cooker

[GLENNSPARK]

my boss has asked me to work out the design current for a 10.8Kw 3 phase cooker for a commercial kitchen...
Now then....according to the osg (p97) table 1b column 3 it states that its to be 100% full load then 80% for other appliances on this circuit. So 10.8 Kw equates to 47A so is that 47A spread across the 3 phases? He wants to use 4mm 5 core hi-tuff and a 32A tripple pole MCB....

 

[GLENNSPARK]

sorry guys....forgot to mention that the supply cable will be in a trunking and the length of run is 28 meters...........

 

[telectrix]

have you considered diversity?

 

[GLENNSPARK]

have you considered diversity?

Evening Tel..
well yes i did by using the first 10A rule then 30& of the rest but then i realised it was a commercial situation and as such this doesn`t apply. Looking at types of premises in the table i looked for small shops offices and business premises which dont allow a diversity factor for the first load.....so i am working of full load which is 47A rounded up......this pre phase would seem huge to me si methinks that its spread across the 3 phases so then 4mm would be too big....2.5 would cut it...would it not with a 20A tripple pole MCB.?..........

 

[GLENNSPARK]

would not the CSA of the neutral be an issue though? not being ballanced etc?.......

 

[GLENNSPARK]

When he rang up i asked him about insulation on the cable run and he said there wasn`t any...

 

[Engineer54]

Is this the only cooker unit in this commercial kitchen?? The Reg's diversity section is purely there for guidance, and rarely matches what is actually present in the work place!!

 

[GLENNSPARK]

evening eng...
Am afraid i havn`t seen the premises myself so i cant say....
My thinking was that if he`s going for 4mm 5 core and the load is across all 3 phases then 4mm seems a bit exessive to me

 

[GLENNSPARK]

see i`m looking at rating factors n all which for trunking (method A) gives it at 20A for 2.5 CSAs........

 

[GLENNSPARK]

no it dont....sorry eng its 18A...silly me..lol.......

 

[GLENNSPARK]

Iv done it again....3d time its ref. B and its 21 A for 2.5 CSA 3 phase...lolffs...

 

[GLENNSPARK]

Ok 20A it is and I dont know how many other cablkes are in this trunking as i havn`t seen the job yet.....better if i look first and get it right in my mind as to what i should be doing here as i still think that 4mm is too much..........

 

[Engineer54]

First off on commercial kitchen installations, the diversity considerations take in to account multiple cooker units, where in your case there could well be just the one unit. Commercial cookers tend to get a real bashing being used for many hours a day ...every day!! So i wouldn't see any problem using the 4mm cable your Boss is considering, it will also allow for an upgrade at a later date, it the business grows.

The thing is on cooker diversities is that they are real, and therefore should be taken into consideration when sizing it's supply cable and protective device. Give your Boss the choice of the two scenarios based on BS7671 and let him decide which to go with... Personally i'd stick with the 4mm and a 20/25A breaker, it sure ain't gonna go into overload is it?? ...lol!!!

 

[GLENNSPARK]

First off on commercial kitchen installations, the diversity considerations take in to account multiple cooker units, where in your case there could well be just the one unit. Commercial cookers tend to get a real bashing being used for many hours a day ...every day!! So i wouldn't see any problem using the 4mm cable your Boss is considering, it will also allow for an upgrade at a later date, it the business grows.

The thing is on cooker diversities is that they are real, and therefore should be taken into consideration when sizing it's supply cable and protective device. Give your Boss the choice of the two scenarios based on BS7671 and let him decide which to go with... Personally i'd stick with the 4mm and a 20/25A breaker, it sure ain't gonna go into overload is it?? ...lol!!!

well thats clear then eng. Thanks mate and sorry n all for that roundabout the capacities farce.....believe me i do know better than that...lol.....wouldn`t leave it though till i was sure in my own mind at what i was looking at..lol....

 

[GLENNSPARK]

see what i was thinking n all is because it aint singles and i dont have the say on neutral size ...say 2.5 phase/s and 4mm neutral.....then 4mm 5 core is the way...yes?.......would i be thinking correct here ?..larger neutrals that is for phase imballance/s??.....

 

[Engineer54]

Your over thinking!! lol!! So long as the neutral is the same size as the phase conductors it's a good-un!!

 

[GLENNSPARK]

Your over thinking!! lol!! So long as the neutral is the same size as the phase conductors it's a good-un!!

Thanks again eng....just wanted to be clear....and now i am..
Thanks.
Glenn

 

[Knobhead]

would not the CSA of the neutral be an issue though? not being ballanced etc?.......

Neutral current won’t be a problem. If your boss is prepared to cough up for 4mm I’d not argue with him. Big is beautiful in my book.

 

[Inteificio]

Don't forget to rate up to take in to account the temp, kitchens can get VERY hot, especially behind an oven.

I tend to use SY when wiring ovens as the additional protection is worth it in a kitchen.

Regarding the neutral current you have confused me. There is a good chance the oven would be phase balanced (it would be if I designed it), so the neutral is theoretically not required. So why do you think you need a larger neutral than line? 4mm 5 core SY would be what I would do, but if singles I wouldn't drop the N/E to what the calcs say (prolly a lot smaller).