Oven and hob on same MCB and diversity.

[HappyHippyDad]

I've just spent an hour getting bogged down with the maths trying to work out diversity for 2 cooking appliances on the same circuit (32A MCB) and same isolation switch,

I thought I would give my workings so others can use it as I wonder if some just assume you can still use the 15kW maximum, which you can't.

In fact the the maximum for 1 cooking appliance seems to be 19.3kW (as long as the switch doesn't incorporate a socket), however, on with 2 appliances....

Diversity is 10A plus 30% of the remaining amps (plus 5A if a socket) but this is for each appliance even if on the same circuit.

Therefore for an oven and electric hob the maximum current we have is:

[10A + ( 0.3 x Q) ] + [10A + (0.3 x Q) ] = 32A

we don't know the maximum size of the cooker and hob (that's what we're trying to find out), so I have just used the letter Q (Q is the current of either the hob or cooker minus the 10A)

Therefore

The above equation is the same as

2 x (10A + 0.3Q) = 32A

20A + 0.6Q = 32A

0.6Q = 12A

Q = 20A

We know that each appliance is allowed to draw a max current of Q +10A, which is 30A. There are 2 of them so 60A.

60A x 230w = 13.8kW (approx)

Therefore maximum combined power for 2 appliances on same circuit with same switch (without a single socket) is 13.8kW.

I think you'll all have had enough now, but the maths works out as a maximum combined power of 10.4kW allowed if the cooker switch incorporates a single socket. (Equation is 25A + 0.6Q = 32A)

If you haven't guessed. I have the day off work! ?

ps.. would you check the maths please @happysteve

 

[Lucien Nunes]

HHD I know you are always keen to analyse and get to the bottom of what's going on and I wish more people would do that, rather than relying on rules of thumb and guesswork. OK, in this case I wasn't in agreement that your method was an accurate model but it prompted some discussion and comparing of theory and practice and I think that is often lacking. People do stuff on paper to pass exams, and then nail stuff to the wall, but don't make enough of a connection between them. IMO the real trick is not to get too fixated on regs, which are just the particular locally-recognised standards that we generally work to, but rather to look at the physics that makes electrical stuff work. Heating elements obey ohm's law, not BS7671!

E2A: But, next time, please don't use Q for an unknown current. Capital Q is charge, usually in coulombs. Either x (the general unknown) or I (for current, the actual quantity that was unknown in this case) would suit. Since Q=It, using Q for current makes time disappear, which could cause wider issues with the universe.

My thought is it should be treated as one device if they are off one common isolator

I can see what you are getting at, i.e. one cooking station controlled as a unit from a single point, but referring to the isolator specifically seems arbitrary as it has no effect on how the appliances are used.

 

[DPG]

New reg for the 19th edition: 'You should endeavour not to make time disappear'

 

[pc1966]

E2A: But, next time, please don't use Q for an unknown current. Capital Q is charge, usually in coulombs.

Or tuned circuit quality factor!

 

[Lucien Nunes]

Now I understand why a 5-ring hob is better than a 4-ring hob - higher Q! They should get rid of all that resistance in the elements and it would go on for hundreds of cycles.

 

[HappyHippyDad]

HHD I know you are always keen to analyse and get to the bottom of what's going on and I wish more people would do that, rather than relying on rules of thumb and guesswork. OK, in this case I wasn't in agreement that your method was an accurate model but it prompted some discussion and comparing of theory and practice and I think that is often lacking. People do stuff on paper to pass exams, and then nail stuff to the wall, but don't make enough of a connection between them. IMO the real trick is not to get too fixated on regs, which are just the particular locally-recognised standards that we generally work to, but rather to look at the physics that makes electrical stuff work. Heating elements obey ohm's law, not BS7671!

E2A: But, next time, please don't use Q for an unknown current. Capital Q is charge, usually in coulombs. Either x (the general unknown) or I (for current, the actual quantity that was unknown in this case) would suit. Since Q=It, using Q for current makes time disappear, which could cause wider issues with the universe.



I can see what you are getting at, i.e. one cooking station controlled as a unit from a single point, but referring to the isolator specifically seems arbitrary as it has no effect on how the appliances are used.

Haha... I first put an X in the equation, as you rightly say this is the standard letter used for a variable, but it looked like this.... 0.3 x X, I realise It should just be written 0.3X but I wanted (at least initially) to show the multiplication sign. 0.3 x I looked too much like 0.3 x 1, or with the multiplication sign removed 0.3I, now looking like 0.31.

I then used 'Y', which I think I should have stuck with. Q shall never be used again in such a form!?

 

[Lucien Nunes]

But Y is the symbol for admittance, the reciprocal of impedance (Y=1/Z). Many capital letters stand for quantities and should not be used generally.

Its very often possible to distinguish 0.3I from 0.31 automatically by context (0.3I has dimensions of current but 0.31 is dimensionless.) However if an explicit multiplication symbol is needed to avoid ambiguity or for visual clarity, there is a choice of the asterisk * (from ASCII-based programming) or a full stop aka period or centre dot which is the conventional algebraic symbol to use on paper. Conventions that use the centre-dot for decimal fractions theoretically use the period for multiplication and vice versa. I would understand 0.3I, 0⋅3.I, 0⋅3*I etc as equivalent. In vector algebra the two multiplication operators ⋅ and x signify different operations, but again it would be clear from context that the distinction doesn't apply here.

 

[Midwest]

I've not read through thoroughly all the replies, but when I was calculating diversity for two appliances, I would use two calculations. With one free standing, it would be one calculation.

I based my rational on that the later are sometime supplied with a plug, with very simple cooking activity and the former could have multiple zones for a hob and two or three or more zones for the oven part. Much more cooking activity.

That said, I once had a Range style electrical cooker, which had 3 ovens, manufacturer required 40A supply. With all three ovens going at full pelt, never pulled more than 14A.

 

[Lucien Nunes]

Ours only has two ovens but with both of those on, the grill, all four rings and the lights, it uses about half an amp.

 

[pc1966]

Ours only has two ovens but with both of those on, the grill, all four rings and the lights, it uses about half an amp.

Half an amp? Have you got on of those 11kV cookers?

 

[telectrix]

Ours only has two ovens but with both of those on, the grill, all four rings and the lights, it uses about half an amp.

obviously it's gas or youse fiddled the meter.???

 

[Lucien Nunes]

The burners are gas, yes, but for some reason the clock, lights and rotisserie are electric.

 

[Mike Johnson]

I wonder who decided in the EU to use a comma instead of a period between the € and C, i.e in €3,00?

 

[pc1966]

I wonder who decided in the EU to use a comma instead of a period between the € and C, i.e in €3,00?

It is common in France and Germany (at least) to use the 'point' as a thousands separator and the comma as the decimal place, opposite to our convention. Why it turned out that way I have no idea...

 

[Midwest]

It is common in France and Germany (at least) to use the 'point' as a thousands separator and the comma as the decimal place, opposite to our convention. Why it turned out that way I have no idea...

Too many frogs legs and too much sauerkraut methinks ?

 

[HappyHippyDad]

I realise we have come to the end if this thread, however.... as coincidence happens a slightly different yet similar scenario has arisen.

The customer wants 2 x separate single ovens next to each other with an induction hob above.

The previous answers in this thread suggest treating a separate hob and oven as a single appliance with regards diversity, and I can see that makes sense. We can't just keep adding an appliance on and classing it as one unit though, as single units have thermostatic controls. How would you apply diversity in this case? I'm inclined to use my initial maths here and class it as 2 appliances.