Split load board Switch rating

[candleman]

Hi Guys,

First post here so bear with me on this one.

I've been looking at the 17 th edition dual rcd boards you can buy at the usual sheds and I can't help wondering if the main switch is underated if you use all the mcb's supplied. Heres my logic.

Both RCD's feed from the main switch outlet side.

They come with 4 x 32a, 2 x 16 amp, 3 X 6 amp, 1 x40 amp

Working with diversity for what i would consider a typical install

1st ring downstairs = 32A no diversity allowed

2nd ring upstairs = 40% of 32A=12.8A

3rd ring kitchen = 40% of 32A=12.8A

Assume 4th 32 amp breaker is for 12.5kw cooker= 10A+30% remainder =10A+13A = 23A total

3.5KW immersion @16A = 3500/230 = 15.2A no diversity

2nd 16 amp spare

1st lights D/strs @ 6A @ say 8 x 100W= 800/230=3.5Ax66% = 2.3A

2nd lights u/strs @6A say 6 x100W= 600/230=2.6Ax66%= 1.7A

3rd lights outside @ 6A say 2x500w=1000w/230=4.3x66%=2.9A

Shower @ 40A say 9.5kw=9500/230= 41A no diversity allowed

Add all these up and we get 143.7A potentially through the mainswitch rated @ 100A.

I know we can assume that not all the circuits will be loaded at the same time but it's nearly 50% over the main switch rating.

Am I missing something obvious?

Candleman

 

[Zeespark]

Candleman, out of all that lot, the highest power consumption would be the shower plus a few bits on one of the rings at worst.

AndyGuinness

 

[Revved Up Sparky]

I would suggest the main switch is not overrated as it has the same rating as the DNO main fuse which is usually 100A nowadays.

Perhaps it would be an idea to fit the board, ask the householder to switch on appliances to simulate normal conditions then put a clip=on ammeter onto the tails to measure the load. Any overload issues could then be addressed.

 

[Larnacaman]

You have high lighted the reason why most, if not all Design Engineers never use the Reg's diversity recommendations and tables. They bear too far into the safety zone, for want of better words.

The figure you have calculated according to the current diversity Reg's table will never be seen, ...and that's a FACT!! Diversity of connected loads needs experience, lot's of it.

One of the ''over'' estimated loadings are from ring circuits, especially those covering bedroom and other low usage areas. Even lounge/dining room (ground floor) rings can be over estimated to the tune of 60% in some cases. Kitchen rings can be a different kettle altogether, even then it depends on what's being connected to that ring, and what's unlikely to be used at the same time.

The only Diversity factor that has proved itself for over 60 years is for cookers, ...how long for in the future, i don't know!!!

It's very common for prejudices and fears to get in the way of realistic diversity loadings of an installation, many not believing the low figures that an experienced electrical engineer can produce.

 

[Andy™]

Am I missing something obvious?

yes. service fuse. which are normally 60 or 80A.

 

[NozSpark]

The main problem with diversity is that it takes into account the circuits and not what is being used..

For instance..

You rewire a house that has 1 x 30A socket ring, 1 x 30A Cooker circuit and 1 x 5A Lighting circuit.... after diversity that's something like 50A

Now your new circuits include 3 x 32A socket circuits (Kitchen, D/S & U/S), 1 x 32A Coocker circuit and 3 x 6A circuits (D/S lights, U/S lights and smokes).... after diversity it's prob >100A

but it's the same house with the same ccupants and the same appliances, so how has the maximum demand changed (just because it's available doesn't mean that it's going to be used)

BTW

I tend to install 16A or 20A radials downstairs or upstairs, not 32A rings

 

[Manator]

I would have to agree, most supplies to domestic homes are 80amp at most, some are still 60amp.

I believe special location did a very interesting post about diversity especially after a rewire, or CU change that really did make interesting reading.

For instance a CU change with 2 5amp circuits and 2 30amp circuits do not become any greater on demand than they did before.

 

[candleman]

I would tend to agree with you all, the actual loadings in real life would tend to be considerably lower than calculated values which is where experiece comes in. And having once worked in the R&D department of one of our leading electrical manufacturers I know that stated ratings are considerably lower than the actual values that will cause the equipment to fail at, and No manufacturer would allow his product to go on sale if they were knowingly underrarted.

But what got me thinking was this article published by NAPIT

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

which in the second last paragraph states "...This means that the total current of each final circuit after allowing for diversity where diversity is allowable must be less than the current rating of the main switch"

If something were to go wrong and ligation followed who's side would the H&S executive take, the "With experience of real life" electrician or the people who have input into writing the regs?

