Cable size and diversity

[FiremanT]

Me again.

I am tryng to get my head around diversity factors. have spoken to three electricians, and am geting dfferent opinions. I am aware that some things are subjective and people have different views on regs. A lot of my colleagues, gasmen, refuse to allow the concessions allowed in the REGS, prefering to adopt a "zero tolerance" stance, even whn Regs state otherwise (personally I beleive that a lot of those suffer from lack of confidence in their knowledge and experience, so play the CYA card to the extreme, but punters end up payng fr unecccessary work))

To sum up, I will be having installed a full Boscfh Kitchen set up:

Oven 2.4Kw
Oven (with microwave 2.8Kw
Induction Hob 7.4Kw
Warming Drawer appraently up to 600Watt (cannot find appliance specific wattage)
Extractor Fan 233 Watt

There will be no sokets on the circuit(s)

The first guy suggested 3 new radial circuits to eachoven and the hob, presumaby also feeding the fan and warm Draw

The others said that was too much.

I understand that the diversity factor is ((Total Load - 10A) * 0.3) +10A . Apologies if that formaula is wrong, Ijust worked it out from info available. I assume that one works on the TL being all five appliances, or is it actually different fr the fan and warming drawer?

That gives me, I think, 27.53A ?

If I am correct, then I only need one breaker, of whch I have an existing spare. Is that correct, my gut suggests that it would be split, but would be better, obviously, financially not to have to fit a second CU.

I know, think, that the DV affects the cable size, but cannot see how that is calcuated. The longest run from the Consumer Unit (to the hob & fan) is about 10M, to the ovens it is about 5M. ther is a 2M run that will be inside a small duct, otherwise all will be surface.

In my head, if I could run 10mm from the CU to a dual oven connector unit such as

( https://www.tlc-direct.co.uk/Produc...l_v2&msclkid=aceaf40cc6b8122b33f99d2c3a3b183e),

then to to the Hob, dropping the cable size to individual appliance switches.

I am perfectly happy to accept that I am wrong. I need to knw wether I need to run two circuits, or if one is okay, and is the existing 40A breaker appropriate?

I am not working currently, so want to do the house bashing myself, then have the right guys to do the clever bits.

Thanks for reading, and for any help.

 

[robinbanks]

You'll need another protection device at the point you drop cable size. You also wouldn't necessarily apply diversity to a fan, because diversity is about supplying loads that vary (ovens get up to temp then cycle the heater element on and off to maintain temp) rather than draw the same continuously.

Can you take a look at the installation methods and tell us what they will be along the entire run, as it sounds like you have a mix. What a cable is surrounded by has a significant impact on its carrying capacity

 

[FiremanT]

Thanks, Robin.
The first 2M is surface, inside the meter cupboard.
Then it will be in a 2M boxed skirting , not tightly compacted, but also containing 15mm heating pipes. It will then go through a single skin wall.
The rest of the runs will be surface clipped behind cabinets with at least 50mm service space.
When you say “protection device” do you mean a FCU to which the appliance will be connected?

 

[securespark]

I am not working currently, so want to do the house bashing myself, then have the right guys to do the clever bits.

Have you checked with the spark or sparks if it's OK for you to choose and run cable for them to connect up and sign off as their own?

 

[robinbanks]

To be fair, running a cable and nailing it to a wall takes minutes, so you aren't really saving much by doing the cable work, and I don't think you were asserting that you wanted to do all the connections, so there's probably only about 20 quid of professional time you're saving (don't forget, you might fuss and faff, getting it perfect, and take 3 hours to do what your crash bash bosh spark does in 30 mins because he does it all day every day and can foresee and avoid the problems you'll have to hit and work through)

You time would be well spent creating the route, drilling holes in walls, finding and marking studs that accessories will be fixed to, but I wouldn't look at it as "if I get the spark to agree to let me do X and Y I could halve the cost of the job"

Maybe go with whichever one is amenable to "what parts of this can I do to make it quicker and easier for you and cheaper for me?" And see if you get answers between "none of it" and "buy this this and this, run it between here and there, screw the backplates in place but leave the rest of the connecting to me"

 

[FiremanT]

To be fair, running a cable and nailing it to a wall takes minutes, so you aren't really saving much by doing the cable work, and I don't think you were asserting that you wanted to do all the connections, so there's probably only about 20 quid of professional time you're saving (don't forget, you might fuss and faff, getting it perfect, and take 3 hours to do what your crash bash bosh spark does in 30 mins because he does it all day every day and can foresee and avoid the problems you'll have to hit and work through)

You time would be well spent creating the route, drilling holes in walls, finding and marking studs that accessories will be fixed to, but I wouldn't look at it as "if I get the spark to agree to let me do X and Y I could halve the cost of the job"

Maybe go with whichever one is amenable to "what parts of this can I do to make it quicker and easier for you and cheaper for me?" And see if you get answers between "none of it" and "buy this this and this, run it between here and there, screw the backplates in place but leave the rest of the connecting to me"

I take your point.

