Diversity and the resilience of circuit protection.

It does not do that Broadgage, it has intermittent short bits see OP. As it is not damaged so far, why do you think it might become so? If it got to say 100C due to some “accident” with some of the load, what changes except the life of the cable? The point is, and I know you don't like it, the whole electrical distribution system runs like this with very little trouble. At 800A, a 600A fuse, and a piece of 0.3 sq inch cable, the cable may get quite a lot hotter than the rated temperature, and this happens under the streets every day. The 70C limiting temperature in BS7671 is based on many factors, some of them being potential cable life, the other being continuous ratings 24/7. I suggest a quick test for you to try. Get a bit of 1.5 T&E cable, put it in a 32A fuseway and put 6kW of load on the end. Set a timer. How long does it take until the cable is at 70C, and how long until you get a bad burn? I suggest you do not allow it to melt but that is at about 150C. At 130C the plasticiser may smoke a bit, and the cable becomes significantly stiffer, but nothing too horrible. This experiment is not recommended for the inexperienced as it is obviously somewhat dangerous. Beware not to touch the cable!

The point of my comments is simple: either diversity is allowed or it is not (just in case). As long as what happens is fully understood there is no problem. The trouble that I see is that many do not understand what actually happens, and as I have said many times before, the time factor it takes to heat a cable. The dissipation per metre of domestic cables is quite low, a few watts at rated current and there is quite a thermal mass, particularly from the plastic. Strangely copper has an untypically low specific heat. As is often said of many things, time is everything.

Do I understand that you think that diversity should not be permitted, Alan? See the paragraphs above.

davezawadi (David Stone): 
The commercial microwave (or perhaps microwave / conventional) oven is very unlikely to take more than 5A ON AVERAGE …

My combination oven can use a full 3 kW, but preferably according to the instructions, not for more than 5 min. Much more than that and the food is likely to be ruined anyway - it will do a one portion baked potato in 8 minutes. I suppose that even in the busiest pub kitchen, it is unlikely to be on more than off, so yes, 5 A seems about right.

I suppose that if you live on ready meals, the microwave might draw more for longer, but then you wouldn't be using all those other appliances.

Proper appreciation of diversity in a kitchen requires a proper understanding of cookery!

Well i looked at the OP`s 94KW and thought “what?”.

I added it (assuming I understood correctly) and got 21.1KW.

I think the age old calculation for cookers holds pretty good in a normal domestic setting and I have no problem with it.

But surely it was designed for up to say 13KW or maybe 15KW ish sort of max load.

A traditional cooker had an oven, a grill, a hob of say 4 rings plus occasional use of a kettle perhaps.

If, in a modern kitchen, this is all taken care of by separate appliances then I see no need to adjust that calculation.

Where the OP states 6 appliances and the enhanced chances of using all together for some time then I think it might be prudent to tweak the calc a little bit.

In fact the sensible spread of arranging to two circuits rather than one might be appropriate.

Anyway back to the calc.

21.1 KW is 87.916A if we take 240 as the voltage considered might have been used as the manufacturer`s advert rather than the so called 230V nominal.

Take 10A leaves 77.916A .

30% of 77.916 A is 23.375A .

Add back on the 10A and we gets 33.375A.

If you want to multiply that by 240/230 then you might feel that 34.826A be more appropriate.

Either way you get to the very high end (or exceed it) of the well tried standard equation and you might decide a better “fix” might be the first 12A and 35% of the remainder be given a thought or some such tweaking.

There are some on here who will have graphs/charts/research to hand who will probably come up with a better solution (or indeed might say the old formula still holds good).

I take DaveZ`s comment about some Electricians not understanding diversity. but add caution, two reasons for it 1/ the cct in question and 2/ modern trends such as higher power electric showers, car chargers to name but two when we are loading a total installation loading might be in order.

It`s the age old chestnut of One house 100 sockets on one 32A circuit is often OK.

If we make it spread around to 2 or 3 circuits then the total load will not increase but however the max permissible load might well do. For one family living in a house this is never a problem but if , for a time, you temporarily have two or three families living in a house (example over a christmas break perhaps) then you might need to consider things a bit more.

I got my tin hat on now , waiting to be shot down ?

ebee: 
 

Well i looked at the OP`s 94KW and thought “what?”.

I added it (assuming I understood correctly) and got 21.1KW.

I think the age old calculation for cookers holds pretty good in a normal domestic setting and I have no problem with it.

But surely it was designed for up to say 13KW or maybe 15KW ish sort of max load.

A traditional cooker had an oven, a grill, a hob of say 4 rings plus occasional use of a kettle perhaps.

If, in a modern kitchen, this is all taken care of by separate appliances then I see no need to adjust that calculation.

Where the OP states 6 appliances and the enhanced chances of using all together for some time then I think it might be prudent to tweak the calc a little bit.

