Calculating maximum demand

[owls]

In my attempt to ask a billion questions on this board!!!...

Here's the next corker thats puzzling me.

I'm attempting to work out maximum demand in my house using OSG guidelines

My lighting circuits draw a maximum of 975W so 66% of that gives 3.3A
Fixed alarm circuit drawing a maximum of 3A
Cooker (not connected to a cooker at present so i've just used the basic figures) 10A + 5A for the associated socket outlet.
Final ring circuits 3 each drawing 32A (So thats 32 + 12.8 + 12.8)
Plus my shower circuit 7.5k (Max draw 32.6A)

Anyway to cut to the chase

By my calculations that gives 111.5 A
And i've got a 100A main distributors fuse (As well as the main switch)

Does the maximum demand need to be under the rating of the fuse or do you record the value regardless?

Or (this is more likely) am i getting my figures wrong?

Hope someone can help

Thanks

 

[the_jinj]

You can use your judgement with diversity - will the cooker be on with someone in the shower and all the lights on?

 

[owls]

You can use your judgement with diversity - will the cooker be on with someone in the shower and all the lights on?

Yeah totally i agree, the problem i'm having is wondering what figure i'm supposed to the record.

The guidelines specifically state not to simply write the rating of the main circuit breaker as the maximum demand.

Thanks for replying

 

[the_jinj]

The regs don't really help with this - I reckon put the full figure down - as long as the main cable is protected suitably by the fuse there isn't a problem. You are also meant to take into account any future growth in the installation so who knows where it ends!

Or if you want to you could diversify down to 100A as you see fit and put that figure down - there is no set way to do the diversity so down to your judgement.

EDIT - and at 111A how long would it all need to be on to blow that fuse

 

[owls]

I've managed to come up with the figure of 99.24 A

Mainly by classing the Shower circuit as "Power and heating" rather than "instant water heater".

Thanks a lot for the advice, sometimes it just really helps to get another opinion.

 

[the_jinj]

I've managed to come up with the figure of 99.24 A

Mainly by classing the Shower circuit as "Power and heating" rather than "instant water heater".

Thanks a lot for the advice, sometimes it just really helps to get another opinion.

What about when I add another socket and 50 downlighters in there.....

 

[owls]

this house is officially having no more sockets or downlighters!

 

[Adam_151]

Switch the shower on, a few lights, boil the kettle, and have an 'average' assortment of appliances on, and use a clamp meter on the tails.

Assuming nearly 60A for the rings is silly, you are not going to be using that many appliances at once, the upstairs and downstairs rings are separate for convience, not because they are heavily loaded, infact the only heavy-ish load is the kitchen, and you could probably have one single ring for the whole house cos the others have next to no load on them...

Quite a lot of appliances in the kitchen once up to temp come in and out with a thermostat, the kettle doesn't draw power for that long, the washing machine and dishwasher draw power only for set parts of their cycle, the humble tumble dryier doesn't do any favours for max demand, and will munch 2kw constantly for over an hour... (so all in all, the kitchen probably doesn't draw as much as you imagine anyway...)

And if you do work out demand too a little too low, and its exceed a little now and again, its not the end of the world... the 100A main fuse will take 150A at least long enough to shower

The max demand here according to the OSG is 116A, the service fuse is 60A, it works fine and has done for years (probably because max demand is no where near 116A in reality... I'd say that our current draw probably peaks at about 60A... but thats just a rough guess)

 

[plugwash]

the diversity guidelines in the regs are just guidelines and they probablly assume that the rings will be serving near thier max allowed floor area (which in modern domestic they very rarely are).

normal approach on domestic is if the service fuse holds then don't worry about it.