Design current: help working it out and how to do it

[MrS]

Im a little confused about working out design current.
If for example I wanted a lighting circuit and I wanted 6 lights on it. I would count up the watts for each light (assuming the minimum as 100) and then divide it by 230V to get Amps. If I had 6 lights all at 100W each is my design current 2.6A?

Do I apply diversity to this figure before I say its my design current? If so, that would be 0.66% (I think its 0.66% for lighting) and so would be 1.72A?

If it’s the above one of 2.6A I would then use a 3A or a 6A mcb and so is that my design current? Is it the maximum that will be on that circuit?


Lighting seems simple once the above is cleared up, but what about socket outlets?
How do I work out the design current for a ring of socket outlets? Do I have to find out everything that’s going to be plugged in, or think of everything that might be plugged in, then find out how many watts that appliance takes, then divide it by 230 to get the Amps? Do I apply diversity before I get my design current?


The kitchen then also gives me a little confusion as this will carry a greater load than any other socket outlets in the house. How do you work out design current for a kitchen? Do you once again have to work out what appliances are going to be used and work out the total watts then the amps?


Please help me get my head round this.
I do have an OSG but seem to have missed this and it doesn’t seem to be listed in the contents at the back.

 

[TONY144]

you have to work out maximum demand and then apply diversity

 

[comms]

Osg table 1A & 1B.
Demand & diversity page 84 osg.
As others have already said you do sound a little out of your depth are you at college? if so say so you may well get a little more help, at the moment people will just assume your on the cadge for free advice for a rewire job, it's ok having the books but you do need to know how to use & interpret them.

 

[MrS]

sorry i should have said. yes im going to be doing my 16th edditon at college and i am doing some backgroung reading before i start. I always like to arm myself with knowledge before jumping into things. i was confused on a few things and so did a search on google and this forum came up. i did a search through here and not all of my questions were answered. i have also tried to apply things to real life situations and see how things would work but it does seem some people have assumed im after free advice.

i will read that section in the OSG again and see if it makes more sense.

at least i now know that you find the maximum load and then apply diversity to get the design current. this is the reason my calculations when i got to the voltage drop section didnt work out properly because i was assuming the design current to be wrong.

if i make any more posts i will explain the situation first just to clear things up.

 

[andy]

but it does seem some people have assumed im after free advice.

all advice here is free...

 

[dingbat]

You will not get very far with just the OSG because it is designed as a on-site (big clue in the name there) reference for working electricians.

Try John Whitfield's "The Electrician's Guide to the 16th Edition of the Wiring Regulations - BS 7671" by EPA Press. It is much more of a textbook and actually explains in some detail what you need to know.

If you are out of your depth in college that's because the 2381 course is not designed to teach you about electrical installation theory. It is an industry course whose primary aim is to update electricians trained under previous versions of the regs. Passing this will not make you an electrician, although you might just be able to struggle your way round a discussion of the regs.

 

[MrS]

for lighting dising current, you add up all the power of the lights. you take each one to be a linimum of 100w even if its lower, then apply diversity of 66% to get the design current.

a lighting circuit supplying 6 lights at 100w each would mean that (6 x 100) / 230 = 2.6A to which you then supply diversity and you get 1.7A design current

cooking appliances. you work out the current that the appliance uses, the you apply diversity by taking the first 10A, then add on 30% of the rest and add on 5A if there is a socket outlet on the end of the cooker circuit.

socket outlets. this im still not sure on. it says that is is 100% demand of largest point of utilisation and then 40% of all other points. how can you know exactly whats going to be plugged into sockets in the house?

 

[andy]

cooking appliances. you work out the current that the appliance uses, the you apply diversity by taking the first 10A, then add on 30% of the rest and add on 5A if there is a socket outlet on the end of the cooker circuit.

FWIW - i would not apply too much diversity to a cooker circuit, otherwise you get problems at xmas etc when everything on the cooker is on

 

[dingbat]

socket outlets. this im still not sure on. it says that is is 100% demand of largest point of utilisation and then 40% of all other points. how can you know exactly whats going to be plugged into sockets in the house?

Here we go... You don't what's going to be connected. That is one of the reasons we use ring final circuits for domestic circuits.

You've also read the diversity rules wrong. On page 87, it's not item 10 you want, it's item 9, which says "100% of current demand of largest circuit..."

Seriously, get the textbook.