How to connect this built-in oven properly?

[tinnews]

I am installing a new built-in oven and hit the usual problem of the horrible connectors they always seem to come with. However this one seems even worse than usual and I'm a bit stumped as to how to do a proper job of connecting it.

I have a dedicated 32 amp MCB for the oven and (electric, induction) hob and it has a 6sq mm cable running to the kitchen units.

However there is absolutely no way that one can connect 6sq mm cable to the oven. The connection block is too small. The only cable that I think I can reasonably connect to it is 2.5sq mm because the screw terminals are such that only single, solid wire can be reliably connected to it.

There's not enough room for any sort of termination on the cable. I've tried and I have a wide variety of crimps in many shapes and sizes, while you can terminate a cable with a 'ring' that will fit under the screw the body of the crimp is too fat to fit through the slot provided for the cable.

The oven is rated 2.3kw so 2.5sq mm is perfectly adequate, the problem is that it's not ideal for allowing the oven to be slid in and out and it's not properly protected by the 32amp MCB.

What options do I have? I suppose I could have a local FCU for the oven but it seems a bit overly complex and discrimination might be a problem. I can't see *any* way to connect flexible cable to the oven so I think it's stuck with at least a short length of 2.5sq mm. The best I can do at present is to take the short length of 2.5sq mm to a junction box (or terminal block) on the back of the oven and run a more flexible cable from there. Can one get high temperature 2.5sq mm cable with solid conductors that I could rate at 32 amps? That would be be the neatest solution I guess.

Or maybe I should take the back off the oven and replace their crap connector!

[ARWadsworth]

There is a very good chance the 2.5 T&E will be adequately protected by the

32A MCB.

However I now suspect that you have just unleashed the pillock from Birmingham and we can now expect a load of ranting and bollocks about the dangers (made up ones) of installing unfused spurs using 2.5 T&E on a 32A circuit.

Our only hope is that he is in his straight jacket or that nursie has heavily sedated him tonight to stop him going again out without permission armed with a hair drier, hi viz jacket and a clipboard. (Last time he went out it took 3 nurses to pry him off the speed camera he was making love to, and then they had to presure wash the camera post clean after they had done so)

PS Are you sure you cannot get 2.5m flex to fit (maybe with a bootlace ferrule on the end of the flex)

[A.Lee]

You need to separate the oven and hob, and have 2 switches, one for each. I presume that 32A is enough for the both? You spur off the 6mm cable with a 13A FCU to supply the oven. You can run 4mm cable from a connector box/joint to the FCU if you cannot terminate the 6mm in a normal FCU. You dont need heat resistant cable, it doesnt get very hot at the back of the oven. 2.5mm from the FCU to the oven is fine.

[John Rumm]

No need if it can only draw 2.3kw peak...

It will be adequately protected by the 32A MCB - since in this application you are only concerned about fault current protection.

You could use a FCU. A 13A fuse would normally discriminate with a 32A MCB upstream. However the chances of it ever operating are slim anyway.

If you can get 2.5mm^2 T&E into the terminals then you are likely to be able to get a 2.5mm^2 heat resistant flex in there as well.

Note however if you were to fit a FCU, then you could reduce your flex size to 1.5mm^2:

best I can do at present is to take the short length of 2.5sq mm to a

Sounds like you are over thinking this one ;-)

[dennis]

This sounds much better than what ARW said. I would go with what Mr Lee says.

[js.b1]

Do you have something like an MK 32A cooker switch & 32A cord outlet behind units, ie, originally for a conventional standalone electric oven?

32A cord outlet supplies units... Cord outlet -> 6mm FTE -> Jn-Box ... Jn-Box -> 6mm FTE -> hob ... Jn-Box -> 4mm FTE -> FCU -> 1.5mm Flex -> oven 2.3kW oven cable...

- 1.5mm Flex in Butyl / 90oC PVC

- Commonly used for storage heaters, immersions etc.

