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Discuss diversity on a cooker ring in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

K

kita

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I only completed one year of a city and guilds sparky course so I'm not sure if I should be posting but.... I'm aware of part P, have a basic understanding of the wiring regs and know that work in kitchens by uncertififed installers needs to be reported to the local BC.

A friend is replacing his hob and cooker. I want to know if a new ring needs to be installed. Here are the details:

Currently there is a dedicated 32A supply for the fixed hob and cooker which only have one power cable (the 'cooker ring'). The run is about 12 meters from the consumer unit, via a single switch. I believe that the cable is currently 6mm twin and earth, but it could be 10mm (its from a 1997 install). What he wants to install is a fixed hob (6.6kw), oven (3.5kw) and warming oven (0.8kw), and he believes that they all have seperate power cables. At 230v the peak load will be just under 50A.

-Taking the units as a whole, what would a single cable and protection need to be rated at, allowing for diversity? Am I right in thinking that the proper way to split the supply into three is via a junction box? Does each 'spur' need its own protection?

-There is a nearby 13A socket, running off a seperate ring. What is the forum's opinion on removing the socket and using this ring to run a fused spur for the oven, provided that it doesn't overload the ring (leaving the 'cooker ring' for the hob and warming oven)?

In short, can the units be installed using existing wiring without running a whole new 50A supply?

Either way the work will be done by a qualified sparky and/or inspected by the BC. I just want to know so that he gets a proper quote.

Cheers
 
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E

EasyFox

  • Thread Starter Thread Starter
  • #2
Currently there is a dedicated 32A supply for the fixed hob and cooker which only have one power cable (the 'cooker ring').
So that'll be a radial circuit & not a ring final then

What he wants to install is a fixed hob (6.6kw), oven (3.5kw) and warming oven (0.8kw),
With diversity 21amp if no socket on the ccu, if it has a socket add 5amp.
6mm is fine.


Am I right in thinking that the proper way to split the supply into three is via a junction box
I'd use a double cooker outlet available from scolmore.

There is a nearby 13A socket, running off a seperate ring. What is the forum's opinion on removing the socket and using this ring to run a fused spur for the oven, provided that it doesn't overload the ring (leaving the 'cooker ring' for the hob and warming oven)?
Isolation of the oven via ccu?

can the units be installed using existing wiring without running a whole new 50A supply?
50amp supply isnt needed

Either way the work will be done by a qualified sparky and/or inspected by the BC. I just want to know so that he gets a proper quote.
Yeh right, just get it notified & tested when your mate has done it ;) !!!!
 
D

DadofTwo

  • Thread Starter Thread Starter
  • #3
If it's just a straight swap of appliances then does it actually need to be notified? I know the appliances are of a higher rating but there isn't actually any changes being made to the Kitchen Wiring?

Unsure on that one.
 
K

kita

  • Thread Starter Thread Starter
  • #4
OK thanks everyone

Easyfox I wasn't fibbing, if it needed a radial circuit upgrade, neither of us would have touched it. He's got money but doesn't like to waste it. So if the work was just the fitting of a new cooker outlet (which I think still is notifiable DadofTwo) then yeah I would do it for him.

But I think there has been a misunderstanding....

-I wanted to know the rated draw, allowing for diversity, of all three appliances combined (hobs, main oven, warming oven). 21 A (there is no socket on the CCU, its up by the ceiling) seems really low for a 50A max. Easyfox were you referring to just the hobs?

-If 21 A is right then the 6mm cable does not need to be replaced, and the 30 amp MCB on the consumer unit provides adequate protection for all three.

Otherwise:
-The hob (6.6kw) and warming oven (0.8kw) can be run off the existing 6mm radial with the 32A breaker, connected via a switched double cooker outlet (Thanks Easyfox) at worktop level. 2.5mm cable for the warming oven, 6mm for the hob. This circuit will be isolated by the new CCU. The hob and warming oven DON'T need individual protection.

-The main oven (3.5kw) would need a spur from the ring main, 6mm cable in plaster, with a 15 or 20A FCU. The oven would be isolated by its own switch on the FCU. Again 2.5mm cable from the FCU to the oven. Rather than install a new spur could we remove an existing socket outlet and install a FCU instead.

