Diversity

[Spearmint]

Thought I'd throw this out there and see what others think. We're working working on a boiler house and storage room separated by an internal wall which is all a separate building to the house. Supply is coming from the house into the new building as a sub main and contains...

2 x 32A MCB ring mains, one for sockets around the inside of the boiler house and the other for the inside of the storage room.

1 x 16A MCB to an immersion of unknown rating (hasn't been ordered yet).

2 x 6A MCB - one for the lighting for both rooms and the other is for the boiler + stats etc.

...all this from a 40A incomer (I'm unsure what the house supply is, presuming 100A).

I'm not part of the CU install, just had a nosy on how the others we're getting on but tried practicing working out diversity after getting a good look at the DB and I work it out to be over the 40A on the incomer.

RFCs: 32A @ 100% + 32A @ 40% = 32A + 12.8A = 44.8A

Have to assume full 16A for the immersion as it isn't on site yet, no diversity allowed so: 44.8A + 16A = 60.8A so far.

Lights: 6 x 100W = 600W / 230V = 2.6A x 66% = 1.7A - 62.5A so far.

Boiler: Unknown load, all controls though so barely anything I imagine, have left this out.

So even after leaving the boiler circuit out I'm at 62.5A on a 40A incomer.

I'm I missing something? Surely this can't be right?

 

[Octopus]

Thought I'd throw this out there and see what others think. We're working working on a boiler house and storage room separated by an internal wall which is all a separate building to the house. Supply is coming from the house into the new building as a sub main and contains...

2 x 32A MCB ring mains, one for sockets around the inside of the boiler house and the other for the inside of the storage room.

1 x 16A MCB to an immersion of unknown rating (hasn't been ordered yet).

2 x 6A MCB - one for the lighting for both rooms and the other is for the boiler + stats etc.

...all this from a 40A incomer (I'm unsure what the house supply is, presuming 100A).

I'm not part of the CU install, just had a nosy on how the others we're getting on but tried practicing working out diversity after getting a good look at the DB and I work it out to be over the 40A on the incomer.

RFCs: 32A @ 100% + 32A @ 40% = 32A + 12.8A = 44.8A

Have to assume full 16A for the immersion as it isn't on site yet, no diversity allowed so: 44.8A + 16A = 60.8A so far.

Lights: 6 x 100W = 600W / 230V = 2.6A x 66% = 1.7A - 62.5A so far.

Boiler: Unknown load, all controls though so barely anything I imagine, have left this out.

So even after leaving the boiler circuit out I'm at 62.5A on a 40A incomer.

I'm I missing something? Surely this can't be right?

Your calcs look about right BUT the rules for diversity DON'T take into account the number of spate circuits that get installed these days.

I would add up the breakers and divide by 4 or 3 worse case!

 

[Badged01]

RFCs: 32A @ 100% + 32A @ 40% = 32A + 12.8A = 44.8A

What the heck you gonna be pulling on sockets in a boilerhouse and storage room that your estimating at 50A??

 

[Spearmint]

Your calcs look about right BUT the rules for diversity DON'T take into account the number of spate circuits that get installed these days.

I would add up the breakers and divide by 4 or 3 worse case!

Fair comment, I'm doing level two so all new to me and can't see past the end of the OSG! That's my problem I think lol.

So 32A + 32A + 16A + 6A + 6A / 3 = 30.7A

That works!

 

[Spearmint]

What the heck you gonna be pulling on sockets in a boilerhouse and storage room that your estimating at 50A??

Just practicing diversity calculations with the OSG, in real life I suspect the only thing that will be running at any one time will be the boiler controls - no lights, no immersion, no sockets.

 

[NickD]

What the heck you gonna be pulling on sockets in a boilerhouse and storage room that your estimating at 50A??

Does that matter? Is it not just calculated on protective device current rather than design current?

