Diversity CU rating

[apprentice87]

Hi, here is a good one for you all!! Now then, i am an aspiring electrician, studying [myself] for my 17th ed, and i try to get things right.

Anyway, Someone asked me today if it was ok if the total connected load to a CU, was more than the main switch rating [say 100A]

I said no it most certainly was not ok, as the incomer had to be sized to switch the total connected load BEFORE any diversity had been applied.

I then found something on the internet from NAPIT that claims yes you can. [apply diversity that is]

Here is the article concerned;

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

Now then, I DO NOT CARE WHAT NAPIT THINK. I always understood that a switching device had to be rated for the full connected load, even better, at the very bottom of page 96 of the OSG it says "It is important to ensure that distribution boards or consumer units are of sufficient rating to take the total load connected to them without the application of any diversity"

Since if i burn someones house down, the prosecution is going to say that, "he did not adhere to the instructions issued by the IEE, and M'lud, they wrote the wiring regulations which can be relied upon in evidence" so lock him up for 35 years, i think i would rather listen to the IEE than NAPIT.

What you all think???

 

[Andy™]

its been done to death.

there is nothing to stop the total value of all the MCB's adding upto more than the rating of the incoming supply.

typical example

a house has 1x 30A ring main and 1x 5A light circuit. total 35A

i re-wire it with

3x ring (up/down/kitchen). 96A

up/down lights - 12A

boiler - 6A

total 114. yet a few days earlier it was perfectly fine running at 35A even though nothing has changed

 

[matt.leung]

if u add everything up in a house hold with no deveristy, it will always come to more than 100Amps, but what is the likleyhood of everything drawing full power at the same time.

U need to apply deversity to each circuit after the main switch and not before, otherwise it would be impossible to carry out the installation.

 

[Andy™]

and then the DNO use somehting like 2KW per property for their calcs....

 

[Manator]

I think you may be getting a little confused as to which circuits can have diversity applied and which can not. Instant water heaters, storage heaters and the like for instance should have no diversity applied because the total demand over a long period would be greater than the main switch could handle if it was a large load. The shower however could have diversity applied because the service cable will take time to heat up and the shower will be turned off probably far sooner than the cable heats sufficiently to trip the main fuse. Diversity is important, and largely misunderstood, if no diversity was applied most new installs would require 3ph supplies, given the splitting of circuits and the number of available ways. When in fact the demand is just the same as before, but the circuits have been split. To see how diversity works try plugging the kettle in and the shower, then add the microwave, record the reading with a clamp meter, and watch what happens when each appliance finishes its cycle, you will soon see it is almost impossible for a standard house to ever be over the allowable maximum demand of the service fuse.

 

[matt.leung]

i have always read it that for a shower circuit deversity can not be applied.

 

[Zeespark]

If you want to see what a fuseboard looks like without diversity applied look at a US/Canadian fuseboard. 36 odd circuits for one home.

Andy

 

[Manator]

@ Matt

IEE Guidance note 1 , you are correct table 1 states no diversity allowed, however if you are designing the circuits and distribution, you could apply a demand factor of 80% of the final load, it does not bring the total demand down by much but it is a well known design method of calculating the total designed demand including allowable diversity.

 

[gordy71]

On a cert do you write down the max load of the circuits with diversity applied, or just the main switch rating?

 

[Andy™]

On a cert do you write down the max load of the circuits with diversity applied, or just the main switch rating?

fuse will often be lower than main switch

 

[SPECIAL LOCATION]

Now then, I DO NOT CARE WHAT NAPIT THINK. I always understood that a switching device had to be rated for the full connected load, even better, at the very bottom of page 96 of the OSG it says "It is important to ensure that distribution boards or consumer units are of sufficient rating to take the total load connected to them without the application of any diversity"

Since if i burn someones house down, the prosecution is going to say that, "he did not adhere to the instructions issued by the IEE, and M'lud, they wrote the wiring regulations which can be relied upon in evidence" so lock him up for 35 years, i think i would rather listen to the IEE than NAPIT.

What you all think???

Well I think

1/ You need to pay more attention to what Napit say.. (and not assume you are more knowledgeable than they are...)

