Ovens and 30A fuse/feed

In a word diversity - circuits are usually designed for the current they'll usually have to carry for a reasonable length of time, rather than any instantaneous theoretical maximum.

Cooking appliance circuits are traditionally rated at 100% of the first 10A, 30% of the remainder, plus 5A if the cooker control unit included a socket outlet.

So with the old 5.1kW oven - 22A say - the circuit need only be rated for 13.6A  - so there's a fair bit spare for the kettle and microwave.

Even with the new 6.2kW oven (27A say) you'd be looking at needing just over a 15A circuit as far as the oven is concerned.

You could argue that a simple oven has less diversity than a conventional full cooker, but even so there's probably a decent margin to play with.

   - Andy.

Well in reality a 30 or 32 A breaker does not break at 30 or 32A - that is the current it can hold all day without distress. It may take 1.5 times this for half an hour or more, and twice this for long enough to boil a kettle and make toast.

This is OK, as very similar time constants apply to cables heating up - they do not instantly melt, rather they get rather overheated over many minutes.

If you have the chance to uprate the circuit - i.e. fatter cable and larger breaker, or splitting the sockets onto another feed, that would be sensible if you have the kitchen in bits already, especially as you are increasing the load, but it will probably not actually give any trouble as it is.

Mike.

@AJJewsbury, thank you for your quick reply. After some googling and youtube videos I understand electrical diversity a little more.

Am I correct in thinking a kettle wouldn't have a diversity factor applied? From my understanding - the current is on from switch on to boiling point.

What about microwaves? Is there a diversity factor? I'm guessing they would pulse the waves - just like an oven thermostat? 

Thank you,

Paul

@mapj1, thank you as well. The kitchen isn't in bits at the moment, just looking to change the oven. I would have thought ovens would have become more efficient / use less energy compared to 20 years ago, but most use more.

I won't need to worry when the oven is delivered and the installers fit it for £90. I think built in double ovens at roughly the same size gave or take a few millimetres. 

Thanks again,

Paul

robopj

Diversity is not only about current, it is also about the time of operation of a load. This factor is complex and rarely taught to electricians, but is an important consideration for design.

A kettle boils in a few minutes, much too quickly to show much temperature rise in a cable. The same with a microwave, we usually use it for short periods on full power, but the same applies. Fuses and MCBs also have a “time - load to trip” characteristic, and all these together make domestic diversity very safe, trips do not normally occur however much you use the kitchen appliances together. The overall average consumption of the kitchen is much less than the total rating of all the appliances added up, and it is this average that best describes the heating effect of cables or the tripping of MCBs or blowing fuses. The power dissipation in a metre of cable is quite small, a few Watts maximum, and the thermal mas significant, so it takes a long time to heat, maybe an hour or more at maximum rated current.

As you do not trip your MCB the installation is perfectly satisfactory, and it is very unlikely the cables get overheated. I am more concerned with this £90 installation charge, what exactly do they undertake to do? I suspect that they just connect the oven to the nearest available power and leave, which may be somewhat unsatisfactory.

You also say you expect that modern appliances will take less power, not more. The consumption is fixed by simple physics, for example, boiling a kettle with less power probably takes more energy than doing the same job quickly, or at best the same amount, because heat loss is more. An oven has always been fairly well insulated, but now the consumer wants a glass door, that has very little insulating value, even if there are two layers because the space in between is well ventilated to keep the outside surface cool to the touch.

Very little is quite as simple as it seems!

David CEng etc.

robopj: 
Am I correct in thinking a kettle wouldn't have a diversity factor applied? From my understanding - the current is on from switch on to boiling point.

What about microwaves? Is there a diversity factor? I'm guessing they would pulse the waves - just like an oven thermostat?

Yes, a kettle will go flat out until the water boils and then it will turn itself off.

A microwave on anything less than full heat does pulse - you can often hear a relay clicking on and off.

FWIW, I don't think that the installation charge is excessive because the oven has to be delivered; it has to be unpacked; I would expect that the old one will slide out and the new one will fit without too much bother; there will be two operatives to carry it on ‘elf ’n' safety grounds; and most importantly, they take away the old one and may well have to pay to get rid of it - hopefully by recycling and not straight into land fill.

Am I correct in thinking a kettle wouldn't have a diversity factor applied?

No it still could. The amount of diversity would vary with the situation, but the main thing about a kettle is that it's only on for a relatively short length of time - a couple of minutes rather hours - so the cables don't really get the time to warm up (as David mentioned). The heating effect is smaller with larger cables - so the 5A allowance for a socket that (traditionally) fed a 3kW kettle on say a 30A circuit is entirely reasonable.  Also as you consider more “things” there's generally less chance of things being on at the same time, which also increases the amount of diversity you can apply.

It's a similar situation with motors - we usually size their circuits for their normal running current - yet they can take several times that current during starting (which can last several seconds) - with no ill effects.

As an extreme example, DNOs consider a typical house to have a maximum load of about 2kW - when averaged across many houses.

   - Andy.

@davezawadi, @ChrisPearson,

Thanks for your replies. Excellent points. 

The oven has free delivery. 

The installation charge covers the below. I think it is a generic serivce / charge. 

Disconnection of your old appliance (regardless of fuel type)
Connection of the new appliance to existing electricity supply within 1.5m of the installation
A basic demonstration of your new appliance
Removal of all packaging

In my case, it should be relatively simple because a double oven is already in situ: turn off power;remove two screws; slide forward; disconnect cable; remove old oven; position new oven; reconnect cable;push back; fix with two screws;turn on power. I'm not qualified to connect a 32A device.

There's also a £20 recycle charge for the old oven.  We’ll remove your old appliance & packaging. The materials will be reused or recycled to create less waste & impact on the environment

I could investigate getting an electical contractor to do the same install work but I guess it may be the same amount. Co-ordinating visits etc takes time. Attempts to find other tradesmen are proving difficult at the moment. If there is an issue with the new oven, it would involve another visit by the supplier (at their cost), and then another visit by the electrician, probably at my cost which I may need to pass onto the supplier.

robopj: 
 

@AJJewsbury, thank you for your quick reply. After some googling and youtube videos I understand electrical diversity a little more.

Am I correct in thinking a kettle wouldn't have a diversity factor applied? From my understanding - the current is on from switch on to boiling point.

What about microwaves? Is there a diversity factor? I'm guessing they would pulse the waves - just like an oven thermostat? 

Thank you,

Paul

In my view, no formal “diversity factor” should be applied to individual appliances that are plugged into socket outlets. Future users may select different appliances, or have different patterns of use.

However long experience shows that a single 32 amp circuit suffices for any normal use of the portable appliances used in a normal domestic kitchen.

Washing machine, fridge, kettle, toaster, microwave oven, clothes iron, and probably a few others will in total use a lot more than 32 amps, but in practice will be fine in normal household use.

Additional circuit capacity might be required for EXACTLY THE SAME appliances if subject to exceptionally heavy use in a very large household, such as say a shared student house.

If the wall switch has been replaced with double socket and you cannot isolate the oven locally in the kitchen the installer may refuse to do the job and may walk out without doing the job.