 

[sparks1234]

It's a load of cobblers really working out load, particularly with sockets, who knows what's going to plugged in, we should all be pulling a shed load more than 100 amps if we went by the OSG

 

[GLENNSPARK]

well....heres a story that may interest you:

It was a couple of weeks ago that a guy in the markets in leeds that is a regular customer of mine rang up saying that he wanted to run some carribean cookery classes in the basement kitchen in the corn exchange....he said he wanted to cater for 10 places to make it work (viable)...he said he wanted to run 10X portable double hobs at 2.5KW each...the council electrician in the corn exchange had told him that he could only run 3 of these at once off the ring (32C)...or it would pop the breaker..

now, he had one of these units available for me to get a general characteristic assessment on it...so i stuck some juice in it, got both hotplates up to tempreture and clamped it....Max draw was 11 A...but fluctuated from that figure down to 0 A...in fact it only hit max draw for about 2 secs in every minute ...as the stats kicked in & out...

went back next week where he had brought 5 of these units...so got them all plugged in and clamped the legs at the board......Max draw was 21.5 A after a half hour assessment....

told the guy the ring was good for 7 units..spread about evenly around the ring....so what Mr council electrician was talking was a load of old chud really...wasn`t it...

at this point the client dropped on that he wanted a 2.8 KW water heater as well.....so told him the ring was good for 5 portable hobs + the water heater

the other 5 portable hobs were:

2X coming off a 16A radial

and the last 3 X coming off 3 extention leads that i connected up to a sub board (fed from the panel that all the other stuff was fed from)....

and guess what....got a text that evening from him explaining how well both his cookery lessons had gone and thanks for all the help & advice...

so just goes to show actually how much diversity can be applied...doesn`t it..

Hope this helps..

 

[Badged01]

Does that matter? Is it not just calculated on protective device current rather than design current?

Kind of does if your estimating what front end you want -- 40A or 100A -- although at the end of the day if you get the diversity wrong the breaker 'pop's with the overload. Glenns illustration gives a good example of how knowledge and experience is more applicable than books (e.g. OSG) for diversity though!

Although in its defence the OSG does say this -- the designer may vary the values given based on his knowledge of the installation. So it depends how you see the rooms being used and 50A is a lot of juice!

 

[Knobhead]

So all the earache you gave me was worth it, Glenn.

Did you do an averaged KWHr reading? Hope you did because that would blow holes in any diversity calculations.

 

[SPARTYKUS]

You worked with hard facts based on gathered evidence Glenn..... Much better

 

[Engineer54]

Fair comment, I'm doing level two so all new to me and can't see past the end of the OSG! That's my problem I think lol.

So 32A + 32A + 16A + 6A + 6A / 3 = 30.7A

That works!

For the time being while you are still at college, stick to the OSG way of calculating things for diversity, it'll get you past C&G exams. Then after, when you have your C&G certificate under your belt, ....''Totally'' forget everything you were ever taught that referred to the BS7671 way of doing things (apart from cooker diversity formula). No-one, and i do mean NO-ONE, in the real world uses them.

For a typical domestic installations, adding together sensible CU breaker ratings, and dividing by 4 (some take the largest breaker rating and divide by 4 the rest, ...i don't!!) is about as accurate a figure you will achieve without putting an amp clamp on the tails....

 

[telectrix]

i add them up and then multiply by 0.4 coz. i'm pessimistic.

 

[Engineer54]

Either or, multiplying by 0.4 will give you a higher figure than dividing by 4....

Example...

32A SOCKETS (up)
32A SOCKETS (down)
32A COOKER
40A SHOWER*
16A IMMERSION
6A LIGHTING (up)
6A LIGHTING (down)
20A GARAGE

= 184A X 0.4 = 73.6A
184A / 4 = 46A (or 144 / 4 = 36 + 40* = 76A)

Think we both know what the likely actual maximum current draw is going to be on a typical domestic, .....yep far nearer to that 46A than the two 70+ amp examples.

 

[NickD]

....got a text that evening from him explaining how well both his cookery lessons had gone and thanks for all the help & advice...

Hope you got some dumplings and plantain and goat curry as well...jeez, I'm starving now.