I want to get some views on this please...

I came across a cooker circuit the other day with a max load of 13.5 kW (Oven and Induction Hob fed via single isolator) on a 6mm T+E fed via a Type 1 40A MCB...therefore max load could conceivably be nearly 59A...I do not have the time/current curve characteristics for a Type 1 MCB, but using the Type B curve as an example, a 40A Type B MCB could sustain 58-59 amps for 10,000 (or indefinately)...The oven/hob will not obviously draw full load continuously, though it could be greater than the MCB rating for a length of time which could be thermally detremental to the 6mm cable... Is my logic right here?

Additionally, I was able to discuss this with the designer/installer (one and the same) of the circuit, and he said that he used diversity to justify the 6mm cable, and if my logic above is right, I cannot help thinking that he should have either split the two cooking devices across two individual 6mm circuits, or he should have uprated the cable, on the basis that his design should be based upon full load current rather than using diversity to justify a smaller cable.

Yooj

Is the cooker not such that the oven and grill could not be in simultaneous use? whilst the sum of the elements may add up to the 13.5kW what would be the actual maximum simultaneous load?

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Regards

Martyn.

Only a mediocre person is always at their best



www.electrical contractors uk.com

Hi Martyn,

Oven has a separate Oven and Grill therefore conceivably they could both be on at the same time. There is also an Induction Hob, which could have all the elements on as well...The Oven is 6kW the Hob is 7.5kW, both of which are on the same circuit.

Yooj

I think all you need to do here is drop the mcb rating down to 32A.
The cable should be fine.

Thanks Mike,

As a retro-fit, a 32A would be a good solution...What about at the design phase...do you think using Diversity to justify smaller cabling is justifiable in this instance?

Yooj

Did he" use diversity to justify a smaller cable" or perhaps
the installer could argue that he used diversity to assess the required cable size rather than to justify that smaller cable

What is the point of using diversity if we then start being concerned about a possible full load current
Isn't diversity an attempt to estimate what would be that full load

If by that you mean keeping the 6.0mm cable rather than installing a larger size, yes.

Using diversity to determine a design load is the whole point of the exercise - it's just a finacial tool to produce an economic installation.

You also need to consider the time constants involved in cable heating - a cable carrying full load may take up to an hour to reach working temperature (ie 70C for PVC insulated).

Is it likley that the oven/hob combo could sustain full load over this period - ie liberating 13.5kW of heat energy into a space for an hour.

Generally the answer would be no - so you have two concepts, the load is not usually full load and that load is not usually constant - this gives us the idea of a thermally equivalent load - ie the load if sustained constantly that would give the same heating effect for a conductor as the actual load profile - this thermally equivalent load will usually be much lower.

As an example think of say 4 x 2.5 kW hand driers connected to a 32A ring final circuit - a total connected load of say 40A - is the circuit at risk - in most cases not, but you would to really understand how the system operates in terms of the number of handriers in use at any time and the period for which they are in use - ie you need to know the probability if you were going to adopt this as a design. It may well be OK in a small office building but might be risky in a motorway services

Regards

OMS

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Failure is always an option

Thanks chaps for all the input, much appreciated...I have suitably changed my viewpoint on this.

Yooj