Complex Installations demand calc

[Michaelwgroves]

I've treated myself to the IET Electrical Installation Design guide. I've been looking at section 3.5 Complex Installations. There is a calculation for calculating demand based on number of dwellings in a group.
Please see picture of relevant page.
Seems simple, sub domain is 66.44A, demand factor (g) for 8 dwellings is 0.22
The calculation is therefore 66.44 + 0.22(8 -1)66.44 = 168.76A
So my question is what do I need to type into the calculator to get the magic answer of 168.76A ?

As a side note can you see the image okay?

 

[Michaelwgroves]

Looks to me like link is not working, how about now
20180114_172513344_iOS.jpg - https://1drv.ms/i/s!ApQQCRGg1ka_gv4PRpTYjGK5EMA17w

 

[Wilko]

Hi Michael - not sure I understand correctly as the equation in OP equals 168.76A ... but you have that already (?) . I can see the pic now, so I'll take a closer look at it.

 

[buzzlightyear]

I've treated myself to the IET Electrical Installation Design guide. I've been looking at section 3.5 Complex Installations. There is a calculation for calculating demand based on number of dwellings in a group.
Please see picture of relevant page.
Seems simple, sub domain is 66.44A, demand factor (g) for 8 dwellings is 0.22
The calculation is therefore 66.44 + 0.22(8 -1)66.44 = 168.76A
So my question is what do I need to type into the calculator to get the magic answer of 168.76A ?

max demandhttp://www.napitonline.com/downloads/CP%204%2007%20P%2010-11%2016th%20Diversity.pdf

 

[Andy78]

Looks like a misprint to be honest.
Factor g is usually a multiplier for diversity so wouldn't make sense to add it. This seems to get you to the printed answer though

(66.44*0.22)*(8-1)+66.44

Workings:
66.44*0.22 = 14.6168
14.6168*(8-1) = 102.3176
102.376+66.44 = 168.76

I would send the IET an email asking for clarification. Could be a misprint or you could have a counterfeit copy.

 

[Ian1981]

Andy is correct and it is a misprint as I have the same book.
In fact there’s several mistakes in it such as giving the max zs of a type b 32 amp mcb as 1.44 ohms then directly underneath or above this example it gives the correct figure of 1.37 ohms.
Also uses old reference methods such as reference method 1 and reference method 3 etc.
Still a decent book tho.

 

[buzzlightyear]

I would send the IET an email asking for clarification. Could be a misprint or you could have a counterfeit copy.

the IET they do not make mistakes .
arthur and the round table will see to that.

 

[Wilko]

Without having the book or the standard I'll put my tin hat on and say "it looks ok to me"
100% of one plus factor times the rest (?)

 

[SparkyChick]

66.44 + 0.22(8 -1)66.44

That isn't a misprint in the mathematical sense... it translates to:-

66.44 + (0.22 x 7 x 66.44) which does indeed equal 168.76A

If the values are wrong that's a different matter but mathematically it is correct.

 

[Andy78]

66.44 + 0.22(8 -1)66.44

That isn't a misprint in the mathematical sense... it translates to:-

66.44 + (0.22 x 7 x 66.44) which does indeed equal 168.76A

If the values are wrong that's a different matter but mathematically it is correct.

Now I'm admittedly not the best at maths and never have been but that just confuses me.
The way it is written in the book does not equate to the way you have written it in your third line in my mind. Help a guy out to understand that ?

 

[Michaelwgroves]

My 11 year old daughter has just taught me about BODMAS. Brackets, Orders, Divide, Multiply, Add, Sub
I now agree with Sparkychick,

 

[Andy78]

My 11 year old daughter has just taught me about BODMAS. Brackets, Orders, Divide, Multiply, Add, Sub
I now agree with Sparkychick,

My 12 year old sons have just taught me their version, BIDMAS, which also now makes sense. Some things I just never grasped. Don't think I'm wired the right way.

Not as bad as when they got given long division problems at age 9 and I had to teach myself it for the first time to be able to help them. What a week that was !

 

[SparkyChick]

It's about the order of evaluation and some basic algebra.

If you have A=2 and B=3... AB=6. This is an implied multiplication. And is either not shown as an operator or is shown as "x" or in the modern age "*".

So A x B, A * B and AB are mathematically identical, all giving the value 6. The order of evaluation is as @Michaelwgroves has stated... Brackets, Orders, Divide, Multiply, Add, Sub

For the most part, divide and multiply are interchangable as are add and subtract.

So using:- 66.44+0.22(8-1)66.44 as an example

Step 1 is evaluate the brackets (8-1) = 7
Step 2 is evaluate the multiplications 0.22 x 7 x 66.44 = 102.3176
Step 3 evaluate the addition 66.44 + 102.3176 = 168.7576

And to see it using something simpler:-

1 + 2 x 3 should equal 7 but if you get it wrong it might be 9, which would equate to (1 + 2) x 3

Hope that helps for anyone that doesn't have kids handy to explain it to them

Indices and orders for those that aren't familiar are things like 2 squared and square root.

Edit:- You may also see "." being used to indicate multiplication but this is less common as it has the potential to be confused with the dot product in relation to vector and matrix math.

 

[Ian1981]

The way it’s written in my book page 42 is 66.44-0.22(8-1)66.44=168.76 A
So the - should be an addition.

 

[SparkyChick]

In my copy it is definitely 66.44 + 0.22 (8 - 1) 66.44.

Which is as I've described. 66.44 + (0.22 x (8 - 1) x 66.44)