As KME says.. you as the electrical engineer have got to make a reasonable assessment of maximum demand of the installation you are working on...
It is NOT a precise calculation because every installation is different..
also with domestic the same property could have a change of occupancy and max demand could go higher - or - lower because of the new requirements of new owners.. although the physical installation may be the same!!!?
which ever way you look at it DIVERSITY has to be taken into account!
which leaves two basic options...
1/ asses individual circuits applying diversity and add em up!
2/ assess the whole installation.. & take a percentage..
Method 2/ in a DOMESTIC! environment...
(commercial & industrial would need to be assessed on a bit more stringent manner)
RULE OF THUMB!
see page 97 On Site Guide,
Table 1B
Point 9.
Standard arrangement of final circuits in accordance with Appendix 8.
100% of largest circuit rating + 40% of every other circuit!
Personally I think this method can be a tad on the high side?
e.g.
1 x 40a shower
1 x 32a cooker
2 x 32a sockets
2 x 6a lights
1 x 16a imm heater
1 x 20a garden shed/garage.
(100% x 40a) + (40% x (32a + 32a + 32a + 6a + 6a + 16a + 20a))
=40a + (40% x 144a)
=40+57.6
=97.6A!
say for example if the shed is not in use 97% o the time... 20a is NOT going to be drawn! :|
and if the immersion heater is ONLY used as a back up to if the central heating packs in?....
thats another 13a out of the picture!
amending the above rule of thumb..
you could say...
(100% x 40a) + (40% x (32a + 32a + 32a + 6a + 6a))
=40a + (40% x 108a)
=40+43.2
=83.2!
Whereas compare with Green H method...
0.42 x 184a =
77a
which it probably is somewhere around 80a ?
Hopefully this helps illustrate how tricky it is...
BUT you must do a reasonable check. as KME pointed out,
check what IS in the property.
Not just what the cut out fuse rating is!
I have been on call outs where the main fuse has blown...
so in those cases Max demand.. MUST of exceeded cut out rating!?
; )
IMHO... Applying a bit of common sense, using the skills & knowledge you have learnt, you should be able to get a reasonable estimate.
Guiness Drink