applying diversity

[ianphilip007]

hi guys

ive got myself a little confused here when trying to calculate a max demand figure for the installation below.

60amp TNCS supply to a bungalow.

1x 30A Ring Main
1x 30A cooker unit(with socket)
1x 40A Shower (8.5kw)
2x 20A Radials
1x 5A Lights (7x100w)

using table 1B app 1 OSG

ring main (30a) no diversity = 30A
cooker unit(30a) 10A+30%(fl)+5A =24A
shower(40a) no diversity = 36.9A
radial(20a) 40% = 8A
radial(20a) 40% = 8A
lights 66% =3.04A

using these figures it comes out at a diversified current of 109.9Amps what am i forgeting as this cant be right can it? do i have to apply diversity to the diversified currents

when i use my usual method of adding up the MCB's and then multiplying by 0.4

40+30+30+20+20+5 x 0.4 = 58 Amps

which is what id expect. any help would be appreciated.

 

[malcolmsanford]

Ian the 0.4 is exactly the method I use and was shown to me by a DNO engineer many years ago as a good rule of thumb, and something I always use as that a rule of thumb, so IMO mate that looks just about right.

The thing is the shower. Quite right the OSG do have the take that it is a fixed load, and so no diversity, I always felt that was the right way to think when you look at it as a final circuit, but not in an installation as a whole.

As said before the OSG is a good guide, but it is just that, and even this tells us in the section on diversity that there are many ways to treat the subject, and eventually the amount of diversity done is purely down to the designer and experience.

 

[ianphilip007]

thanks Malcom

i usually times the total of the breakers by 0.4 (rule of thumb) for max demand.
when using the OSG table 1B to work out diversified currents would this total figure then be multiplied by 0.4 ? if this is correct then it makes sense.

 

[ianphilip007]

on a similar note ive noticed that the OSG states that "the C.U should be of a sufficent rating to take the total load connected to them with out the application of any diversity".

so in the case above the total load connected would be 125Amps if no diversity applied. i find this very confusing and hard to understand. help pls lol

 

[Sparky Ninja]

With working out the maximum demand and applying diversity I get approx 74.2 Amps. (I have not referred to the book for it though).

 

[Sparky Ninja]

on a similar note ive noticed that the OSG states that "the C.U should be of a sufficent rating to take the total load connected to them with out the application of any diversity".

so in the case above the total load connected would be 125Amps if no diversity applied. i find this very confusing and hard to understand. help pls lol

What current rating is the unit?

 

[ianphilip007]

its 60 amp main fuse, the rating of the mainswitch of the unit is 63Amps

 

[Sparky Ninja]

its 60 amp main fuse, the rating of the mainswitch of the unit is 63Amps

Well I'd advise to either reduce the demand of the shower or increase the nature of the supply.

 

[ianphilip007]

widdler how did you come up with the 74.2A ? is that by using the OSG table 1b and applying diversity or by adding mcb's ratings and x 0.4 ?

 

[ianphilip007]

i calculate the max demand to be 58A when adding mcb's and x 0.4

 

[Bradzz]

just somthing in one of my books

 

[Sparky Ninja]

I use the IET design guide.

 

[Sparky Ninja]

View attachment 5237 just somthing in one of my books

Word for word that is utter rubbish.

 

[Bradzz]

Just somthing in one of my books, the book is called the Part P Doctor, Alan Lynch if you look him up he is always welcome to coments.

 

[nbashir786]

Hi Guys, I see the 0.4 figure being applied as a rule of thumb to determine MD. Where does the 0.4 figure come from other than the DNO..