5.3 kw double oven on a 20 amp fuse with 2.5 cable

[egginyourface]

evening all,

I was told to run 2.5 cable from fuseboard to supply a 3.5 kw single oven. Job is done....

Now they've said theyre going to put in a 5.3kw double oven in.

Question i have is can I apply diversity to this and keep 2.5t+e or do i need to run a new 6mm,

advice would really appreciated

thanks

 

[a1guvner]

They want to change the spec, time for a variation order and some more dollar for you

 

[TaffyDuck]

23 amps. Well technically 20amp fuse still exists so the cable is protected subject to normal de-rating considerations. Pushing it a bit IMHO. I think I`d recommend an upgrade in writing so if/when you have to go back to upgrade your covered. Sometimes these ovens have 5.3kw or something on them and that includes 3 elements say 2x grill and 1x rear circular fan element. As its unlikely or impossible perhaps to have the rear fan oven element on at same time as the grill you would be under 20 amps again

 

[TaffyDuck]

Cant say I can remember a job in a kitchen when it didnt change a bit so i usually bung 6mm in. These flippin electric range cookers are the nightmare at 10-11 kW.

 

[telectrix]

as a1guvner says. alteration in design, extra work, extra cost.

 

[ample current]

the diversity allowance for cooking appliances in a household is 10amps + 30% f.l ( full load if i remember right ) +5amps if the cooker outlet has an incorperated socket outlet.

 

[telectrix]

which gives around 15A without a socket outlet ( top of head as calculator not to hand). so 2.5mm on 20A is adequate. problem would arise if they added a hob or changed to oven for a higher rated one.

 

[ample current]

which gives around 15A without a socket outlet ( top of head as calculator not to hand). so 2.5mm on 20A is adequate. problem would arise if they added a hob or changed to oven for a higher rated one.

nearly telectrix 16.9 amps to be precise.

 

[egginyourface]

there is a hob but its on a seperate 6mm

 

[telectrix]

just worked it out. 5300/230 = 23A

10A + (0.3 x 13 ) = 13.9A. where did 16.9 come from?

 

[egginyourface]

Can someone give me a step by step guide on how to calculate diversity as i havent used it for about 4 years...

 

[telectrix]

1.divide the watts by 230 to give a total load current figure.
2. apply diversity: 10A + 30% of remainder = answer. add 5A if a socket outlet is incorporated.

 

[GLENNSPARK]

should add here that watch out for old BS3036s...as this affects the CCC of cables....as in for 2.5mm flat T&E on a BS EN 60898=27A....for BS3036=24A.........

 

[ample current]

just worked it out. 5300/230 = 23A

10A + (0.3 x 13 ) = 13.9A. where did 16.9 come from?

are you saying its 30% of whats left of the 23 amps after taking the 10 off? I thought the fl (in the osg) meant full load so 30% of 23 amps (not 13amps) ie 6.9 amps plus the 10
equalling 16.9

 

[GLENNSPARK]

its the first 10A...then 30% of whats left......then if theres a BS1363 outlet incorperated into the double pole iso for the cooker....then just add 5A......

 

[SimpleSimon]

So in your case it would be 230/5300= 23a
First 10a than 30% of remainder (23-10=13) 13/100=0.13x30=3.9a. Than add your original 10a on gives you 13.9a, and if you have a socket add 5a so your total would be 17.9a. 2.5 would still be ok but you run future risks if anything changes or they deciede to open their doors to the homeless

(PS this calculation does not take into account any factors for the cable run)

 

[GLENNSPARK]

and if adding up multiple loads on a single circuit.....so lets say that you have several cookers all fed from one circuit...then you would apply diversity to EACH LOAD SEPERATELY...then add up the total......

 

[egginyourface]

ok, so 30% of the 23 amps is 6.9 amps


so 16.9 amps as you said in your earlier post right

 

[GLENNSPARK]

this of course will dictate the size of overcurrent device you will use at origin....which in turn will give you the correct cable size to use.....installation method, ambient tempreture, grouping and length of run taken into account of course.....

 

[egginyourface]

sorry i meant 30% of the 13 amps


The cable run is about 12metres long and run in yt2 trunking with a 10mm earth for the gas. Only about 1 metre of it is actually buried

 

[telectrix]

ok, so 30% of the 23 amps is 6.9 amps


so 16.9 amps as you said in your earlier post right

NO. 10A + 30% of remainder (13A, not23A)

 

[ebow72]

there is a hob but its on a seperate 6mm

Might be a stupid idea and I don't know the rating of the hob, but is there enough slack of the 6mm, to swap over to the cooker and vica versa...
I am still learning so please don't make me brew up

 

[GLENNSPARK]

it might be a start to look at app. A starting page (109) in the osg (green)....table A1....

