Diversity Calculation - oven/hob - help appreciated

[jonwestuk]

I am in the process of buying a new Induction Hob for my kitchen (replacing an electric ceramic hob) but before I finalise my decision on which hob to buy, I want to fully understand the type of change that will be required to the wiring of my house and therefore the likely cost from my electrician. I would therefore very much appreciate some help calculating the diversity of my oven and potential new hob, to determine whether I will need an additional 6mm radial circuit or not.

The situation is as follows:
- Current wire is a 6mm Twin and Earth connected to a 32amp RCBO. There is no insulation between the fuse box and kitchen.
- Oven is a single oven rated at 3.48kw
- Preferred hob is 7.4kw

Total Load without diversity = 10.9kw (10900w/240v = 45.4 amp)

So I have read that the diversity factor for cooking appliances is: 100% of total demand up to 10A, + 30% of the remainder. I guess this is per appliance so:

Oven = 3480w/240v = 14.5 amp
Oven With Diversity = 30% x (14.5amp - 10amp) = 1.35amp

Hob = 7400w/240v = 30.83 amp
Hob With Diversity = 30% x (30.83amp - 10amp) = 6.25amp

Total Load with Diversity = 10amp + 1.35 amp + 10amp + 6.25amp = 27.6 amp
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This would mean theoretically so long as I don't use the Ovens Pyro mode at the same time as all 4 hobs on 'super quick boil the water' mode simultaneously, a single 6mm with a 32amp RCBO should cover my requirements.

That said, ideally 2 x 6mm radials would be preferred and prevent the unlikely situation where the draw is greater than 32 amp for more than 10 seconds or so, but my understanding is that this is more usual in an industrial kitchen, where loads are more constant, mine is not. Even on xmas day, the reality is that the oven is maintaining heat at 180 so working only intermittently and the hob wont be on max for the same reason.

Does my logic work, is there anything else that my electrician is likely to raise as a reason for installing a further 6mm radial?

Next step will be to call him and check he agrees with the logic and so is happy to conduct the work on this basis!

 

[EFLImpudence]

You count the hob and oven as one:

10900/240 = 45.42A

30% of 35.42 = 10.62

10.62+10 = 20.62A (+ another 5A if there is a socket on the cooker switch.)

One 32A circuit will be fine. In fact up to 19kW without the socket, 15kW with.

 

[jonwestuk]

Splendid thanks. I will speak to my sparky tomorrow.

 

[flameport]

ideally 2 x 6mm radials would be preferred and prevent the unlikely situation where the draw is greater than 32 amp for more than 10 seconds or so,

One circuit is entirely suitable.
Totally wasteful and unnecessary to have two.

If you put 50A through a 32A circuit breaker, it would take about 15 minutes before it tripped, and that assumes the current of 50A was applied continuously for the whole time, something that will never happen with hobs/ovens/cookers.

The stories about people tripping things on Christmas Day with ovens and hobs blazing are exactly that - just fictional stories.

 

[Iggifer]

Other than a job I did recently where a well known electrical retailer beginning with C had fitted a plug in induction hob that they’d swapped for a gas hob. Every time the woman used more than one ring the lighting MCB would trip.

Turns out because it was just an ignitor before, the original spark had used the lighting circuit and the hob installer hadn’t even checked what circuit it was on

 

[ban-all-sheds]

Splendid thanks. I will speak to my sparky tomorrow.

Might be better to speak to a sparky who knows about diversity with cooking appliances.

 

[John D v2.0]

Might be better to speak to a sparky who knows about diversity with cooking appliances.

Are you saying eflimpedence doesn't?

 

[ban-all-sheds]

No - I'm saying that if the OP has had to come here to ask about things he feels unable to ask his electrician about or ask him to do, then it might be better if he used an electrician who did know about these things.

