Maximum demand

[GrannySmiff]

Hi,

A while ago I remember somebody mentioning a quick way of working out max demand with out doing it properly and it came out roughly the same as if you work it all out. It was something like add up add the rcb's and divide by 4 or something.

Anybody know what I am on about or was is this a figment of my imagination.

Thanks.

 

[m4tty]

Take the largest breaker out. Add the rest then multiply by 0.4 then add the largest breaker on. It's in the design book which looks like osg. It's grey

 

[GrannySmiff]

Nice one, just worked out what I worked out earlier properly and it came out exactly the same. is this always the case can we go by this as gospel?.

Thanks.

 

[Mark.W]

How does it work then if the largest breaker is a 32A, and we have 4 of them?

 

[m4tty]

I know what your saying mark it's a bit hit n miss but that way is endorsed by iee in their books

 

[Mark.W]

I don't dispute that at all mate, it was just a question? I always take maximum demand depending on size of Main switch, If it's a 100A switch I put down 100A as Maximum demand! That's the way my old boy taught me anyway!

 

[SirKit Breaker]

I add the lot up and times by 0.4, but then i am a bit thick with the old maths, so i keep it like myself....SIMPLE

 

[ezzzekiel]

I always take maximum demand depending on size of Main switch, If it's a 100A switch I put down 100A as Maximum demand!

lot of people do that but it is wrong, max demand after diversity can be applied by either following tables or using rule of thumb.

you couldnt ask for a single phase supply as your demand was 100a when it turned out you needed a 3 phase supply.

 

[GrannySmiff]

well my example was this.

1) 32a shower (wrong I know but its at college training board) 8.5 kw, no diversity
2) 32a ring final no diversity for first ring
3) 32a sub main to rcd cu with a 16a radial and 6a light circuit with one light. no diversity for first radial
4) 16a sp motor 370w no diversity
5) 6a light circuit, with 4 lights plus the outside one makes 5 in total so 500w apply diversity of 66% and that makes 330

so totals for each circuit are
1)36.9
2)32a
3)16a for the radial and one light consider with other light total
4)1.6a
4)1.4

added up comes to 87.9

the quick method of above is 32a 32a 32a 16a 16a 6a 6a take away one of the biggest leaves 32a+ 32a+ 16a+ 16a+ 6a+ 6a = 108 x 0.4 = 43.2 + 32 = 75.2

that is a bit of but close earlier I forgot to take away the biggest and just multiplied them all by 0.4 then added the biggest back into it and it came to 88 exactly right. Is there any chance that that is how to do it. Or is it the fact I am counting the 32 supplying the sub main which has a 16 and 6 on it messing up the calcs?.

Cheers.

 

[Risteard]

Diversity is really down to judgement.

For example, if there is a lighting circuit where all lights are going to be on at the same time then there is no sense in applying diversity factors to that!

 

[davelerave]

you could have a workshop with 10 machines

only 1 man working 1 machine at a time

or 10 workers and 10 times the demand

calcs are useful but judgement is required

 

[Jay Sparks]

All this info sounds great. The last place I worked for doing testing on new rewires and part rewires they told me to put down half the size of the main cut out as the max demand. i.e 100a cut out - 50a max demand. Because I was pretty new to testing I didnt question it as they were sending the certificates to NICEIC and I must of done over 150 of them. But now I know to take out the largest then add the mcbs up and times by 0.4 then add the other mcb on is a big help. But like some have said personal judgement also has to come into it.

Cheers Lads

 

[GrannySmiff]

How many of you guys would turn down a job if it involved taking the maximum demand over the cut out, when in reality we know its prob not going to happen.

In the regs book if you look at the time graph thing for the cut out (can't remember the bs no something like 1361) for a 100a it could take 200a for about 50mins before it goes.

So lets say you calculate maximum demand to be 120 officially do you walk away or just put down 120. I am talking domestically here.

 

[ezzzekiel]

just done rewire which came out as 122a after diversity 80a at cut out and was used for my annual reassesment and was told to put 80a as max demand so go figure

 

[keniff]

I don't dispute that at all mate, it was just a question? I always take maximum demand depending on size of Main switch, If it's a 100A switch I put down 100A as Maximum demand! That's the way my old boy taught me anyway!

when i had my assessment they said never put the incomers fuse rating as maximum demand because that means you are saying this installations demand is up to the max and nothing else can be added. i mean it makes sense if you had a little house and had just a lighting circuit and ring main and cooker for example and you put max demand at 100 amps theres no way these would be able to pull 100 amps!! i always take a view on the job in hand, i guess this goes for the switch aswell hope this helps someone