NkW hob cable...

- 6.0mm FTE - 60oC PVC (or 90oC 6242BH off Ebay)

- 6.0mm Flex - Butyl, High Temp PVC, Silicone (CPC, Farnell, Conrad)

TLC do a good range of junction boxes, 6mm cut length, 4mm cut length,

1.5mm high temp flex.

Check what spec cable the hob requires, that may make things a bit more interesting.

Easy. Done. What you moan'n about. Your cat is eating the children whilst you are thinking about it :-)))

[ARWadsworth]

How do you know what I said? You claim you do not read my posts?

Have you invented a time machine so that you can see posts from the future? That means you can claim that you were replying to a post of mine by seeing another posters reply (by someone that you do not claim to blacklist) to my post?

All it takes is someone that you have not blacklisted to reply to my post with the words "dennis you are a bell end" to make the future come true

Any volunteers to make this post?

[tinnews]

The induction hob is 7.5kW (if we get the one we're thinking about getting).

I did wonder about this, maybe I am being a bit paranoid! :-)

Yes, but the terminals won't hold flex properly. They are just round[ish] head screws with a plastic guard around them, no washers or anything, if you put flex under them it just squeezes out. The only reason that 2.5sq mm works is because a pre-formed U stays under the screw.

Could be!

[rde42]

The worst thing is that nursie probably has to clean his horrible connector.

[John Rumm]

The hob won't be on the reduced capacity cable feeding the oven though...

Still if you do the sums:

7.5 + 2.3kW = 9,800W = theoretical max combined load of 42A

Now apply diversity: 10A + 30% of remaining 32A = 19.6A

(assuming no socket on the cooker switch, around 25A if there is).

In reality, not a big load.

When you think that you routinely run more powerful devices, on longer and thinner flexes without concern...

Either twist the strands together and loop it round, or stick a bootlace ferrule on the end, which will make it substantially the same as T&E solid core to terminate. (on 1.5mm^2 there ought to be space to spare!)

[tinnews]

But not with just 32 amp circuit protection surely? There will be a

13 amp fuse in the plug to protect the cable.

[Adam Funk]

I'm not looking for an argument (or even a good session of contradiction), but I was under the impression that 2.5 T&E wasn't rated for 32 A in any situation, and that you ought to use heat-resistant (rather than "ordinary") cable coming into an oven. I'd like to know why those notions are wrong.

[A.Lee]

As Adam has pointed out to me, it is a known load, so for that oven, there is no need for any overload protection. Clearly, it is sensible to have o/l protection, but not necessary.

The wire input is at the bottom corner (generally) on built-in ovens, so the cable is unlikely to get much over 30-40 degrees, hence no need for heat resistant cable.

Alan.

[John Rumm]

You are right its not...

However in this case, we are not designing that part of the installation to carry 32A, or anything close to it.

This is one of those situations where the responsibility for fault and overload protection can be split. The fault protection must always be at the origin of the circuit, and must in the event of a fault ensure that the circuit is disconnected quickly enough to avoid (possibly further) cable damage (5 secs typically on a fixed appliance). Overload protection is also usually at the origin of the circuit and provided by the same protective device. However in some cases the overload protection can be placed elsewhere in the circuit, or for that matter omitted altogehter, if it can be imposed by some other design criterion.

In this circumstance the fault protection for the 2.5mm^2 cable will be adequately provided by the 32A MCB (much the same situation as applies with a spur in 2.5mm^2 from a 32A radial wired in 4mm^2 for example). There is however no need for explicit overload protection since in this case it is imposed by the design i.e. the oven can only draw a maximum of 10A, and there is no way for someone to unwittingly add additional load to the end of its cable.

Ideally it should be - and high temperature flex would be the cable of choice. However there are two common problems with that, its not always easy to buy in the required size[1], and second it won't always fit in the connector on the oven. This means that in practice many ovens / cookers etc get wired in T&E. By selecting a size significantly in excess of that indicated just by the current demand, you also allow for significant de-rating due to the high ambient temperature. While it gets warm in the oven enclosure, its unlikely to actually go above the insulation temperature limit of the "normal" PVC.