 

[Steven P]

As a general rule of thumb, if you have an electrician to do the work then let them do thw work. They do this every day and will know off the top of their head exactly what you need. In the grand scheme of things if you need to go up a cable size the cost isn't that great - but they will just do that for you.

As for diversity, it is a black art at times (not just house electrics, industrial as well). If it was me, I woud consider what you are doing. Oven is either all on or all off - so take that as its fulload current, 14.5A - there is no diversity when there is only 1 item \(assuming a single oven). Since you have the ratings for each device seperatlyyou can consider them seperatly.

For the hob then you can apply a diversity factor, I woud say 2 rings on full at anyone time because that is a likely situation, put the potatoes on in one pan, put the carrots on in another, put the chicken in the oven to cook Happens in my hoiyse every Sunday. So 15A.

Add these together, 29.5A. Thats within the 32A supply and is quite normal.

 

[JohnW2]

Oven is either all on or all off - so take that as its fulload current, 14.5A - there is no diversity when there is only 1 item \(assuming a single oven).

There is diversity over time as well as 'space'/items. Whilst an oven element (like a hob element) will only be on or off at a particular point in time, it will switch between being on and off under thermostatic control, and the average load over any significant period of time will be far less than the full 'on' load.

Since you have the ratings for each device seperatly you can consider them seperatly.

Considering them separately largely defeats the concept of diversity. The whole point about having several things independently cycling on/off under thermostatic control is that this means that, when considered together, they will usually not all be 'on' simultaneously - so that, again, the average total load over any significant period of time will be relatively low. The more such items one considers together, the more does does the concept of diversity work 'as it says on the tin'.

Kind Regards, John

 

[EFLImpudence]

As a general rule of thumb, if you have an electrician to do the work then let them do thw work. They do this every day and will know off the top of their head exactly what you need.

Unfortunately that is not always the case.

In the grand scheme of things if you need to go up a cable size the cost isn't that great

It already is; 4mm² at method C is adequate for 32A.

As for diversity, it is a black art at times (not just house electrics, industrial as well)

Not really.

By a different method you have arrived at a figure just 1.9A greater than the accepted method.

Do you therefore not think that the accepted method, used for over half a century, is satisfactory and you may as well do it that way?

 

[JohnW2]

By a different method you have arrived at a figure just 1.9A greater than the accepted method.

Indeed he has, but if he really believes in the principle he is applying (that each oven/hob ring has to be treated separately, and assumed to be 'on' for 100% of the time, then he is really being dangerous to his own beliefs by assuming than no more than two hob rings will ever be used simultaneously.

He has arrived at roughly the traditionally-calculated after-diversity current by 'ignoring' two of the hob rings, but that is more 'lucky' than anything else.

Kind Regards, John

 

[EFLImpudence]

Well, it's not right but that depends what is done with other combinations of appliances.

Assuming (?) four hobs, then by using what is, in effect, "50%" instead of "10A + 30% of the remainder", his method will over estimate the figure for all appliances over 8kW - plus no allowance on the oven.
Below that - under the usual diversity value of 17A - it will underestimate but I suspect his caution will mean he would not be putting 4.8kW ovens on a 10A OPD (nor we on a 13A).

 

[ban-all-sheds]

If it was me, I woud consider what you are doing. Oven is either all on or all off - so take that as its fulload current, 14.5A - there is no diversity when there is only 1 item \(assuming a single oven). Since you have the ratings for each device seperatlyyou can consider them seperatly.

Please explain why you would consider the load of a separate hob and oven to be different than if they were fixed together in the same housing as each other and called a "cooker".

Also, consider, say, a range cooker with two ovens, a grill, and a 5 "burner" hob, all in one package with one connection to the supply. How many cooking appliances is that?

Then consider two separate ovens, a separate grill, and 3 Domino hobs (2, 2 & 1 "burners"). How many cooking appliances is that?

 

[ban-all-sheds]

Unfortunately that is not always the case.

In that case they are not really electricians, are they.