mm2 'trunk' or the individual flexes. There is a massive, some would say too big, safety margin in the tabulated cable ratings.

[Mr Pointy]

This seems quite a thorny topic over on DIYnot as well.
https://www.diynot.com/diy/search/96755854/?q=oven...

One poster did link to a 45A Dual Cooker Outlet Plate which looks useful:
https://www.alertelectrical.com/click-polar-45a-ea...

The general opinion over there seems to be that diversity does apply to ovens, in which case the OP could simply connect his new ovens to his existing 45A switches. I believe the load for each oven corrected for diversity is 10 + (0.3*(15-10)) = 11.5A. I think 23A on a 6mm supply cable is within limits, depending on the run length & installation method of course.

[Smiler.]

Initforthemoney said:

fking hell.

Quite.

[jackofall84]

OP - As per usual on these questions I've seen a lot of nonsense replies - I'm not sure why people bother posting an answer when they don't have the faintest idea...anyway, I'd break it down like this:

Firstly work out the max current draw of the 2 ovens...Power = Volts x Amps so (2*3600)/230 = 31.3A, so if you've got both ovens on at the same time warming up from cold its asking a lot from your 32 Amp breaker, get a sparky in to swap that breaker for a 40A or 45A breaker or swap it yourself if you feel confident, will take 5 - 10 minutes to fit and the breaker will cost less than £5.

Secondly, according to the IEE 4E1A table the worst case installed scenario for 6mm cable is 45 Amps. So all wiring and switches can remain as they are perfectly safely.

Simple

[Mr Pointy]

jackofall84 said:

OP - As per usual on these questions I've seen a lot of nonsense replies - I'm not sure why people bother posting an answer when they don't have the faintest idea...anyway, I'd break it down like this:

Firstly work out the max current draw of the 2 ovens...Power = Volts x Amps so (2*3600)/230 = 31.3A, so if you've got both ovens on at the same time warming up from cold its asking a lot from your 32 Amp breaker, get a sparky in to swap that breaker for a 40A or 45A breaker or swap it yourself if you feel confident, will take 5 - 10 minutes to fit and the breaker will cost less than £5.

Secondly, according to the IEE 4E1A table the worst case installed scenario for 6mm cable is 45 Amps. So all wiring and switches can remain as they are perfectly safely.

Hilairious. Where did you get 230V from? Voltage harmonisation is a fantasy. What about protecting the short run of cable from the 45A switch to the ovens? You've put them on a 45A breaker now.

jackofall84 said:

OP - As per usual on these questions I've seen a lot of nonsense replies - I'm not sure why people bother posting an answer when they don't have the faintest idea...anyway, I'd break it down like this:

Well you got that bit right.

[Smiler.]

jackofall84 said:

OP - As per usual on these questions I've seen a lot of nonsense replies - I'm not sure why people bother posting an answer when they don't have the faintest idea...anyway, I'd break it down like this:

Firstly work out the max current draw of the 2 ovens...Power = Volts x Amps so (2*3600)/230 = 31.3A, so if you've got both ovens on at the same time warming up from cold its asking a lot from your 32 Amp breaker, get a sparky in to swap that breaker for a 40A or 45A breaker or swap it yourself if you feel confident, will take 5 - 10 minutes to fit and the breaker will cost less than £5.

Secondly, according to the IEE 4E1A table the worst case installed scenario for 6mm cable is 45 Amps. So all wiring and switches can remain as they are perfectly safely.

Simple

Table 4E1A is for single-core thermosetting cable with a 90°C insulation.

Domestic cable is still almost all multi-core thermoplastic with 70°C insulation, so the correct table is 4D2A, worst case current carrying capacity being 32A.


OP, a 32A type B or C MCB is at the limit of the max connected load, so if you intend to run both ovens from cold, you might see it trip under overload, but it's highly unlikely.

Assuming that you don't change the MCB to 40A, you need to check the cable type & size of the flex into the ovens. If it's 4mm² 3-core ordinary flex, at 4.0mm², it should be good for 32A. If it's 2.5mm², then 25A.


So first, establish what the cables fixed to the overs are.

If you need to add an MCB for each one, make it 16A, but this might be difficult to accommodate & you will need ready access to it..

Also, have a look (as far as possible) at how the existing cable is fed. If it's in a void in contact with rockwool or similar, that's the worst case. If it's clipped surface, then it will do 40A. But there is another caveat - how many other cables run alongside it.

For that last point, it really means for a significant part of it's entire length.

Hope this helps.

[Smiler.]

Mr Pointy said:

Hilairious. Where did you get 230V from?

It's the nominal voltage figure used in the UK.