In uk.d-i-y, Pandora wrote:

It's Sunday evening. Mum's doing the washing/drying/ironing, Dad's cooking
lunch and four teenage children are showering/doing homework/watching TV.
(Putting aside the debate about stereotypical representations) I think that
this family could draw the following loads simultaneously:

12.5A (Washing machine)
Only for the small proportion of time it's heating water; "modern" machines
use little water compared to older ones. And few have elements above 2kW.
So a better figure for the peak draw would be 9A; and with some 15% of the
time spent heating, rather than spinning, tumbling, etc., the average figure
will be more like 3A.

14.5A (Tumble dryer)
Not as high as that, but could be 12A, and unlike the washing machine will
be sustained for a longer time - 60-90 minutes.

6A (Iron)
Yes, but again there's a thermostat. If Mum's ironing her linen tablecloth
with the steam going full on, you might see an 80% duty cycle; if she's
just passing it over her Janet Regier with the iron set a bit under the
one-dot position, it'll be a 5-10% duty cycle...

1.5A (Fridge)
3A (Freezer)
Unlikely to be that high for either, and again will be smoothed by thermostat
action.

12A (Dishwasher)
Same comment as washing machine: heating and drying account for say 50% of
the duty cycle, the rest is spent spraying the already-hot water round the
washdosh and rinsing its contents with cold water; so average down to 5-6A.

10A (Cooker (electric))
Yes, maybe that high if we've got all 4 plates on the go and something in
the oven (each plate 2kW and maybe 1kW oven, but thermostats clicking in
and about bring the average down near the figure you suggest).

6A (Microwave oven)
Ooh, that's a big one, as the bishop said to... but seriously, most MWs
are in the 600W-800W range, so 3A rather than the 1.5kW which 6A would mean.
And is Dad cooking for real (on the cooker), or just heating something up
(microwave)? Shirley he's not using both?

8A (Electric kettle)
Yes, while it's on; but not for more than 4-5 minutes in an hour.

2A (Hi-Fi Stereo, surround sound system.)
2A (2 x Computer)
1A (2 x 17" monitor)
6A (Hair dryer)
Hang on. If Dad's cooking, Mum's ironing, and the kids are doing homework
on the three computers you've enumerated *and* watching teli *and* got the
Hi-Fi on the go, who's left to wash the hair?
0.5A (TV)
1A (Computer)
0.5A (17" monitor)
2A (Shower pump)
2A (Extractor fan)
Hmm, it's the cat in the shower?

12.5A (Immersion or water heater)
3.5A (Central heating pump)
What, immersion *and* CH on the go at once? And there's a 'stat on the
immersion; and that's a monster CH pump - steady-run current will be well
under 1A.
3A (Fan Heater 1/3hp)
10A (Lighting)
Possible but profligate!

5A (Sundry electrical loads)

That's a total 124.5 amps. OK, we can argue that their approach to energy
efficiency could be improved. In terms of rating a supply, however, a single
phase 100A supply seems inadequate.

How do my load estimations look?

They're notably pessimistic; but they do show why (a) new builds usually
have a 100A mainfuse these days, rather than the 60A which used to be
standard; (b) illustrate why the main incomer is a cartridge fuse rather than
an MCB (well, there's cost reasons too, and the vulnerability of an MCB to
kids turning them off if they're in an outdoor-accessible meter cupboard).
(The wired fuse allows getting on for twice the nominal rating to be drawn
for a good few minutes before it blows, though it'll rupture within
milliseconds with a serious short circuit.) In particular, lots of the
loads you mention are thermostatically controlled, and you list the
peak rather than the average. The heating effects in the circuit cables
(from meter tails down to final circuits) take a while to kick in, so
in practice we don't see houses either blowing their main fuses or
melting/drooping PVC cables all over the place.

There's also a difference between a single-family house - the case you've
suggested here - and "houses in multiple occupation", i.e. converted into
flats/bedsits. For a bedsit conversion, it's more likely that there'll
be multipe fanheaters/hairdryers/kettles and all sorts going on at once
- in the evening as the occupants come home, say - and Good Practice says
you apply lower "diversity" factors (that's RegSpeak for "not all the
possible loads will be on at once") in such a case, or in a school domestic
science "lab" where all cookers/oven/baby-Bellings *will* be on at once,
than for a domestic installation.

HTH, Stefek