Loading Calculations

[sparky692]

HI Fellas. Just a quick question for a brain box out there )
Got a classroom that's been refurbished , that im testing. They have a Dorman Smith 3 phase 6 way Db with a 125 Amp main switch.
Does anyone know the calculation to work out the total loading on each phase. There are 12 X 10.7KW cookers. 4 on each phase..There are other compliancy issues as well, but think the Main switch isn't man enough.
Any quick reply would be a help.

Regards
Jools

 

[Ponty Massive]

I=P/V......

Dont forget diversity

 

[sparky692]

Thanks for the reply, I am aware of ohms law etc, but how do you calculate the 4 of them. Just add them together it seems.Seem to remember that some percentages calc also apply
If its just total load then its going to be as i said .

10700 x 230 = 46 amps x 4 = 186 amps ?

 

[Adam]

You can apply diversity to them.

Cooking appliances in non domestic installations:

100% of largest load (if they were different, but not in your case),
80% of second largest,
60% of all remaining.

 

[SW1970]

But isn't it highly likely in a classroom that all cookers will be on at the same time? I don't think the diversity rules of thumb apply here.

 

[Adam]

But isn't it highly likely in a classroom that all cookers will be on at the same time? I don't think the diversity rules of thumb apply here.

True, but are they all going to be heating at the same time with all heating elements on?

Perhaps use domestic diversity (10 amps + 30% of remaining load). It is extremely unlikely that all heating elements will be in use at he same time. I can't see students being permitted to use four hobs, grill and oven at the same time.

 

[sparky692]

Yes i can see what everyone is saying but do you not have to allow for "in case ", Thing i dont get is on speaking to indesit who make these , they tell me that they are 10.7kw but thge fitter seems to think thewre only 7.5kw as there fan assisted with ceramic hob. watch this space this afternoon as cant get in till then .

 

[Adam]

Even if all cookers were using all heating elements, you would still not achieve full load (unless you turned all of them on at exactly the same time).

The elements will be switching on and off constantly once at set temperature, so it would be extremely unlikely for all to be heating at the same time.

 

[SW1970]

Maybe it's best for OP to find out the typical and worst case usage scenarios from the client and then build in some extra allowance.

 

[Engineer54]

So who designed the electrical provision for this refurbishment of a domestic science classroom, because he is the best guy to ask!! Like everyone here has suggested, the chances are, that a good knowledge of diversity has been applied to this installation and it's DB....

 

[Darkwood]

You say its a refirb' what no' and rating of the original cookers for which the supply was originally designed, are you seeing significant increase in the number of points or ratings?

 

[telectrix]

time to get the clamp meter out. then you can see what's being drawn per phase.

 

[sparky692]

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.
The original board upstairs has an 100amp TP incomer fitted and has had no problems.
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.
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. 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 , but to back it up at the supply end is 200 amp HRCs . 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
Jools

 

[Knobhead]

Condemn it then, it’s your job (on the line).

You need to adopt a practical approach to diversity. By rights half the homes in the UK would be without power due to blown service head fuses.

Get real!

 

[Engineer54]

Any suggestions apart from get them to rip it out and start again ))

Sorry, but Yes, Get an electrician in that understands diversity and max demand loads

oh!!! the clamp put a load of 87 amps for 2 cookers on full load

Absolute Rubbish!!! How long did these so called 10.7KW cookers at full load, show they were pulling 87A??