max demand

[mule83]

hi, i've been racking my brain and cant seem to work it out, i have an installaton and am trying to work out the max demand, hopefully someone will be able to give a correct figure, i have a main switch of 100A, and the following circuits in the DB.

circuit= Amps / after diversity
1st floor lights= 6 / 3.96
ground lights= 6 / 3.96
extension 1st floor lights= 6 / 3.96
extension ground lights= 6 / 3.96
garage supply= 32 / 22
down ring= 32 / 32
up ring= 32 / 12.8
ext down ring= 32 / 12.8
ext up ring= 32 / 12.8
shower= 32 / 32
cooker= 32 / 15.26

total = 154.6 Amps

i would be grateful if someone would know and tell me how to get this down to a figure of less than 100A.

ive been reading through the forum and see that many people say to just put down 100A, but i know this isnt an explanation to an assessor if he was to come out and check my installation, some help would be very much appreciated! cheers

 

[cleverspark]

Hi Mule,
Well don't expect a black and white answer here as I don't think there is one.
There is many scenario's for this case from what i have seen and also read.
you can do as you have done and used the OSG to the letter, which on most scenario's it would prove that you are using more current then your main switch of 100A, and more probable then not your main incoming fuse of 60A.
I have been taught previously that you do all the calculations as you have done and then apply a further diversity factor of multiplying this demand by 0.4 (cant remember where that figure arrived from?)
This would still give you over 60A which a lot of the times is effectively greater then your main incoming fuse. (even if it is just over and would never blow a 60A fuse)
A lot of people also just put the main fuse rating down (ie 60A).
I guess the most accurate way of doing this would be to use a current clamp and then measuring it, but again, from day to day use the rating may change, so again another dead end.
One thing that also makes me wonder about this is that we have now started to split all rings up and down and possibly kitchen, same with lights etc. But lets say we wired old school with only one ring etc then after diversity we would be no where near the load capacity but still able to use the whole house the same as we can with multiple rings.
So at the end of this waffle, I guess the easiest solution to keep all your records in line is to just use the main fuse figure, again making a mockery of BS7671.
Just my thoughts
Cheers

 

[flameport]

What is in the onsite guide is exactly that - a GUIDE only. The method in there probably worked when houses had 3 final circuits (cooker, sockets and lights) but is quite useless with new installs with 10 or more circuits, as increasing the number of circuits is often just for convenience and safety, rather than additional load.

Other ways to calculate total demand:

Lighting - add up the total wattage of all of them, then apply a suitable factor, e.g. possibly average of 40% of them on at the same time (could easily be less - think about which rooms are likely to have lights on at the same time). Lights are not usually on in the daytime, dimmers reduce the total even further, some lights (toilet, hallway) might only be on for a very short time, etc.

Kitchen socket outlets will have fairly heavy duty appliances connected, but these won't be on 24/7, and even if everything was on at the same time, the total load will be less than you may imagine, as they will only draw the full load current when the heating elements are on.
Fridges and freezers although connected 24/7 cycle on and off all the time, and the average consumption is very small.

Other socket outlets - probably minimal, as most other appliances use small amounts of power. Try adding up the total wattage of all appliances in your home - and then consider would they really all be on at once?

The figures for your 4 ring finals add up to over 70A, or to look at it another way, the same as having 16kW of heating on. No way.

Garage - 22A is over 5kW which is huge (unless you have a 24 hour drive through welding shop or are growing certain plants in there!).

Cooker - 15A when on is certainly possible (could even be more), but will this be on at the same time as everything else?

Showers - yes, the OSG states no diversity allowed as they are either off or on full, but showers are on for a few minutes at a time, and certainly not at the same time as all the other appliances in the house.

You can alternatively base the total load on the area of the property, e.g. watts per square metre. Also consider how many people live in the property, are they all in at the same time, etc.

As for explaining this to an assessor or anyone else - the point is that you can explain how you arrived at the figures, rather than just quoting something from a book. There is no 'correct' answer, it is based on your assessment of the installation and how it will be used.

Finally - consider that most DNOs allow less than 2kW per house when planning power requirements for new housing estates. This is calculated on the average load over a period of time, clearly the total is more than this at certain times, but on average it is very small.
[/i]

 

[Katron]

Very interesting and useful replies.

 

[Adam_151]

Going on what clever&flameport said, try assuming the minimum amount of circuits required, rather than the number actually installed, try for example:

circuit= Amps / after diversity
1st floor lights= 6 / 3.96
ground lights= 6 / 3.96
kitchen ring= 32 / 32
down ring= 32 / 12.8
up ring= 32 / 12.8
shower= 32 / 32
cooker= 32 / 15.26

total = 112.8 Amps

Then look at those figures, and think what you'd have to have going to use that much... and knock a bit off to compensate for the high OSG when figures when applied to an installation as a whole

 

[GaryMo]

Diversity is applied to final circuits and then overall diversity is applied to the installation.
It's generally based on experience and not an exact science.
A middle aged couple with four teenage daughters may inflate the figure compared to an elderly couple for example.

 

[mule83]

just like to say thankyou to everyone who replied, its very much appreciated and been very helpful. i guess ive learnt its not exact or text book figures which is what i first assumed.

after looking at my circuits again i came to a figure of after diversity which = 120Amps, then applying a further total diversity i came to a figure of 86Amps, taking into account what would be used most at peak times, which is based on a family of four. as someone said its not text book, but i should be able to explain my figure. does this 86Amps sound ok?

or is the 40% rule which someone mentioned a better guide, so 120Amps x40% = 48Amps, this figure sounds better to me, which would you suggest i go with?

thank you again for the help

 

[plugwash]

Lighting - add up the total wattage of all of them, then apply a suitable factor, e.g. possibly average of 40% of them on at the same time (could easily be less - think about which rooms are likely to have lights on at the same time).

IMO in a fully occupied house there is a very high probability of all lights being on at once for substantial periods of time. To design a lighting circuit that does not allow for this is IMO totally moronic.