2/And I think you need to go and read the rest of the comment at the bottom of page 96..

The Note refers to table 1B over on page 97..

Specifically rows 6,7 & 8

look down the second column where the little cross symbol that the note refers to..

As Manator says..

its applicable to:-

Water Heaters (thermostatically controlled)

Floor Warming Installations

Thermal storage space heating installations

NOT..

Just CU's in general!!!!

:|

---------- Post Auto-Merged at 20:15 ---------- Previous post was made at 20:13 ----------

On a cert do you write down the max load of the circuits with diversity applied, or just the main switch rating?

What is the question on the cert asking?????

Max Demand?

OR

Switch Rating?

go back have a look again at the certificate wording!

Or does it ask for both..

in different boxes?????????????????????????????? ?:|

 

[gordy71]

[quote name='Andy

 

[SPECIAL LOCATION]

for example try the model form on page 138 of OSG..

Top o the page you have the box for MAX DEMAND

little bit further down you have MAIN SWITCH OR CIRCUIT BREAKER..

including....

No of poles

Current Rating

eh!



was that a penny dropping I just heard??

Guinness :Salute

 

[gordy71]

Kerching! So diversity has to be applied, cheers, it was just one off the top of my head, you remind me of one of those teachers who forces you to think about things then makes you feel stupid:coat Thanks all the same

 

[SPECIAL LOCATION]

I have been reliably informed Stupid loves being felt....

But for the life of me I dunno why Admin calls Admin2 Stupid?!

] :C ; \

 

[Manator]

Working for the answer teaches you more than being given it, it helps you understand the working of the question to answer stages.

 

[apprentice87]

Hi all!! Firstly, Thanks for all the replies!!

Secondly, [and i address this to "Special location"] With the greatest of respect, no, I do not think i know better than NAPIT at all, and am not trying to be clever or smart either, but merely to learn, but surely, does not the sentence in the OSG;

"It is important to ensure that distribution boards or consumer units are of sufficient rating to take the total load connected to them without the application of any diversity"

stand alone????

I do take the point about the wording on certificates about max demand and main switch rating, so i do not know what is the right approach....

So what is the correct approach?? In the BRB [311.1] it states; "For economic and reliable design, the maximum demand of an installation shall be assessed. In determining the maximum demand of an installation or part thereof, diversity may be taken into account"

So, the IEE say one thing in one book they wrote, then seemingly contradict themselves in another.....

Finally, returning to "knowing more than NAPIT" and in particular "I have been reliably informed Stupid loves being felt" I have in front of me a very well known book written by a Mr John Whitfield.

Apparently, Mr Whitfield;

"has many years experience both as an electrical contractor and as a lecturer. He was for many years a senior lecturer and group leader at Norwich city college. He currently concentrates on writing and consultancy. He is a chartered engineer of the IEE and member of CIBSE and is the author of other bestselling guides"

In his book, [page 130 paragraph 6.2.3] [applied diversity] Mr Whitfield states;

"Apart from indicating that diversity and maximum demand must be assessed, the regulations themselves give little help. Suggestions of values for the allowances for diversity based on guidance note 1 are given in [table 6.2]. Distribution boards must not have diversity applied so that they can carry the total load connected to them.

Is Mr Whitfield stupid too???

 

[Andy™]

Hi all!! Firstly, Thanks for all the replies!!Secondly, [and i address this to "Special location"] With the greatest of respect, no, I do not think i know better than NAPIT at all, and am not trying to be clever or smart either, but merely to learn, but surely, does not the sentence in the OSG;

"It is important to ensure that distribution boards or consumer units are of sufficient rating to take the total load connected to them without the application of any diversity"

stand alone????

cant see anywhere in that sentence that says you must work out the total load connected by simply adding together all OPD values.

 

[apprentice87]

Hi again, is not the wording; "total load connected to them" not obviously referring to "total connected load" I.E. everything connected to a system before the application of diversity?

Even more baffled now!!

 

[Andy™]

Hi again, is not the wording; "total load connected to them" not obviously referring to "total connected load" I.E. everything connected to a system before the application of diversity?Even more baffled now!!

go re-read post 2. ratings of MCB's will help, but they are not the only way of measuring the total load.