Electrical experts - please can you check my calculations?

[Penners]

Our new electric range cooker arrives next Wednesday, so I've been trying to check that the existing cabling and fusing is sufficient to support the load. Anyone got any comments on the following, please (e.g Zebra, FF, etc.)? :-

Total power consumption of all the cooker's heating elements: 13.8kW
I believe the diversity factor for cookers suggests a load calculation of 10A plus 30% of the remaining total load.
I calculate the total load as 13800/240 = 57.5A
57.5 - 10 = 47.5
47.5/3 = 15.8 + 10 = 25.8A nominal load

Based on that I'm assuming that a 40A minibreaker in the consumer unit and 6mm^2 supply cable will be a proper spec. I'd be very grateful for any comments to agree or disagree with that, please!

 

[Flyfisher]

As I recall from discussions with our electrician who wired our new kitchen, with electric cookers there are diversity and, with modern units, load sharing things to consider.

This IET thread might be of interest: http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=60160 and suggests that a 32A supply over 6mm2 cable is adequate for a 14.9kW cooker.

Looking at it from another angle, if you have a modern CU with MCBs then the 32A MCB will adequately protect the existing 6mm cabling and the worst that could happen is nuisance tripping if it turns out that the cooker really does demand more power - and diversity guidelines suggest it shouldn't.

I'd say your calculations and assumptions are correct but there's no need to step up to a 40A MCB as 32A should be adequate (and more standard).

 

[Zebra]

The first part of your calculations are largely correct. However, nominally the voltage is now 230v (I know they haven't actually changed the voltage. Nonetheless........)
Therefore,
13800 / 230 = 60A
You take the first 10A, add 30% of the remainder, as you said:
10 + (0.3 x 50)
= 10 + 15
= 25.
Very similar to what you came up with.
Yes, a 40A MCB would do it. Even a 32A MCB would do it.

One cannot use 6mm on a 40A MCB as readily as one could before. Although technically it can carry it under certain conditions, the tendency for houses to be better insulated has led to a restriction on the loadings for cables in certain conditions. Many electricians would now install 40A MCBs on 10mm cable by default. However, it may be permissible, depending on other factors, if we look carefully. Are you installing a new circuit, or is this an existing circuit which you are trying to check is OK? What is the length of the run from the Consumer Unit, and is it buried in the wall, under the floor, or clipped to the wall? (Unlikely to be the latter, I assume.) Do you have RCD protection on the circuit? Those are all factors which would affect the calculation. Many electricians would simply say "You can't put 6mm on a 40A any more" and be done with it, of course, but we may be able to prove them wrong.

If you were using a 32A MCB, it would be permitted, and I agree with Flyfisher that it would seem more acceptable. Therefore that may be the best course of action.

 

[Penners]

Many thanks FF and Zebra for those erudite answers.

To answer your questions, Zebra:-

The cooker circuit is existing - it was installed about 10 years ago (by a professional) before we bought the house. It currently ( :lol: ) has a 32A MCB and 6mm^2 cabling. Following the most logical route for the cabling, the cooker is about 12 metres from the consumer unit, but I have no definite idea where the cable actually runs - in the walls or under the floor (it's not surface clipped). The consumer unit (fairly modern, Crabtree) is fitted with a master RCD, and each circuit has its MCB, but there is no individual RCD on the cooker circuit.

As for "the tendency for houses to be better insulated" - that's not an issue! Our Georgian cottage has solid 9-inch brick walls, so other than a good layer of insulation in the loft (where I'm certain the cooker supply does not run) it's poorly insulated (just ask Mrs P!).

 

[Zebra]

Of course, not all houses are well insulated, and the regulations have simply got more complicated to account for when there is and when there isn't insulation in the walls or ceilings. Who knows why they downrated the breaker that can be used with 6mm cable. It's an ongoing trend. Soon we'll be having to use enormous, armoured, fireproof cable to each and every switch. Hmmm, should I be on the grumps thread right now?

So, given that the cable is existing and you don't want to upgrade the cable size, you don't meet the requirements to use a 40A MCB, it would have to be clipped on the surface for that, but you're fine on a 32A, you can use all the normal installation methods then and the max permissible length is 49m.

 

[a twig]

Thanks a twig.

I decided that this question might get overlooked, tagged on to the end of eleven pages of grumps! So I've given it its own thread.

But thank you for responding. I did originally consider leaving the existing 32A minibreaker in place, but - like you - something on the dusty high shelves at the back of my memory muttered that a cooker exceeding 15kW should have a 40A breaker, so I thought I'd uprate to that, even though ours is 13.8kW. However, there's so much rubbish on those shelves that I don't really trust anything up there. Would you be happy to leave the 32A in place?

Definitely I would be happy to.

I'm an E&E engineer by background, plus domestically I've done several kitchens now, from a small cooker with a couple of induction hobs through to massive all electric ranges - never needed more than a 32A breaker. Best test was a Christmas day a couple years back, my folks had all 3 electric ovens in the range going, plus the electric hobs, no problem what so ever.

