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Diversity and the resilience of circuit protection.

Following a report from a well respected Contributor here, I've been tasked with sorting out a few problems in a house.

One of them is a good lesson in diversity for cooking appliances.

A 45 amp circuit breaker, 6mm T+E cable, running in some patches of insulation, so a nominal 34 amps current carrying capacity. Probably a little more, as the insulation is not continuous, and is patchy in the void the cable runs through.

This cable supplies 6 appliances. Oven 3.6kW, hob 6.2kW,frying plate 2.4kW, large microwave 2.6kW, Steamer 5.6kW and a warming drawer at 0.7kW. A total nominal rating of 94kW.

With the current diversity guidance in the OSG, this install complies (forgetting about the overloaded cable) with the hob at 10 amps, and the rest at 30% of their rating, giving a total of 32 amps but it just doesn't feel right. To get a little better control, and fault reduction, I had intended to put 2 extra new supplies in, to divide them up, I've since put one supply in, but the second one is getting rather difficult to install without taking apart the kitchen.

The oven (16amp) was on a plugtop, with no signs of overheating, or reports of problems from the householder. A 20 amp DP switch, which supplied the steamer, microwave and drawer did show a little heat damage, though that could well have been a loose connection.

So it goes back to the pragmatic view, how much can a small family cook at the same time, and there we have the diversity. They say there are 2 hob rings they never use, the fryer is rarely used etc. So the typical load is less than 3kW once the oven has warmed up.

Even with the Christmas day dinner cooking, I cannot see much more than 30 amps being pulled for a short period, hence no overheating of the cable, and no tripping of the circuit.

The circuit breakers are now 32 amps.

I heard a whisper that guidance on Diversity is changing, has anyone else heard what may be changing?

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0 Chris Pearson over 3 years ago

John Peckham:
I do know what diversity is and I do know it is not an exact science. The connected load is 21.kW which is 91.7A. The previous occupant was a woman living alone. The new occupant is my friend and could very well host a large diner party cooked by professional caterers hired in for the night. A lot of the connected load may be in use at the same time. So in these circumstances how would you design this installation and circuits? Would you put all that kitchen plant on a single 6/2.5 radial?
John, thank you. I can picture it!
If I understand the OP correctly, there are now 2 x 32 A circuits, which should be fine.
Even if you have a couple of cooks, they are not going to use everything at once. We have not been told the age of the appliances, but presumably not ancient. The oven might go on first, but will probably take no more than 15 minutes to warm up. After that, the thermostat will click on and off; and there will be a second burst when food is put in until that gets up to temperature. Similarly, when boiling vegetables, you get the water up to temperature, add them and wait for the water to boil again before turning down to a simmer. Clearly, you do not put your peas on at the same time as your potatoes. If the vegetables are cooked in the steamer, there will be no need for the hob except perhaps for sauces. Similarly, if meat or fish is fried on the frying plate, the cooks will not be using the hob for the same purpose. And so on.
I am curious to know why the sub-mains are so large. Are the buildings so far apart that voltage drop is an issue? And how do you terminate such large cables in an ordinary CU?
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0 Chris Pearson over 3 years ago

John Peckham:
I do know what diversity is and I do know it is not an exact science. The connected load is 21.kW which is 91.7A. The previous occupant was a woman living alone. The new occupant is my friend and could very well host a large diner party cooked by professional caterers hired in for the night. A lot of the connected load may be in use at the same time. So in these circumstances how would you design this installation and circuits? Would you put all that kitchen plant on a single 6/2.5 radial?
John, thank you. I can picture it!
If I understand the OP correctly, there are now 2 x 32 A circuits, which should be fine.
Even if you have a couple of cooks, they are not going to use everything at once. We have not been told the age of the appliances, but presumably not ancient. The oven might go on first, but will probably take no more than 15 minutes to warm up. After that, the thermostat will click on and off; and there will be a second burst when food is put in until that gets up to temperature. Similarly, when boiling vegetables, you get the water up to temperature, add them and wait for the water to boil again before turning down to a simmer. Clearly, you do not put your peas on at the same time as your potatoes. If the vegetables are cooked in the steamer, there will be no need for the hob except perhaps for sauces. Similarly, if meat or fish is fried on the frying plate, the cooks will not be using the hob for the same purpose. And so on.
I am curious to know why the sub-mains are so large. Are the buildings so far apart that voltage drop is an issue? And how do you terminate such large cables in an ordinary CU?
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Diversity and the resilience of circuit protection.

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