Diversity CU rating

[Manator]

apprentice87, I respect your willingness to learn, however I still think you are mistaken in regards to how and why diversity is allowed. First of all maximum demand is calculated by the installation designer, and the appropriate size of CPD and main fuses are designated. Now, because of the advent of domestic properties being outside the control of proper electrical engineers, a set of guidelines were agreed. The result of this was the tables in the IEE publications that show the average joe, how to apply this design diversity without having had to go to university to gain the qualification for the design of circuits and or distribution. You will find that all mentions on diversity, are primarily aimed at domestic properties. Any large commercial or industrial install would have to be specifically designed and the applied diversity is done by the designer to ensure the size of supply is adequate, and cost effective.

 

[apprentice87]

Hi Andy, you say that;

"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"

Well yes, and I take your point about nothing having changed, but going by the OSG again, it would be the three rings [100% of the largest and 40% of the rest] plus 66% of the lighting, plus not sure how they would see the 6A for the boiler. But anyway, that would be; 57.6 for the rings, 7.92 for the lights, and if you just added on the 6A for the boiler that would give a grand total of 71.92A so that would be perfectly ok, as you say just as it was before.

 

[Andy™]

Hi Andy, you say that;"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"

Well yes, and I take your point about nothing having changed, but going by the OSG again, it would be the three rings [100% of the largest and 40% of the rest] plus 66% of the lighting, plus not sure how they would see the 6A for the boiler. But anyway, that would be; 57.6 for the rings, 7.92 for the lights, and if you just added on the 6A for the boiler that would give a grand total of 71.92A so that would be perfectly ok, as you say just as it was before.

thats were common sense comes into it. the OSG is just that - a guide. you have to use your own judgement

 

[Sidewinder]

This is the thing with Diversity, it IS a bit of a black art and is not "taught" in any detail in any courses other then undergraduate.

You have to use your engineering judgement.

 

[Manator]

I have a good experiment for maximum demand in a domestic, connect every item of load currents you can find, and run them for the longest time you can, whilst doing this check with a clamp meter the readings, even with all loads connected I will state my life on it that even if the main fuse is only 60 amp, no load will be able to run long enough to blow that service fuse.

 

[apprentice87]

Hi Manator, thanks for your reply. Yes, i would agree with what you say. They do actually state that the rules for diversity are only guidelines, and that the designer can dream up his own scheme for determining maximum demand if he likes. I suppose you are right too in what you say about the various tables were compiled to help "the average Joe" as you put it, get things right.

I do not like "woollyness" though. See, in the event of any mishaps the insurance company and the people in the courtroom [none of whom will be electricians] will all just say; "he ignored published rules, wave their reference books about, and conclude "therefore he is guilty"

I know they are only all guidelines, but surely they would be relied upon when "pointing the finger"

In a similar way, the "highway code" is not law, but it can be relied upon to "create offences" By this i mean, there is no law at all that says you must drive on the left hand side of the road. Yes i know it says "keep left" in the highway code, but this is not law, just guidelines, however, if you choose to ignore these guidelines and drive on the other side of the road, you will be jailed, precisely because an accident could be caused because you ignored "mere guidelines" that "created" the offence of dangerous driving [That you will no doubt be charged with]

PLEASE PLEASE PLEASE Do not all think i am trying to be clever, I just want to learn.

Is this thing about diversity and total connected load and CU's a common source for debate [in the way that exporting PME earths is, or was] or is it just me being silly/over fussy??

As i say, i just want to learn....

 

[Andy™]

Hi Manator, thanks for your reply. Yes, i would agree with what you say. They do actually state that the rules for diversity are only guidelines, and that the designer can dream up his own scheme for determining maximum demand if he likes. I suppose you are right too in what you say about the various tables were compiled to help "the average Joe" as you put it, get things right.I do not like "woollyness" though. See, in the event of any mishaps the insurance company and the people in the courtroom [none of whom will be electricians] will all just say; "he ignored published rules, wave their reference books about, and conclude "therefore he is guilty"

I know they are only all guidelines, but surely they would be relied upon when "pointing the finger"

In a similar way, the "highway code" is not law, but it can be relied upon to "create offences" By this i mean, there is no law at all that says you must drive on the left hand side of the road. Yes i know it says "keep left" in the highway code, but this is not law, just guidelines, however, if you choose to ignore these guidelines and drive on the other side of the road, you will be jailed, precisely because an accident could be caused because you ignored "mere guidelines" that "created" the offence of dangerous driving [That you will no doubt be charged with]

PLEASE PLEASE PLEASE Do not all think i am trying to be clever, I just want to learn.

