Loading Calculations

[Chr!s]

Hi Guys

this is getting a tad tricky now as i wont sign it off although getting pressure from the builder and his sparks, who "designed" it. The original classroom was upstairs, and had 12 cookers of varying age. some had halogen hobs ,others were , radiant rings etc.some had a rating 0f 11kw while others were 7.5kw.

So why will the loading be any different now other than a few sockets?

The original board upstairs has an 100amp TP incomer fitted and has had no problems.

So why will the new, whats changed?

When applying diversity of 100% for the first cooker then 80% for second and 60 % for the remainder you get a total load of 138amps. now couple this with fact that when arriving on site to test it the cooker panel have sockets fitted without any RCD protection , something that has now been rectified with 12 RCBOs. so we now have to apply an extra 5 amp per unit for the socket outlet.so for the 4 cookers that are on each ring we have a new loading of 158 amps per phase.

If diversity was only that straight forward. Do you think the socket outlets will be drawing 20 amp per phase continuous?

Teacher has said that they do not EVER use the top oven/grill and only 2 hobs plus oven as a max on each unit.Ok so what does one do.

That's why we apply diversity, along with cyclic loading etc.

Its a Dorman Smith DB about 12/15 years old (DBX) 6 way . They dont seem to do either a 160 or 200 amp incomer for these DBs only a 250amp.Then to cap it all we have a 25mm 4 core SWA feeding it .Yes i know its only rated at 124amps ,Not running at 70 C its not, but to back it up at the supply end is 200 amp HRCs , So where is the overload protection?



Any suggestions apart from get them to rip it out and start again ))
oh!!! the clamp put a load of 87 amps for 2 cookers on full load

Initially you will, then the cycles start.
Jools

Your not the designer, so why is this falling on your shoulders?

Certainly in this environment i would be cautious over the amount of diversity, the likely hood of these cookers operating at the same time are greater, but its existing and the fact its been okay tells you a bit about the loading.


So if you use the OSG we have 25 amp including Socket. So each final circuit 32 amp. Distribuition, 4 x 25 x 0.8 diversity, so 83 Amp per phase.

 

[sparky692]

Not sure what you mean by absolute rubbish . but they were on full load and pulling that till they heated up and the rings were on and of but didn't drop below 68 amps for 20 mins. So called cooker are what the manufacturers say. 46 amps.
and a new fuse board is going in. Having turned on all 12 cookers today , it didn't take long for the isolator to "warm up".
Diversity .. mm always a subject that different people look at differently. .One thing i don't want whether its "rubbish" or not is a fire or burn out in a school.So on the side of caution it is.
Thanks to all replies.

 

[Engineer54]

Diversity .. mm always a subject that different people look at differently.

Only to those that don't know how to apply it. Going by your reckoning every cooker on a 32A MCB will be tripping out all over the country during the Xmas holidays or big family gathering. The fact is they don't, and never have done, and most will have a far bigger KW (up to 15 KW) rating than your small 10.7KW units....

Those cookers will never see the full load, you can't seem to get past that fact, you have even had this confirmed by the responsible Domestic Science teacher. There are also elements within the oven sections that cant be utilised at the same time, so it's impossible to see the full ''connected'' load (which is the only rating the manufacturers state) anyway....

 

[sparky692]

Thank you for all your replies to this thread. its been interesting to say the least. And also has taught me not to be so rigid with diversity.The idea i thought for this forum was indeed to ask questions and get answers and general stuff about our "game".Just as a point of interest , 3 different teachers use this room,or will be, The one i spoke to was teaching the younger kids.On speaking to the other 2 , i found out the older pupils regularly use at least 3 rings the oven and top oven as a "keeping warm " place.
so again thanks for your replies.
Jools

 

[snowhead]

Once the School Business Manager, or whatever they call the person that pays bills, finds out how much the electric bill has gone up by, they'll be down to cooking ready meals in the microwave.

I assume the 200a fuses have been changed to protect the cable or is there also a new cable to match the fuses?

Just to reassure yourself for the future try clamping the load when the kids are using the cookers.

 

[Knobhead]

I assume the 200a fuses have been changed to protect the cable or is there also a new cable to match the fuses?

Good point :redface:

 

[plugsandsparks]

Looks like his isolater is the weak link. Often see fuses / breakers way over size for the poor old isolater sitting in the middle. You need to get the overload protection approrpiate for the weakest link in your circuit as has already been said or change it.
If you do set the fuses down a bit, get some spares in.

Generally as others have mentioned when confronted with this type of situation you have to adopt the "real world" approach by observation and measurement.
No book will help.

 

[sparky692]

Yes indeed the real world. The clamp test was done with all cookers on full load for 20 mins.As my clamp only went up to 100amp and it flew past there , we turned of 2 and doubled up. then applied the said diversity.
We have now installed a new fuse board suitable rated. fuse checked, and now i can sleep at night..
its been interesting anyway this thread and again i thank everyone . and i mean everyone for their comments
Jools

 

[dpiers1980]

10700 x 230 = 46 amps ??????

10700 / 230 = 46.52 amps