In fact the sensible spread of arranging to two circuits rather than one might be appropriate.

Anyway back to the calc.

21.1 KW is 87.916A if we take 240 as the voltage considered might have been used as the manufacturer`s advert rather than the so called 230V nominal.

Take 10A leaves 77.916A .

30% of 77.916 A is 23.375A .

Add back on the 10A and we gets 33.375A.

If you want to multiply that by 240/230 then you might feel that 34.826A be more appropriate.

Either way you get to the very high end (or exceed it) of the well tried standard equation and you might decide a better “fix” might be the first 12A and 35% of the remainder be given a thought or some such tweaking.

There are some on here who will have graphs/charts/research to hand who will probably come up with a better solution (or indeed might say the old formula still holds good).

I take DaveZ`s comment about some Electricians not understanding diversity. but add caution, two reasons for it 1/ the cct in question and 2/ modern trends such as higher power electric showers, car chargers to name but two when we are loading a total installation loading might be in order.

It`s the age old chestnut of One house 100 sockets on one 32A circuit is often OK.

If we make it spread around to 2 or 3 circuits then the total load will not increase but however the max permissible load might well do. For one family living in a house this is never a problem but if , for a time, you temporarily have two or three families living in a house (example over a christmas break perhaps) then you might need to consider things a bit more.

I got my tin hat on now , waiting to be shot down ?

Many multi storied houses in multiple occupation originally designed for just one family can nowadays be occupied by many families and also single people, living full time in every room. A good chance of overloading there.

I saw the above in Milton Keynes where three story houses were let out to many tenants all sharing the facilities. No real maintenance or upgrading was carried out on the original electrics to cover the greater use and loading.

Z.

Of course some diversity is allowable with the multiple cooking appliances as described. I still feel that a single 32 amp circuit is inadequate for the listed appliances.

The steamer and the frying plate alone come to slightly more than 32 amps, and those appliances could draw full load for some minutes whilst reaching operating temperature. Those two appliances would probably be acceptable on a 32 amp circuit, but with all the other appliances also, I remain doubtful.

I also can not agree that a 6mm cable in thermal insulation is acceptable on a 45 amp MCB, even with the insulation being patchy and not continuous.

The absence of any thermal damage to an existing cable does not in my view justify continued use on an oversized MCB.

broadgage: 
The absence of any thermal damage to an existing cable does not in my view justify continued use on an oversized MCB.

Moreover, how can you inspect a cable which is buried in insulation. ?

OK, we have some ideas. Let's try something else. I am designing a factory electrical installation, the added up nameplate consumption of all the metal cutting machines is 500 kVA. What size supply, distribution, etc. should I choose? There are machining centres, lathes, milling machines and similar all over the place. It may reasonably be expected that they will all be used at once, some automatic ones 24 hours a day. In some ways, this is quite comparable with the kitchen problem. I decide on a main board and a number of submains around the floor each fed from an MCCB, each of the sub-boards will feed a local group of machines. Nothing unusual there. Should I allow any diversity, and how much? The question is to consider the real load, and why it is very unlikely to ever be 500 kVA. Something to keep you awake tonight!

Some might like to compare a domestic and commercial kitchen in the same way. In reality, how are they different?

You look Chris, but most in insulation are in difficult places such as behind plasterboard. You consider the installation, for example, the loads on cables in the roof space may only be LED lighting, although someone may have sneaked in a shower cable. Do you need to inspect every inch of it for “damage” or burning etc. (Some will be sure that it must be black and charred but you know that it is not)? How?

Has anyone tried my experiment homework yet?

davezawadi (David Stone): 
 

OK, we have some ideas. Let's try something else. I am designing a factory electrical installation, the added up nameplate consumption of all the metal cutting machines is 500 kVA. What size supply, distribution, etc. should I choose? There are machining centres, lathes, milling machines and similar all over the place. It may reasonably be expected that they will all be used at once, some automatic ones 24 hours a day. In some ways, this is quite comparable with the kitchen problem. I decide on a main board and a number of submains around the floor each fed from an MCCB, each of the sub-boards will feed a local group of machines. Nothing unusual there. Should I allow any diversity, and how much? The question is to consider the real load, and why it is very unlikely to ever be 500 kVA. Something to keep you awake tonight!

Some might like to compare a domestic and commercial kitchen in the same way. In reality, how are they different?

Well, some machines will be out of use due to breakdowns. Some will be out of action due to being set up or having new material made available for machining. Some will be on a light duty  cycle where not all functions are in use. Some will be off at a change of shift of operators. Some will be out of service whilst being adjusted etc. Some will be out of use whilst being reprogrammed etc.

Oh, and if one machine is doing some heavy metal removing,  another might just be automatically positioning its work for the heavy process so is not using maximum current at that time.

Z.