Is this thing about diversity and total connected load and CU's a common source for debate [in the way that exporting PME earths is, or was] or is it just me being silly/over fussy??

As i say, i just want to learn....

i would imagine that is in the road traffic act etc, and so would be law

 

[apprentice87]

No, i do not think it is any road traffic act, they just rely on the highway code to create offences. For example; the speed limit on the motorway is 70mph. There is no law that states you have to slow down when it is foggy, but you could be prosecuted for doing maybe 50, [even though this would be perfectly legal] because the highway code advises you to slow down in adverse weather conditions and thereby creates offences if you do not [such as dangerous driving] It would be no good pointing out that "it says 70 on that sign over there!!]

 

[Sidewinder]

apprentice,

You are wrong on one thing and you are right in the same sentence.

IF you end up in court over a diversity calculation, then you will not necessarily have en electrician as an expert witness for the prosecution.

You will almost certainly be up against an electrical engineer, almost certainly a Chartered Engineer (CEng) and member of the IEE, though he may be an Incorporated Engineer (IEng) though probably still MIEE.

You will NOT be able to bluff your way out, he will know his onions from his shallots!

He will know more about diversity, and he may well have even sat on the regs committee, or be an adviser to said body.

It is doubtful that an electrician would be qualified to provide expert witness testimony for the prosecution, without additional qualifications.

However, please note, in the same vein as your posts I'm not trying to be "clever" or "come down on you" I am just hoping to explain the "ways of the world" as it were.

Please keep with the questioning as the thread I suspect will turn out to be very helpful to all learners.

HTH

---------- Post Auto-Merged at 23:17 ---------- Previous post was made at 23:16 ----------

apprentice,

The last post that you have made, I think you have the wrong end of the stick.

Think about the relationship between EAWR & 7671 as the same as the RTA & the Highway Code...

 

[Manator]

I actually like your above post, the understanding can only be learned from asking the type of questions you ask, it is easy for any of us to say " well the book says". What you have to look at when you need to determine the maximum demand is how long each individual appliance is connected at the same time. One of the best times to calculate for this is Christmas Day when it is feasible to calculate that an electrical hob will be running for prolonged periods of time.If you have a 60amp main fuse, and your cooker is using 65 amps then you will find that your supply cable will become very warm, prolonged usage, it will become extremely hot. However if we look at the curve for the main fuse we can see that it will run for a determined length of time at above the rated amps, designers will use this curve, to design circuits that will overload the demand but for limited periods, and well within the set curves of the protective main fuse.

 

[apprentice87]

Hi Manator,

You say that;

"I have a good experiment for maximum demand in a domestic, connect every item of load currents you can find, and run them for the longest time you can, whilst doing this check with a clamp meter the readings, even with all loads connected I will state my life on it that even if the main fuse is only 60 amp, no load will be able to run long enough to blow that service fuse"

For years i had a large single phase welding set, i used it fairly often for about 20 years. Not once did the board fuse [60A] blow One day not so long ago, i thought that as i was bored, i would crank the thing right up and stick a clamp meter on the feed just out of interest. Errm 107A!!!! Now, it was only on full throttle if you like a few times, but at more sensible settings it still drew 80 odd Amps and i did that quite often, and with the house loads on top too!!

Now, that i have read characteristic tables for the fuse i realise why! Them fuses certainly take some blowing!! Did not break the meter either!!

Now i got a proper three phase supply, so the house meter and cutout fuse can live out their days in semi retirement!!!!

 

[SPECIAL LOCATION]

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!!

No its not..

As I suggested earlier go back and re-read your quote from the bottom of page 96..

In fact here it is..

Note to table 1B:* In this context an instantaneous water-heater is considered to be a water heater of any loading which heat water only while the tap is turned on and therefore uses electricity intermittently.

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

Both notes refer to Table 1B and

the little symbols preceding the paragraphs indicate from where in the table those paragraphs apply....

and as I have already pointed out this second item '+' ONLY refers to three rows (6,7 & 8) relating to particular types of circuit..

Look for the '+' symbol in the table!

i.e.

Where your cu has these types of circuits you must ensure all of those types of circuit can be supported simultaneously at full power...

Other circuit types and arrangements can have appropriate diversity applied.

 

[Manator]

Now you see, why diversity is applied, and how a designer can reduce this by design so that a domestic having an applied IEE maximum design using the tables,can actually design the installation and even apply diversity to the shower, to reduce the maximum demand.

Its all down to the curves.

 

[apprentice87]

Hi Sidewinder,

Yes, that is what i mean. You say that;

"You will almost certainly be up against an electrical engineer, almost certainly a Chartered Engineer (CEng) and member of the IEE, though he may be an Incorporated Engineer (IEng) though probably still MIEE.

