maximum Demand/ 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 109.9Amps what am i forgeting as this cant be right can it? do i have to apply diversity again ?

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.

 

[geoffa]

hi,
using table 1B app 1 OSG

ring main (30a) no diversity = 30A. My interpretation is 100% of the biggest point of utilisation which can only be 13A then 40% of every other point which would be (30-13)*.4=6.8A. Therefore diversity = 19.8A)
cooker unit(30a) 10A+30%(fl)+5A =24A. It's actually slightly less than this - it's 30% of anything over 10% which is 20A. Therefore diversity (20*.3)+10+5= 21A or 5A less if you don't have a socket outlet)
shower(40a) no diversity = 36.9A. Agree
radial(20a) 40% = 8A. I make it 13A+30% of 7= 15.1A
radial(20a) 40% = 8A. I make it 13A+30% of 7= 15.1A
lights 66% =3.04A. I make it (700/230) * .666 = 2.02A

Total diversity = 109.92A

Note though that on page 95 of the OSG it states, 'the values given in Table 1B, may be increased or decreased as decided by the installation designer concerned' as the 'values given in the Appendix are intended only for guidance...'

 

[ianphilip007]

hi geoffa

thanks for your reply, are you saying that you calculate the max demand to be 109.92 A

 

[Amp David]

I tend to use the 0.4 way of calculating tbh

 

[Bradzz]

Just somthing in one of my books

 

[JUD]

I tend to use the 0.4 way of calculating tbh

Agreed. Have a look at #9 of Table 1B (100% of current demand of largest circuit + 40% of current demand of every other circuit)

36.9 + ((30 + 30 + 20 + 20 + 3.04) x 0.4) = 78A