UK 16a oven on 20a fused spur from Kitchen ring main

[gm1984]

Hi,

I've just bought a new built-in 3.4kw oven = 14.8A @ 230v. I'm getting 245v so actually 13.8A.

Existing oven plugs into single socket fed from 20A DP switched fused spur.

Ring is small (3 double sockets) with 4 x 20A DP switched fused spur single sockets (oven, washer/dryer, dishwasher, fridge/freezer). B32 MCB protection in consumer unit, all wiring 2.5mmsq clipped.

Was wondering can I just change oven socket to a 20A flex outlet plate or do I need dedicated supply/run to hob cooker plate both of which a pain?

 

[loz2754]

Might be easier to exchange the oven for a lower power one.
Otherwise, the best option is a new circuit, correctly rated for the oven.
A switched fuse connection unit is rated for a maximum of 13 amps.

 

[Lucien Nunes]

3.4kw oven = 14.8A @ 230v. I'm getting 245v so actually 13.8A.

This doesn't make sense. The heating element is resistive so a higher voltage will result in a proportionally higher current. If it is rated at 3400W at 230V its resistance is (230)² / 3400 = 15.6Ω. At 245V it would take 245 / 15.6 = 15.7A and dissipate (245)² / 15.6 = 3860W and the thermostat cycle on-time will be shorter to compensate for the higher heat input.

This was presumably not designed to plug into a UK socket-outlet although might have been designed to be fitted with a European 16A plug. We have seen examples of appliances where the English instructions have overlooked the fact that the UK doesn't use 16A socket-outlets domestically.

I have no idea what a 20A fused spur unit looks like since the maximum fuse rating for a BS1363 accessory is 13A. Please clue us in! Does this thing actually have a fuse? Not that it needs one as it's feeding a socket-outlet.

It is permissible, I believe, to connect a load to a ring using an MCB up to 16A although not a fuse. Perhaps someone with the regs to hand could quote? There are two other potentially heavy loads in use on this ring and I am not suggesting that this is a good idea, plus it runs counter to the recommendation that heavy fixed loads have their own circuits or at least circuits in which their full load is accounted for without competing with other appliances. But as far as the actual method of connection goes, a 16A SPSN MCB and flex outlet plate might be technically compliant. Comments? (excluding 'it should be on its own circuit' as I've already said that.)

 

[gm1984]

Thanks for replies. P=IV, V=IR limit of my calculations as DIYer.

I realized after posting I was making pig's ear of it. I have changed socket to 20A flex outlet plate and changed DP 20A switch (not fused ) to 13A fused spur as temporary measure to get oven working and cable protected. No blown fuses so far (not tried pyrolitic cleaning or rapid heat).

I'm going to run to 32A cooker switch/hob outlet plate with 4mmsq this weekend before I put up with temporary solution.

DIversity calculation hob 3.4kw and oven 7.2kw plus socket = 26A. Please correct me if I've made a James of it.

Having read other threads (here and IET) general consensus is this is ok and there is no need for separate 16A MCB for oven, a few dissenting voices though.

 

[littlespark]

Having read other threads (here and IET) general consensus is this is ok and there is no need for separate 16A MCB for oven, a few dissenting voices though.

Dissenting voice here as well.

Anything over 2kW should be on its own circuit.


Would you be in possession of a clamp ammeter by any chance?
That would give you a rough idea of the true current when the oven is on.

 

[James]

DIversity calculation hob 3.4kw and oven 7.2kw plus socket = 26A. Please correct me if I've made a James of it.

should I be offended?

 

[timhoward]

P=IV, V=IR limit of my calculations as DIYer.

Not a bad start at all. You know more than you think you do! For resistive loads like heating elements you just need to take the two you know and lob them together:
P=IV and I=V/R
P=(V/R)V
P=V squared / R
Rearrange to get R= V squared / P

 

[gm1984]

Dissenting voice here as well.

Anything over 2kW should be on its own circuit.


Would you be in possession of a clamp ammeter by any chance?
That would give you a rough idea of the true current when the oven is on.

So I cannot put 16a oven on same 32a cooker circuit as hob? Both 4mmsq to dual outlet plate and existing 6mmsq to cooker switch and 32a MCB in consumer unit.

Is the problem that although cable protected the oven isn't?

 

[Lucien Nunes]

You would need to check the manufacturers' instructions to confirm the maximum permissible rating of fuse or MCB for the oven. Some do indeed stipulate a maximum of 20A in which case tacking it onto a 32A circuit without fusing down won't do, as a fault could cause an internal wiring burnout.

 

[gm1984]

You would need to check the manufacturers' instructions to confirm the maximum permissible rating of fuse or MCB for the oven. Some do indeed stipulate a maximum of 20A in which case tacking it onto a 32A circuit without fusing down won't do, as a fault could cause an internal wiring burnout.

Thanks, I've triple checked and only thing in detailed user manual is "means of disconnection must be incorporated in fixed wiring." No mention of protection, I've got switch on cooker plate and on MCB as disconnection points.

Only thing on installation diagram is 220-240v 50/60hz.

I'm going to crack on with dual cooker outlet, upgrade flex to 4mm, restore socket and dp switch for my next 13A oven .

Last one before I go, new flex for oven will need to run behind/under a floor cabinet. Does this need to be in trunking?