 

[Flyfisher]

Presumably, the diversity effect is greater with a modern appliance as each over is likely to be better insulated, this although an individual oven may be rated at, say, 3kW it would only draw that continuously while it gets up to heat and thereafter would be switching on and off just to maintain temperature. I know that our 3 year old dual oven is better insulated than our old one as we used to be able to use the upper, small oven for keeping things warm, even when not switched on, just from the heat leakage from the lower oven - but now the top oven only becomes lukewarm unless actually switched on. I guess this contributes to overall efficiency, less average power consumption and, therefore, a greater diversity effect - although it must be quite difficult to accurately model the diversity, hence the rules of thumb I suppose.

As for modern insulation practices affecting cable rating regulations, I hadn't thought of that but now mentioned I can understand it. I wonder if anyone has ever insulated an old house so effectively that the installed wiring then failed to meet the latest regs? :lol:

I've also pondered Zebra's point about the regulations being seemingly 'tightened up' with each new edition and wondered where this might lead in the future. Funny old world.

 

[a twig]

My in-laws laughed when I ran the connections out to their summer house in 6mm armoured cable which was the thickness of my thumb, then internally wired the two sockets in 6mm LS0H. All protected by an RCD back in the house.

I did point out that it's better to be safe than sorry, particularly in a summer house made from nicely seasoned and oiled pine shiplap with polystyrene insulation, and with the father in law was threatening to run all sorts of hefty garden appliances off the sockets, so I'd much rather over spec everything than under spec it. The 16A breaker should stop him going too nuts though

I think the regs are slowly coming round to a similar point of view - people will always find new and inventive ways to use things in ways that were not thought of during design and install. Unfortunately Darwinism isn't socially acceptable anymore, so you have to design to ensure the safety of the lowest common denominator.

Given that a survey I saw recently said 46% of young males between 16 and 23 don't know how to boil an egg, I think the bar is pretty low.

 

[Penners]

Cooking an egg is easy. You just leave it on an overloaded electric plug.

 

[Tony.]

I think your 6mm will be more than ok with the applied diversity.

 

[Penners]

OK, well many thanks again to all you experts for your views and advice on this. I'm delighted that the existing installation will be fine for the new cooker.

So far as Mrs P is concerned, it will of course be entirely my own expertise that has worked all this out - unless anything goes wrong, in which case it's down to you lot! :wink:

 

[Flyfisher]

My lips are sealed (taps nose). Say no more.

 

[Penners]

If that's a hint for a bung, FF, more ruthless men than you have tried and failed...

 

[Flyfisher]

:lol: :lol: :lol:
Perish the thought, just a bit of male solidarity lest Mrs FF should discover the source of my 'expertise' as well. Us chaps must stick together. :wink:

 

[Penners]

Amen to that!

 

[Penners]

Many thanks to the electrical illuminati who helped me with their advice. The new cooker is now installed and working.



A small anecdote to accompany the picture:-

It took me an awful lot of faffing around, cutting, sanding and modifying the kitchen units, to get the thing snugly fitted into the kitchen. After three days of b***ering about, I finally got it fitted and wired up yesterday evening. The instruction book (yes - I actually read it! ) said that all three ovens should be turned on full for half an hour, in order burn off any manufacturing oils, etc. So I dutifully turned them all on at maximum heat.

Almost immediately the entire house was plunged into darkness! My heart sank and I wondered what on earth I had done wrong. I groped around for a torch, and went to inspect the consumer unit.

All the breaker switches were up! It was a power cut!!

Never was the word 'PHEWWWW!' more meaningfully uttered!

 

[Feltwell]

:lol: :lol: :lol:

Looks great Penners. More importantly, is the Chief Domestic Engineer happy? For if she is not, you by default will not be either :wink:

I spent quite a while sorting out an electrical problem on my motorhome, had to order parts so it took a while, got it finished, plugged everything back in then realised the two very hefty habitation batteries (that power all the non-base vehicle bits) were totally dead. "Oh Christ", think I, "What now? What have I done wrong?"

Then I spotted that Mrs F hadn't closed the wardrobe door properly - the courtesy light must have been on for at least a couple of weeks and killed the batteries. Phew!

Thankfully 4 days each on a smart charger and they came back to life, otherwise it would have been over £200 for new batteries......

 

[Penners]

is the Chief Domestic Engineer happy?

We don't a CDE in this house, Feltwell. We have a Domestic Menial and an Ayesha. I presume I don't have to explain which of us is which?

However, yes - SWMBO is very happy, and cooked me an excellent seafood lunch today. So all in all, it was worth £GULP that it cost.

I was amused by your motorhome story, except that you appear mistakenly to have blamed Mrs F for the problem. Surely that was your fault, wasn't it?

 

[Feltwell]

Ahh yes, silly me. If I'd have emptied the wardrobe when we got home then she wouldn't have needed to do it and wouldn't have left the door ajar.
I was probably told to do it but wasn't listening as usual.

:wink:

 

[Penners]

I was probably told to do it but wasn't listening as usual.

I have a catch-all excuse for such situations that you may find useful.

"I was unable take in what you said, dear, because my all of my circuits were irrevocably committed to calculating the meaning of life, the universe and everything."