Recording Max Demand

[Jonnyboy 68]

Hey guys... a bit of help pls from all the senior sweats.. from a very keen but green spark !!

In relation to recording Max Demand on the cert... Having gone through the whole installation and applied diversity as per the OSG ..I still find my Max Demand exceeding my CU main switch and suppliers cut out of 100A ?? I've read a number of threads but I'm not sure what to record ??

I think this is right ..The property is a bungalow ..I've got ;

16 luminaires @ 100w = 6.96 x 0.66 = 4.59A

Main Ring = 32A

Kitchen Ring = 32 x 0.4 = 12.8A

Radial to Garage = 16 x 0.4 = 6.4A

Radial to Outside Sockets = 16 x 0.4 = 6.4A

Cooker 40Kw = !0A + 30% of 30A = 19A

Shower 10.8Kw = 47A

TOTAL = 128A

Help me lads ... What figure would i record on the cert ????

Ur help is appreciated !!!

 

[sparkytim]

Thats a mighty big cooker ! 40 kW ?

The 10.8 kw shower is probably 240 Volt rating= around 9.6 kw @ 230 Volt

and in practice you are allowed to exercise judgement on these things.

OSG Pg 95 last paragraph.

 

[mikep]

What are you cooking with that cooker is it for Mr Kipling !! surely 180 amps not right You mean 4Kw

I'd just say add them and times them by 0.42 Thinkthat comes out at 54amps

But will try and get me book when I come back, going out in a min !!

Regards, Mike.

 

[m4tty]

16 luminaires @ 100w = 6.96 x 0.66 = 4.59A

Main Ring = 32A

Kitchen Ring = 32 x 0.4 = 12.8A

Radial to Garage = 16 x 0.4 = 6.4A

Radial to Outside Sockets = 16 x 0.4 = 6.4A

Cooker 40Kw = !0A + 30% of 30A = 19A

Shower 10.8Kw = 47A

Assuming these are the CPD's used

Lights 6A

Main Ring 32A

Kitchen Ring 32A

Garage Radial 16A

O/S Socket Radial 16A

Cooker 32A

Shower 45A

You can take the highest one out which is 45A Shower then add the rest up, times by 40% then add the highest one back on. Its in the IEE Design book which looks like the OSG but has a grey front cover.

134A x 0.4 = 53.6A + 45A = 98.6A

Thats assuming ive guessed the CPD's that you have there.

Hope that helps

Cheers

Matt

 

[mikep]

Beat ya MATTY !!

But forgot to add the shower headbang

And how come only 2 ring mains. Bungalow !!

 

[m4tty]

Lol Mr Pitts both had the same idea. CheersGuinness

 

[Jonnyboy 68]

Ha ha ... cheers boys !!!!!

An thanks for pointing out my glaring mistake on the cooker .. if i ever get any jobs at 'kingsmill' The max demand will be a breeze for me !!!!!

Seriously .. thanks lads.. It seems that there are a number of different formulas in use and as a newby I'm just trying to settle on one that I am comfortable with, is easy to calculate and realistically accurate.

I'm finding this forum very useful and picking up some great tips that they neglect to mention in the classroom... just goes to show that there's no substitute for experience

Anyway... until my next 'hoofed up' thread ..... Anyone for Scones !!!!!

Ta

 

[SPECIAL LOCATION]

Just to add to what the other guys have said..

Heres another thought for you to keep in mind when considering MAX demand..

With the 17th edition and multiple RCD protected installations, there may often be MORE circuits than actual load an installation demand actually need..

but they are there to MINIMISE INCONVENIENCE IN THE EVENT OF A SINGLE FAULT.. e.g. Reg 314.1

Consider a simple hypothetical example

Old style 4-way wylex box

30A - Cooker

30A - Sockets

15A - Imm heater

5A - Lights

Total 80A!

Rule of thumb 42% max demand 33.6A

New CU is fitted.. but lights and sockets are both split up and down...

and a new additional dedicated smoke alarm circuit added, so now..

32A - Cooker

32A - Socket#1

20A - Socket#2

16A - Imm heater

6A - Lights#1

6A - Lights#2

6A - Smoke Detectors.

Total 118A!!!