[1] Most wholesalers have stock of 2.5mm^2 which will cover pretty much any oven, however many do not carry the 4.0mm^2 that would be required for many hobs.

Your notions are not actually wrong as rules of thumb, its just there is a little more design finesse one can apply to take into account the specifics of the situation.

[tinnews]

Thanks for this detailed clarification, just what I needed to feel that the way I have wired it is satisfactory.

[Adam Funk]

Right, it's the fault protection that I was thinking of, but (a) I didn't realize a 32 A MCB was low enough for fault protection on

2.5 mm^2, and (b) what about a fault inside the oven that takes it over 10 A? Do we assume the oven has internal fault protection?

Good point.

Thanks for the explanation.

[Adam Funk]

Right, but I was thinking more of fault protection (see John Rumm's reply & my reply to that).

Good point.

[ARWadsworth]

more detail.

It needs 160A to trip a 32A MCB quickly enough to meet fault protection requirements.

But 160A is NOT the maximum fault current ir is a mimimum current. The fault current will be much higher than 160A in most cases. A house close to the substation could have a very low ELI and well over 1000A could pass through the cable in the event of a fault (even on a 6A lighting circuit).

If the manufacturer of the oven specifies a 13A fuse needs to be fitted then you must fit one.

Can you suggest a way that a 3kW oven could fail and allow greater than a

27A current (the maximum 2.5 T&E can carry) for any length of time?

[John Rumm]

Yup, fault protection (i.e. short circuits L to N or L to E) is a *very* different ball game from overload protection. You are dealing with very high instantaneous currents way in excess of the normal duty rating of the conductors. Hence you need enough fault current to cause the protective device to open quickly, and you need the cable conductors to survive long enough to ensure that happens when subjected to adiabatic heating (i.e. so fast you can't take into account any losses to the surroundings).

So with a 32A MCB:

table shows that a current of 160A or more is needed to cause it to operate in its "instantaneous" mode (i.e. its magnetic rather than its thermal response).

Say you take a fault scenario of the insulation on the oven element breaking down completely and the element shorts to earth. Say this presents a total Earth Loop Impedance of 0.8 ohms at the fault position. That ELI would give rise a fault current of around 230/0.8 = 290A. So enough to open the MCB in a nominal 0.1 secs. The weak link of the chain with T&E is the earth conductor which is only 1.5mm^2 in this case. You can check that it will survive long enough to clear the fault with the adiabatic check:

s = sqrt( i^2 x t ) / k

Where s is the cross sectional area of earth conductor required, i the fault current, t the time to clear the fault, and k a factor empirically derived to factor in the physical response of the cable.

So you get:

s = sqrt( 290^2 x 0.1 ) / 115

(where 115 is the k factor for PVC cable, see table [1])

s = 0.8mm^2

So we known that the 1.5mm^2 in the T&E is >= s, hence it is adequate.

Here is where you need to be careful with language... if you really mean fault, then its covered by the above - as in my example.

However you also need to assess the risk of overload - i.e. a sustained period drawing more current than the circuit was designed for.

In this case, while there are a number of fault current scenarios that may occur in the oven, there are no realistic scenarios where the oven could start drawing current significantly in excess of its design current for an extended period of time. There is also no way for a user to easily do anything that could add to its load.

(there are additional failure modes for things like ovens that result in excess earth leakage, but only at very low current levels rather than fault levels)

No, not in general. (Although as a general rule, if an appliance needs additional protection beyond that expected for its supply conductors then it should include it).

[1]

[Mike Tomlinson]

En el artículo , [e-mail] escribió:

When that has happened to me in the past, I've added another washer under the screw head. That prevents the stranded wire from squeezing out as the additional washer doesn't turn with the screw head.