Or am I getting this all wrong.....

Happy easter
 
S

Shakey

  • Thread Starter Thread Starter
  • #5
Easyfox was correctly using the cooker diversity formula which is

1st 10A
30% of remaining amps
plus 5A in s/o in CCU

so 6.6kw + 3.5kw + 0.8kw = 10.9kw

so I = 47A

so 10A
+ 30% of 37 = 11.1A

no s/o so total Ib = 21.1A

30A trip

6mm tw&e (assuming method 1) = 47A

so Ib In Iz is satisfied.

So put a duoble pole CCU in, leave the perfectly adequate trip and cables are they are, and because you havent changed circuit cable sizes, changed cable routes etc then it is non-notifiable

Oh, and do you have a copy of the OSG?

if so, open it, and write the following in the inside cover:

"As a first yr apprentice, when a very experienced sparks (such as Easyfox) tells me how it is, then i should bow to his experience and knowledge and listen to him, instead of doubting him":eek::eek:

i think it may help you in the future.........;)
 
E

EasyFox

  • Thread Starter Thread Starter
  • #6
when a very experienced sparks (such as Easyfox)
Why thanks Shakey .... ego is swelling as I type:D

Having re-read my post I did come across a tad sarcastic at the end, so appologies for that Kita. And maybe I should have put the formula up as Shakey kindly did rather than expecting you to take the 21amp as gospel.
 
K

kita

  • Thread Starter Thread Starter
  • #7
all clear now! thanks for your help shakey and easyfox. I wasn't trying to sound narky, I just wanted to understand the thinking. but still... I don't mind a little bow scrape bow scrape :)
 
J

johnelectrical

  • Thread Starter Thread Starter
  • #8
Just a thought, diversity is there to enable you to calculate assumed maximum demand
you wouldn't actually use a cable which can only handle 21 amps to supply a possible 50 amp load would you?
 
S

Shakey

  • Thread Starter Thread Starter
  • #9
Just a thought, diversity is there to enable you to calculate assumed maximum demand
you wouldn't actually use a cable which can only handle 21 amps to supply a possible 50 amp load would you?
John, are you suggesting that diversity is just 'pie in the sky' stuff?

assuming 2.5 tw&E, method 1, = 27A

but of ciurse it doesnt melt or burst into flames at 27.1 A

0r 28A, or 30A etc etc

this is to keep it witihin its normal 70 degress operating tempereature

as discussed, the cooker wont pull maximum current for any length of time, which is what diversity is all about

10 double sockets on a ring means a possible load of 260A

but you can have 20 sockets or more if you like (within 100m square)

so what -400A? 500A?

even with a ring at method 1, thats only 54A worth of cable - for a 400 or 500A load?

its called diversity!!!!!!!

so yes, we would use the calcs for real applications (and do, everyday);)
 
R

randym68

  • Thread Starter Thread Starter
  • #10
no way , straight swap means no increase in current being used, from what you have described id say an increase in power, therefore rewire. and notify before work commences, or be a competent person, then you wont need to ask the question. fairly standard install.
 

telectrix

-
Mentor
Arms
Esteemed
i go with foxy and shakey on this. just modify outlet/s to suit appliances and leave the circuit alone.
 
R

randym68

  • Thread Starter Thread Starter
  • #12
ok here is an install i've never done and was hoping someone here (ie shakey or firefox) would confirm my understanding. some thing feels wrong but all seems fine from my reading of OSG and regs.