 

[WPD Lad]

Might be a stupid idea and I don't know the rating of the hob, but is there enough slack of the 6mm, to swap over to the cooker and vica versa...
I am still learning so please don't make me brew up

Hobs are normally rated higher than ovens, something in the order of 7kw

 

[WPD Lad]

If its in a conduit surely it's easy enough to swap out for 4 or 6mm

 

[TaffyDuck]

Diversity on a cooker works fine until Xmas day when they trip out. personally I think regs are out of date on this one. Take a 10 kW electric range cooker. 10kW/230V= 43 amps. 10 amps plus 30% remaining load (approx 9.9amps) =19.9 amps so 2.5mm twin and earth on a 20 amp, dont think so. "At the installers discretion" is a term we ought to use more. On top of that you`ve got an appliance that draws for long periods of time allowing heat to build up in the circuit. Its not like a kettle that runs for a couple of minutes at a time. 6 or 4mm on this OP specs every time for me,

 

[Dave 85]

Bung it on the 2.5 on a 20A breaker with 20A DP iso. You're happy, regs are satisifed and client doesn't feel shafted and may actually call you again. I really don't understand this attitude of 'charge the client for an unnecesary job just to punish them for changing their mind.' Repeat business is good no?

 

[GLENNSPARK]

Diversity on a cooker works fine until Xmas day when they trip out. personally I think regs are out of date on this one. Take a 10 kW electric range cooker. 10kW/230V= 43 amps. 10 amps plus 30% remaining load (approx 9.9amps) =19.9 amps so 2.5mm twin and earth on a 20 amp, dont think so. "At the installers discretion" is a term we ought to use more.

well on such a load then...yes....it would be up to the designer as to the right action to take here.....short of treating it as a commercial load...100% of the first load....80% of the second and 60% of remaining appliances....

 

[ample current]

so what does the fl mean in the OSG (red one page 97)?? and i quote "10A +30% of fl of connected cooking appliances.........."

 

[GLENNSPARK]

full load...

 

[TaffyDuck]

well on such a load then...yes....it would be up to the designer as to the right action to take here.....short of treating it as a commercial load...100% of the first load....80% of the second and 60% of remaining appliances....

Well I was taught to build in redundancy and that diversity was to do with total demand not individual circuits but I may be wrong there.. May be considered old fashioned by some but Ive lost count of the times Ive come across burnt out 2.5mm on an oven. Whatever the regs say the buck stops with the installer, hoping the client doesnt cook much is daft in my book. I have always used 4 or 6 on an oven, might be over engineered in some peoples eyes but its a better quality job

 

[WPD Lad]

The 2.5 hasn't been de rated at all, we're assuming it'll have the full ccc, if it's in a conduit I'd stick a larger cable in

 

[ample current]

full load...

So why is it 30% of the remainder and not 30% full load
it says 10A + 30% fl not remainder :dizzy2: or am i missing something????? or just being a bit dim.

 

[TaffyDuck]

So why is it 30% of the remainder and not 30% full load
it says 10A + 30% fl not remainder :dizzy2: or am i missing something????? or just being a bit dim.

OSG page 110 10amps + 30% of remainder

 

[GLENNSPARK]

diversity becomes a problem when people start adding all connected loads together and then applying diversity....doesn`t work as the overcurrent device and associated cabling will be woefully undersized/underrated.....but as has been said before....the "regs" as we see it...that is BS7671 and anything seconded to it.....OSG GN3 etc are but guidance notes after all and as such are open to question as regards issues like this.....of course it is wiser to go over rather than under in terms of protective devices and cable/s and theres the issue of futureproofing as well....but how far do you take it?....Diversity is there for us to utilise ...given the guidance laid out in BS7671....its just a case of how and where....like you wouldn`t apply diversity to such a thing as an instantainious water heater would you.....as someone who posted on this forum about 3 weeks ago was going to do.....until advised not to of course....

 

[ample current]

OSG page 110 10amps + 30% of remainder

I take it thats the green one.

 

[GLENNSPARK]

if you look it is refering to 10A +30% fl in which it means you take the full load and apply the first 10A then 30% of whats left......

 

[Gordon]

The diversity applies to a cooker incorporating oven, rings, grill and assumes that they won't all be drawing full power in one go. likelihood
is that a double oven will draw full current at some time. Better to upgrade cable accordingly.

 

[TaffyDuck]

Table A1 is concerning diversity on individual circuits and states 10amps + 30% of remainder. Table A2 is diversity of a whole system and states 10amps +30% Full load in excess of 10a

 

[GLENNSPARK]

The diversity applies to a cooker incorporating oven, rings, grill and assumes that they won't all be drawing full power in one go. likelihood
is that a double oven will draw full current at some time. Better to upgrade cable accordingly.

yet again....looking at the appliance first...or if the appliance isn`t on site at the time of design....then gathering info (make model) etc from the client and doing a bit of research on it.....find out the KW rating etc...so as to calculate/design accordingly....isn`t that the difference between installers and electricians.......