Rule of thumb 42% max demand 49.6A

So theoretically by replacing the CU and adding a Smoke Det circuit we have increased max demand by 38A or 16A

Which obviously hasn't its just the appliances and loads are divided across more circuits!

Guinness

 

[Jonnyboy 68]

Cheers trailerboy..... ur makin a lot of sense to me

It seems that some estimate the 'actual' load as it stands via the standard calcs and others calculate the 'potential' load by the sum of all the rated MCB's ...

Out of interest mate .. where can i find out about the 42% rule of thumb ??

Told you ... All questions me !!

Cheers

 

[binky]

Last time I looked (which was a while ago) cookers are 10A plus 5 A if cooker point is fitted with a socket. The max demand calculation is more a guide figure, it's about making you think about the likely max demand on the installation and frequently exceeds the 80A main fuse or 100A main switch, and as Special quite rightly points out, the more circuits you fit the higher the figure calculated - which is why I tend to regard it as more nonsense than use (especially within domestic premises) as I like to split a house into more circuits rather than less!!!

The 42% thing is an alternative calculation method which I understand is favoured by the DNOs, and is also the calculation I use, cos it's easier than the other methods.

 

[Jonnyboy 68]

Cheers binky .... all good mate !!!

So ur team will be playing in a division lower nxt year too eh mate ... just like the mighty burnley !!!

not to worry pal ... the beer still taste the same eh !!!!!!!!!!

Cheers mate

 

[SPECIAL LOCATION]

Cheers trailerboy..... ur makin a lot of sense to meIt seems that some estimate the 'actual' load as it stands via the standard calcs and others calculate the 'potential' load by the sum of all the rated MCB's ...

Out of interest mate .. where can i find out about the 42% rule of thumb ??

Told you ... All questions me !!

Cheers

The bottom line is ..

"THERE IS NO ONE-SIZE-FITS-ALL CORRECT CALCULATION"

Maximum demand has to be based upon the specialist knowledge of the person evaluating that particular installation, and knowledge of the persons who will be using that installtion....

See Paragraph 2 of page 95 Appendix 1, Max Demand, One Site Guide.

Just to give you more food for thought...

As well as individual circuits and their connected loads, you do actually need to take account of the times of day and the duration that these loads are in operation..

( The only accurate way would be to have mains monitoring equipment recording power consumption over a set period of time. )

For example consider the following circuits on another hypothetical installation.

A 9.0Kw electric shower (39amps), protected by a 45A MCB.

A 7.0kw electric oven (30amps), protected by a 32A MCB.

Three 2.7Kw night storage heaters (11.7amps), protected by 3 x 16A MCB's.

Lets say we use the 100% of highest rated circuit + 40% of all other circuits to calculate max demand...

Calculating using fuse ratings we get 45+(0.4x(32+16+16+16)) = 77amps

Calculating using load currents we get 39+(0.4x(30+11.7+11.7+11.7)) = 65amps

(Already dropped Max demand down by 12amps)

But now lets consider an imaginary family who uses these circuits...

2Adults + 1Child...

The family are very routine in their daily patterns.

a) They have two showers each per day, 10mins max; one in the morning between 6:30am & 7:00am when they get up and one between 10:00pm & 11:00pm each evening before bed.

B) They have a fully cooked meal each day between 5:00pm & 8:00pm using 65% of the ovens capacity. (Approx 20A)

c) All three of the storage heaters come ON at 11:00pm and off at 6:00am

SO...

considering Max demand now...

The shower, cooker and Storage heaters are never on at the same time.....

The shower is only taking 39amps for 10 minute duration max.

The storage heaters are taking 35.1amps for 7hour duration.

The cooker is lower than both so can be ignored!

So the reality of the max demand for these circuits that will cause max heating and other loading on the incoming supply cable is the 35.1amps of the night storage heaters because they will be running for hours at a time!

Although the shower is a higher current the short duration means that you could reasonably consider it less of a demand on the supply cable than the storage heaters.

So we could say max demand is the 35amps... well below the calculated 65 or 77 amps!

(If our family moved out and a larger family moved in the demand will be totally different)

So max demands not purely related to the circuits on their own!

HTH

:coffee