situation:- conversion of an old pub to HMO. 12 individual bedrooms with en-suite toilet shower and basin each (no electric water heating anywhere). shared 2 kitchen/dinning/ common room with total 3 combined oven + hob ( 6kw) + radial socket circuit each. then socket and lighting circuit for corridors stairs etc. Fire alarm system , twin boiler supply. Land lord "wants a separate fuse" (MCB) for each room (so separate light and socket MCB)

so here are my calculations - assumed boarding house/hostel

Lighting = 15 x 300W= 1.3A x 15 = 11.2A

ovens = 6KW = 26A + 21 (80%) + 16 (60%) = 63A

rooms 12 radial = 20A = 20 + 11x 8A (40%) - none are dining rooms = 108A

radial corridors 2 floors = 20A (40%) = 2x 8A =16A

2 radial lighting corridors = 400w = 1.8A @75% = 1.35A x2 = 2.7A

kitchens 2 radial = 20A @ 75% for kitchen / dinning room circuit = 30A

comms radial 16A @ 40% = 7A

Gas boiler 3A = 100%

LV fire alarm assume 1A (much less than this)


all adds up to 243 A , im lucky already have 3 phase onsite 100a per phase.

so split loads between phases, 2 x 12way CU for rooms sockets lights (sub off the DB) , and rest as part of the 3 phase distribution board. on RCCB / RCBO etc as necessary

cant think of anything else off top of my head... but happy to answer clarification

look ok to you or have i missed something
 
no way , straight swap means no increase in current being used, from what you have described id say an increase in power, therefore rewire. and notify before work commences, or be a competent person, then you wont need to ask the question. fairly standard install.
Say i swapped a pendant in a kitchen for a track light with 4 GU10s on, by your reckoning thats an increase in power, therefore a rewire and notifiable work. Seriously?!
 
N

not clever

  • Thread Starter Thread Starter
  • #14
ok here is an install i've never done and was hoping someone here (ie shakey or firefox) would confirm my understanding. some thing feels wrong but all seems fine from my reading of OSG and regs.

situation:- conversion of an old pub to HMO. 12 individual bedrooms with en-suite toilet shower and basin each (no electric water heating anywhere). shared 2 kitchen/dinning/ common room with total 3 combined oven + hob ( 6kw) + radial socket circuit each. then socket and lighting circuit for corridors stairs etc. Fire alarm system , twin boiler supply. Land lord "wants a separate fuse" (MCB) for each room (so separate light and socket MCB)

so here are my calculations - assumed boarding house/hostel

Lighting = 15 x 300W= 1.3A x 15 = 11.2A

ovens = 6KW = 26A + 21 (80%) + 16 (60%) = 63A

rooms 12 radial = 20A = 20 + 11x 8A (40%) - none are dining rooms = 108A

radial corridors 2 floors = 20A (40%) = 2x 8A =16A

2 radial lighting corridors = 400w = 1.8A @75% = 1.35A x2 = 2.7A

kitchens 2 radial = 20A @ 75% for kitchen / dinning room circuit = 30A

comms radial 16A @ 40% = 7A

Gas boiler 3A = 100%

LV fire alarm assume 1A (much less than this)


all adds up to 243 A , im lucky already have 3 phase onsite 100a per phase.

so split loads between phases, 2 x 12way CU for rooms sockets lights (sub off the DB) , and rest as part of the 3 phase distribution board. on RCCB / RCBO etc as necessary

cant think of anything else off top of my head... but happy to answer clarification

look ok to you or have i missed something
OSG Page 95 last paragraph; allows you as the designer to add all the diversified current demands & then apply an additional diversity allowance. But dont use table 1B for the values..

As for the design
How about a panel board feeding a small TP board for the communual areas & distributuion submains to each room where you can install a small board with rcbo's.

As for shakey & foxy they I belive are long gone, not seen either here for ages.
 
R

randym68

  • Thread Starter Thread Starter
  • #15
yep you should be careful , because could be 1mm or old .5 mm cable , could be in insulation, also was the light near a water outlet, is it the right IP rateing, maybe the ceiling is a dropped bulkhead. Imagine people just kept adding more lights never notifying. Kitchens and bathrooms are special locations...more as well but these are the two common ones.... the law under part P refering to BS7671 in special locations swap like for like no exceptions, not my rules i think its over kill , untill you look at cable cals and it makes sence

the whole reason for increase in regulations is an MP's daughter died, because a DIY friend put a metal dish drainer on wall just caught a live conductor, ok for 2 years till she had a dishwasher installed, one day she made a circuit between dish drainer and the earthed casing of the dishwasher drop dead.
 
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