Excessive Maximum Demand and Applying Diversity

[sherratts]

Dear all


I have been asked to quote for some work at a property and am concerned at the maximum demand already present without me actually adding to it.


I would therefore welcome any thoughts on just how much diversity I can apply.


The consumer unit currently has ten circuits as follows:-


2 Nº lighting circuits with 6 A MCBs
1 Nº Immersion heater (assumed to be 3kw) 16 A MCB
3 Nº Socket outlet circuits 32A MCB
1 Nº 9kw shower 40 A MCB
1 Nº Oven Circuit + integral Socket Outlet 32A MCB
1 Nº Hot Tub, rating plate is 25A but on 20A MCB
1 Nº Wise Box wireless remote control distribution box, rated at 40A but on a 20A MCB


The incomer is a standard 100A and the consumer unit is a Contactum unit also rated at 100A


The wise box currently has 1 Nº 2.1kW heater, 1 Nº 1.5kW heater and a total of 460W of lighting.


The job I've been asked to quote for includes adding another 2.1 kW heater to the wisebox including replacing the feed cable to suit – this would bring the maximum demand of the wisebox to 26 A so obviously a new MCB is also required.


The cooker is being replaced by a new double oven which according to the client needs a 20A feed (probably actually a 4.4kW)


We are also adding a new ceramic hob which according to the client needs a 30A feed. This obviously means replacing the consumer unit with a larger one as we have no spare ways.


My problem is that whilst calculating diversity for the new and existing kit, before I even get to the socket outlet and lighting circuits I am at 134 Amps.


Without the additions I believe we stand at over 100 Amps and again this is without the three socket outlet circuits and the two lighting circuits.


I appreciate the OSG states “The information and values are....only for guidance” and that “the values given in table 1B...may be increased or decreased” but the size of the excess here seems too big for me to claim “I know better than the book”.


I did raise my concerns the issue of maximum demand when I visited – I'm on the verge of telling the customer I can't do it.


Any advice would be greatly welcomed.


Cheers


Ken

 

[telectrix]

you must have some right slappers in surrey then.

 

[SW1970]

I use this book....

Electrical Installation Design Guide: Calculations for Electricians and Designers - IET Electrical

...which has a much better section on demand and diversity than the OSG, and always do a turn-everything-on-clamp-test before any new works to help inform the design.

 

[the pict]

Des I agree form filling is a pain, so I got a tester that downloads all the results and put them in all the right boxes does it ****** it chucks results all over the place so back to pen, tea, smoke
Pict

 

[kung]

Agree Clamp for max demand/load and get the job done as said the only time i think you'd be near the 100A is christmas day !
Local then ! Im in Woodley !

 

[wade88]

you must have some right slappers in surrey then.

The finest mate

 

[Sjn electrical]

Hi guys,

I have a similar problem and again Im struggling with diversity. These are the circuits I have

1, Electric boiler, 63a but will be put on a 50a mcb
2, Electric boiler 2, 38a but will be on a 32a mcb
3, Kitchen ring, 32a
4, Rad 1, 16a
5, Rad 2, 16a
6, Cooker, 32a
7, Lights 1, 6a
8, Lights 2, 6a
9, Lights 3, 6a

Im not sure why 2 electric boilers are required, and if ive applied diversity correctly I also feel I have a problem, I know there likely hood of everything running at full demand is unlikely and a new 100a fuse is capable of carrying 120a - 130a but just need some advice.

Thanks

Steven

 

[paddyscouse]

add all you final circuits up take your highest breaker out then x by 40% then add your highest back in thiss will give you your maximin demand and you will know if you can add any more circuits on board (note result should not be rated higher than your suplyers main fuse)

 

[KFH]

I worked out the diversity for my house using the OSG and got >130A. It has run for many years on a 60A fuse, which has got a bit hot but has not blown. Cut the load down a bit now so no problem.

 

[bigbaddave]

add all you final circuits up take your highest breaker out then x by 40% then add your highest back in thiss will give you your maximin demand and you will know if you can add any more circuits on board (note result should not be rated higher than your suplyers main fuse)

At a couple of years later they may have sorted it bud

 

[paddyscouse]

must have plenty of circuits in house if your m/d calculation came out at 130a ? are you shore you done calc correct !

it dose show you that diversity actually works hence your 60a fuse not blowing........

 

[Guest55]

do you even bother to look at the thread date ?
lol , wot a doughnut ;-)

 

[paddyscouse]

not intreasted

 

[ackbarthestar]

do you even bother to look at the thread date ?
lol , wot a doughnut ;-)

I think that max. demand and diversity is an ongoing syndrome that many people have problems with and is often useful to resurrect old threads to see as continuity as opposed to kicking off with new ones.

Now back to those slappers .................

 

[Engineer54]

The ongoing problem for new electricians and the like with MD/Diversity worries, is the totally misleading values and information contained in BS7671 and it's OSG's. And it's not helped any, by having to use this data in C&G examinations etc!! The only example that actually works, is for cooking loads, and that's been around for over 60+ years!! The rest can literary all be slung in the bin for what use they are to any electrician. You'll not find a single Design Engineer even look at this section of BS7671, let alone apply any of it!! lol!!