You will NOT be able to bluff your way out, he will know his onions from his shallots!

He will know more about diversity, and he may well have even sat on the regs committee, or be an adviser to said body"

The thing is, I do not want to bluff my way out of anything, I just want to understand and get things right...

You see, with reference to the above discussion about diversity, Mr Whitfield [in his book]states; "Distribution boards must not have diversity applied so that they can carry the total load connected to them"

and he IS a chartered engineer of the IEE. The OSG states the same [or at least it reads that way to me] but in the BRB 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"

It does not say that this does not apply to CU's and distribution boards.....

I wish i could get hold of Mr Whitfield to ask him where he got his information from or what does he base it on.

I just want to do things the right way!!!

 

[Sidewinder]

Manyana (If I have spelt it right!).

Come back tomorrow & we'll explain more.

You are right & wrong, and not totally either!

It is good that you ask these questions and that you have the appetite for learning.

 

[Manator]

To be honest, this is one of the most honest debates this forum has seen in a ling time. Understanding takes time, the penny drops when you accept the Asian concept of clearing your mind. Think of nothing,now look at the supply characteristics,now look at the maximum you can connect to this, now look at how long you can run this without the supply giving up, there you go you have just designed a supply, applying diversity, quite differently than the guidelines in the tables.

 

[apprentice87]

Hi Special location,

Ahhh I see, What i think you are getting at, is that what they are ACTUALLY trying to convey, is that;

"you had better make sure that, as no diversity is allowable for these particular circuits [the heaters etc] that you are able to allow sufficient diversity amongst the OTHER connected loads that the maximum demand TAKEN AS A WHOLE does not overload the consumer unit.

So their sentence in the OSG would be better worded if it read;

"It is important to ensure that the distribution board or consumer units are of sufficient rating to take the total load connected to them "without having the benefit of the application of diversity to these particular circuits"

I think i understand [i hope!!]

 

[SPECIAL LOCATION]

The thing is, I do not want to bluff my way out of anything, I just want to understand and get things right...

You see, with reference to the above discussion about diversity, Mr Whitfield [in his book]states; "Distribution boards must not have diversity applied so that they can carry the total load connected to them"

and he IS a chartered engineer of the IEE. The OSG states the same [or at least it reads that way to me] but in the BRB 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"

It does not say that this does not apply to CU's and distribution boards.....

I wish i could get hold of Mr Whitfield to ask him where he got his information from or what does he base it on.

I just want to do things the right way!!!

Post #32 above...

There is NO contradiction..

Just you miss-reading Pages 96 & 97..

I have explained above.. :| :C

 

[steptoe]

diversity,

well if you have 20 double sockets, thats a theoretical 260A loading all on its own, so you are gonna need a big T&E ring final cable and CPD,!!!!!!!!!

so we decide we are only going to allow 32A max on any ring final,

then we decide between our 2 lighting circuits a max of 4A each at any one time,

boiler, 6A MCB but the boiler actually only draws 1A and we have a 3A fuse in the SFCU so thats fine,

etc etc etc,

so if we are going to make limits on each individual circuit, then let us also say that in any combination we will only allow it to draw 60A max,

et voila,

do you know what,

that is exactly what the DNO decided and put a 60A fuse in the HED.!

astounding stuff this as to how they knew precisely what I was thinking!!!!!

 

[SPECIAL LOCATION]

Think AGAIN of what these type of circuits are probably doing in the real world..

Shower.. maybe running full whack 15 - 20 mins at a time

Cooker.. maybe running 3/4 full whack 1 hour or 2 tops...

Whereas

Floor heating Thermal storage space heating

running full whack for several hours at a time...

People like to be kept warm...

They don't like the heat going off after 20 mins on a winters day...

BUT the shower & the cooker still only run for shorter period..

Also things like washing machines...

maybe on a 1hour cycle but for a lot of that it may be only running a motor to spin the drum..

The heating bit is only on for a shorter time span!

SO it is NOT just looking at the possible loads and adding them up..

It is serious consideration of what are the feasible SIMULTANEOUS loads that will all be running for longer periods.

:coffee

Go and ALSO read the 2nd Paragraph of page 95..

"There ARE NO hard and fast rules for calculating diversity.."

So you will no be hauled up in court for not following rules that don't exist..

BUT you should be able to show a reasonable appreciation of what the probable maximum loading for a particular type of installation is...

e.g.

Lights in a School / office most probably running full load for many hours a day..

lights in a house... less on for less duration!!!

So its NOT just the circuit type but ALSO the installation environment & usage as well..