But timing is critical - I need to remove the kitchen and have the cables in place before the fitting of the cabinets. (I was going to do the cabinets myself but have a colleague who will do it with my assistance a lot quicker than on my own, at a reasonable rate). So, if you wouldn't mind popping around on the specified evening , probably at short notice, and do the first fix cable work, I will happily give you £20.

 

[Murdochcat]

Thing is modifications to installations as suggested above come with responsibility on the installer to check the installation is up to regs before changes are made.

 

[EFLImpudence]

All your appliance can easily go on a standard 4mm² 32A 'cooker' circuit although you might need larger cable through some of the route - or route it elsewhere.

I would not count the fan in the diversity but it is such a small load there will be plenty of room for it.

So:
Oven 2.4Kw
Oven (with microwave 2.8Kw
Induction Hob 7.4Kw
Warming Drawer appraently up to 600Watt (cannot find appliance specific wattage)

13,200W / 240V = 55A. (actually 52.8A @ 230V but don't bother with that)
45A x 0.3 = 13.5A + 10A = 23.5A

 

[flameport]

Oven 2.4Kw
Oven (with microwave 2.8Kw
Induction Hob 7.4Kw
Warming Drawer appraently up to 600Watt (cannot find appliance specific wattage)
Extractor Fan 233 Watt

Warming drawer and fan are such low power they can be disregarded. Generally such things would just plug in to whatever socket outlets you have in the kitchen.

The other three are no different to a freestanding electric cooker, of the type that's been used on a 32A circuit in millions of homes for decades.

 

[FiremanT]

All your appliance can easily go on a standard 4mm² 32A 'cooker' circuit although you might need larger cable through some of the route - or route it elsewhere.

I would not count the fan in the diversity but it is such a small load there will be plenty of room for it.

So:
Oven 2.4Kw
Oven (with microwave 2.8Kw
Induction Hob 7.4Kw
Warming Drawer appraently up to 600Watt (cannot find appliance specific wattage)

13,200W / 240V = 55A. (actually 52.8A @ 230V but don't bother with that)
45A x 0.3 = 13.5A + 10A = 23.5A

Thanks for that, EFL. I had envisaged having to use a heavier cable, especially as the guy who quoted was using a new CU with 3 MCBO's , so presumably we would have had 3 cables.

 

[EFLImpudence]

Thanks for that, EFL. I had envisaged having to use a heavier cable,

Most people still use 6mm² cable but that is just habit and was for 30A BS3036 rewireable fuses.
On the other hand, if you used 6mm² you would not have to worry about any larger cable in heat sensitive areas.

especially as the guy who quoted was using a new CU with 3 MCBO's , so presumably we would have had 3 cables.

You would have - unnecessary.

 

[robinbanks]

13,200W / 240V = 55A. (actually 52.8A @ 230V but don't bother with that)

How did you divide the same large number by a smaller number and get a smaller result than before? (57.4A@230V, surely?)

 

[EFLImpudence]

The Wattage figures quoted for your appliances will have been given for 240V therefore 13200W/240V=55A.

To calculate for 230V is not simply a matter of dividing 13200 by 230 because If the supply for these items is actually 230V then the Wattage and Amperage will be lower - that's how it works because their resistance is the constant factor; not the Wattage.

So 55A x (230 / 240) = 52.71A (not sure how I got 52.8 before) and only 12100W.

Ohm's Law Calculator

www.tlc-direct.co.uk

Ah. That says 52.8 so not quite right.

 

[trojanhawrs]

The Wattage figures quoted for your appliances will have been given for 240V therefore 13200W/240V=55A.

To calculate for 230V is not simply a matter of dividing 13200 by 230 because If the supply for these items is actually 230V then the Wattage and Amperage will be lower - that's how it works because their resistance is the constant factor; not the Wattage.

I'd be more likely to assume it's given for 230V, that's the european standard.
Either way, induction hobs are far from a purely resistive load (it's in the name) so you can't assume the current drops with the voltage.

 

[EFLImpudence]

I'd be more likely to assume it's given for 230V, that's the european standard.

Never assume.

Either way, induction hobs are far from a purely resistive load (it's in the name) so you can't assume the current drops with the voltage.

Are you saying the current will rise with lower voltage?