calculation for maximum demand

[Peter Woodford]

Can anyone show me the maximum demand calculation or tell me were i can find it

 

[Guest123]

Ok, diversity is as follows:

Lighting - 66% of total current demand for each circuit
Power - 100% of the first ring circuit + 40% of all other circuits
Cooker - First 10A + 30% of remaing rated current + 5A if the CCU has a socket outlet
Shower - No diversity allowable
Instantaneous water heater - 100% of largest heater + 100% of second largest + 25% of of any others
Thermostatically controlled water heater - No diversity allowable
Underfloor heating - No diversity allowable

 

[Davey101]

I'm sure you know, but for less experienced people browsing this forum, those examples are for domestic i.e, a house. There are two other types of premises, basically shops+offices, and hotels+B+B's etc.

Just noticed that you are asking about maximum DEMAND, not just diversity, will have to wait for someone more intelligent ;-)

 

[crazyelec]

on site guide is where it is.

 

[sparkie710]

on site guide page 96 although seeing how im just a boy and i wanted to research this, what is the calculation all well and good saying 66%. but say i went to a job:
cooker x 32
shower x 32
2 x rings x 32
immersionx 16
2 x lights x 6
smokes x 6
from here what do i do and calculate ? much appericiated.

 

[sparkie710]

right scrap my last post but if say you have a 60a with loads of stuff on to much is being drawn what will be the worst outcome ? regards

 

[ezzzekiel]

Or take the largest rated mcb and add 40% of the remaing breakers total rating will give you a realistic value for domestic purposes

 

[esc]

Hi peter, my first thread this after watching for a while. Doubt you'll read this, its so long ago and bet you've got there by now.
As i understand it maximum demand is just the sum total of the CB's supplying the circuits of an installation. eg if our consumer unit had a 32A, 20A and a 16A CB, the max demand would be 68A.

Diversity being the allowances we apply to the circuits to give a more realistic value of how much current we would expect each circuit to carry under normal operation. This is put very well by Lenny above. Do like ezzzekiel's method though, its much simpler, will create some numbers and see how well the 40% rule compares to Lennys.

 

[Welchyboy]

you really need to give careful consideration when calculating maximum demand and diversity, as if you go too strictly on bs7671 guideline amounts then you will end up with huge a demand when in reality the total load will never reach anywhere near that amount, you also must consider the likely use for the installation too, there is not a formulae that will be accurate for every eventuality, for example an upstairs 32a ring circuit in a domestic property will likely never see close to 20A for any period of time let alone 32A!, but in a big kitchen it is more likely to be higher and for a longer duration, adding up mcb ratings will just end up with silly figures, you have to assess the installation and use your experience, things to look for in a domestic property are generally kitchen ring mains, electric showers, immersion tanks, electric cookers and electric heating, things like smoke detectors i wouldnt personally even consider.

 

[Sparky Ninja]

Following the guidelines illustrated in the electricians design guide will always result in a nice low realistic calculation of maximum demand.

 

[brucelee]

One book I NEED to buy yet widdler but may wait for the updated version

 

[La Poste]

Following the guidelines illustrated in the electricians design guide

Is this the book you are talking about?

Electrical Installation Design Guide: Calculations for Electricians and Designers: Amazon.co.uk: Paul Cook: Books

 

[Guest123]

Is this the book you are talking about?

Electrical Installation Design Guide: Calculations for Electricians and Designers: Amazon.co.uk: Paul Cook: Books

I believe so, yes.

It is being updated next year though so maybe hold off for now...

 

[esc]

Well explained welchyboy, thanks

 

[Thomas123]

page 96 and 97 of onsite guide dude

 

[Dan]

Just bumping some threads in the Electrical Wiring Theories and Regulations forum category here on our Electricians Forums. If this specific topic isn't current, just ignore it, it'll drop off the list in no time. However, if it's something you'd like to contribute to, feel free to reply and bring it back into current discussion.