 

[sparkytim]

I am sure Larnacaman will put us right here, but I think DNOs use to work on

9 AMPS or so PER DEWELLING when sizeing up supply requirements for a steet of Houses.

 

[sellers]

max demand and diversity have to fall on a case by case basis.

I found this out recently when working it out for some flats i was pricing. Came out at 170amps per flat using osg way. Using common sense got it to around 90amps. Imagining someone cooking tea while partner is in the shower and wall heaters on.

 

[SPECIAL LOCATION]

Hi Guys, First post here so bear with me on this one.

I've been looking at the 17 th edition dual rcd boards you can buy at the usual sheds and I can't help wondering if the main switch is underated if you use all the mcb's supplied. Heres my logic.

Both RCD's feed from the main switch outlet side.

They come with 4 x 32a, 2 x 16 amp, 3 X 6 amp, 1 x40 amp

Working with diversity for what i would consider a typical install1st ring downstairs = 32A no diversity allowed

2nd ring upstairs = 40% of 32A=12.8A

3rd ring kitchen = 40% of 32A=12.8A

Assume 4th 32 amp breaker is for 12.5kw cooker= 10A+30% remainder =10A+13A = 23A total

3.5KW immersion @16A = 3500/230 = 15.2A no diversity

2nd 16 amp spare

1st lights D/strs @ 6A @ say 8 x 100W= 800/230=3.5Ax66% = 2.3A

2nd lights u/strs @6A say 6 x100W= 600/230=2.6Ax66%= 1.7A

3rd lights outside @ 6A say 2x500w=1000w/230=4.3x66%=2.9A

Shower @ 40A say 9.5kw=9500/230= 41A no diversity allowed

Add all these up and we get 143.7A potentially through the mainswitch rated @ 100A.

I know we can assume that not all the circuits will be loaded at the same time but it's nearly 50% over the main switch rating.

Am I missing something obvious?

Candleman

Yes!!

your assumptions for diversity are about as wrong as you can get!

1/ Are all of you light circuits going to be taking your suggested 66% at the same time that the shower is in use AND the immersion heater is running..

AND some is cooking a full Sunday roast AND you've got 3kw worth of load going upstairs AND 3kw worth of load going downstairs and another 7.3kw worth of load going in the other downstairs circuit???????????????

WTF have you got plugged IN???????????? :O:O

2/ Consider a washing machine for example maybe running a 50min wash cycle...

a majority of that time it is only take a low power, rotating the drum, spinning and slushing stuff about...

Its probably only taking full whack when its heating the water up!!!!

3/ All your TV's Hifis, game consoles, etc.. etc.. take gnats pee worth of lectwickery.

4/ The immersion heater probably on for the bulk of the time at night..

when peoples are in bed NOT using stuff plugged in elsewhere!!!

5/ You assume a property with 3 x 32amp ring circuits!!???

most properties only probably need a 32A ring for the kitchen / Utility..

all the rest could be covered by 20A circuits.

You certainly DO NOT need a 32A upstairs socket ring!!!!!

6/ Try reading the ON-SITE-GUIDE,

Appendix 1 Maximum demand and Diversity.

But don't jump straight to the example tables..

read the boring writing bit on page 95..

ESPECIALLY PARAGRAPH 2!!

which starts.

The information in this appendix are intended only for guidance, because it is impossible to specify the appropriate allowances for diversity for every type of installation and such allowances call for specialist knowledge & experience....etc.

It is NOT for "strict follow to the letter" calculation..

you asses the realistic loading..

Not just adding up percentages off MCB ratings!

7/ Have you put ever put a clamp meter around the incoming tails while a lot of plug-in appliances and lights are switched on...

In a majority of cases you will be hard pushed to get a reading even as high as 15 amps

In fact I just been to my own fuse box and stuck my clamp meter on..

we are drawing a whopping 18.7Amps!!!!!!!!!!!!!!!

thats a Sunday evening, kids in lights on, loads a stuff plugged in..

:| :C

8/ As Manator pointed out an existing property with one 5A lighting circuit supplying ALL of the lamps, doesn't suddenly double its lighting consumption just because you split the existing lights onto two 6A MCB's..

Think LOADS not MCB ratings!

 

[ProDave]

Interesting point though:

Most domestic CU's have a 100A incomer, and can be used with up to a 100A main fuse.

Most 3 phase boards have a 125A incomer, yet are still used with the same, up to 100A per phase fuse.

Why are they not both 100A, or both 125A?

 

[green-hornet]

3 phase are fused per phase, most smaller 3 phase units will be 80amps per phase.

Larger industrial units may have 400amp or more at the incommer.