Diversity Calculation - oven/hob - help appreciated

[JohnW2]

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.

Yep. 8.05kW to be precise

Below that - under the usual diversity value of 17A - it will underestimate ....

Yep.

.... but I suspect his caution will mean he would not be putting 4.8kW ovens on a 10A OPD ....

Well, he doesn't allow any diversity for an oven, so he would probably regard even a 20A OPD as 'marginal'.

... (nor we on a 13A)

That, too, would be pretty marginal!

Kind Regards, John

 

[ban-all-sheds]

If the 4.8kW rating was at 240V then diversity would get you 13A. "Marginal" is irrelevant. 13A is 13A is 13A.

 

[JohnW2]

If the 4.8kW rating was at 240V then diversity would get you 13A. "Marginal" is irrelevant. 13A is 13A is 13A.

As you know, I know that - but EFLI did not say what voltage related to the 4.8kW figure he mentioned.

Although 13A is 13A is 13A, one problem with a single-element 4.8kW oven (if such an animal exists) would be that, at any point in time, it would be drawing either 0A or ~20A. Whilst that probably doesn't matter much with a fuse, were we talking about a hypothetical 13A MCB, I suspect that the MCB manufacturer would say that such a situation was not acceptable.

Kind Regards, John

 

[ban-all-sheds]

That is a good point, but I wasn't assuming a single-element 4.8kW oven.

The X + (X-10) x 0.3 calculation which is commonly used is, AFAIK, "guidance", so maybe there should be recognition of the largest individual component vs device rating, so that a single-element 4.8kW oven is not treated the same as a 2.4 + 2.4 double oven?

 

[JohnW2]

That is a good point, but I wasn't assuming a single-element 4.8kW oven. The X + (X-10) x 0.3 calculation which is commonly used is, AFAIK, "guidance", so maybe there should be recognition of the largest individual component vs device rating, so that a single-element 4.8kW oven is not treated the same as a 2.4 + 2.4 double oven?

Yes, maybe. As things stand, given that this is only "guidance", one is free to exercise one's own judgement in cases like that.

Going back to what I said, I do rather wonder what MCB manufacturers feel about diversity. As we know, by application of the standard diversity calculation, we could put 19kW of cooking loads (~80A) on a 32A MCB. However, it would be inevitable that the current would quite often, albeit usually briefly, be (sometimes considerably) in excess of 32A, with the theoretical possibility that it could sometimes approach the ~80A 'full load' for short periods of time (e.g. if someone turned absolutely everything on, from cold, simultaneously).

Even though the average current over any reasonable time period would be below 32A, I'm not at all sure that the MCB manufacturers, if asked, would say that they found the situation to be 'acceptable' (and I'm far from sure that they test their devices under such conditions).

Kind Regards, John

 

[ban-all-sheds]

[RINGS THE BELL]And then there's the issue of how near to In a device should be loaded when it is adjacent in a CU to a device also loaded to its In.[/AND RUNS AWAY]

 

[JohnW2]

[RINGS THE BELL]And then there's the issue of how near to In a device should be loaded when it is adjacent in a CU to a device also loaded to its In.[/AND RUNS AWAY]

Indeed, and the down-rating advised/recommended by manufacturers in that situation can be quite substantial.

However, the saving grace there may be that they are probably talking about sustained or 'average' loads, so that the transient highs that result when In has been selected on the basis of diversity may not invoke their advice/recommendation for down-rating.

Kind Regards, John