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]

i think you would be hard pushed to exceed 100A. ensure that your tails are 25mm and the earthing and bonding are satisfactory. get the job done. then if the customer has an issue with excess load, it's time for him to dig deep and contact the DNO.

 

[ackbarthestar]

"1 Nº Wise Box wireless remote control distribution box, rated at 40A but on a 20A MCB"

As Telectrix states you will be hard pushed to go beyond 100A unless you are going to hold regular Arc welding parties at the bottom of the garden.

As I remember it, the Wisebox RF controller has 4 outputs rated at 10A fused per output for the 866MHz and the older one 434MHz was 16A unfused outputs. I assume that you are using 1* 2.1kW heater per output? I've never attempted to run 4 *10A loads through them as the small output relays do not look as though they could cope with switching that amount of current.

 

[electromonkey]

How often do you think every circuit old and new will be operating simultaneously and drawing their maximum load?

In all likelihood, never.

 

[telectrix]

all the methods of determining max. load are at best a guesstimate, and err on the side of caution. just don't let them talk you into fitting a couple of 10.5kW showers.

 

[sherratts]

Dear All

Thanks for your very prompt replies, I totally agree with all of them however it does make applying the OSG's diversity table seem pointless.

In reply to ackbarthestar, each heater will be on a seperate circuit with the lighting on the fourth -I would expect all of these to be on at the same time along with the hot tub (which is never turned off but obviously has a thermostat so is not always drawing 20/25A).

What value would you guys recomend I put in my Certificate for the MD? I would propose 100A but that looks as if I haven't worked anything out.

Cheers

Ken

 

[telectrix]

so put 99.9, sorry.i'd enter 90A.

 

[Des 56]

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.

Why not raise those concerns with the customer,outlining the issue and explain that calculating that maximum demand is not an exact science

Highlight the worst case scenario and let them make an informed decision

Turning down the job because of your doubts about supply capability, is not, in my opinion, your best decision,that decision does not need to be taken by yourself

 

[telectrix]

problem is if he frightens the customer off having the work done, knowing the sort of cost involved getting DNO to upgrade a supply. i would think that if the 100A was ever exceeded, it would be of too short a duration to take out the supply fuse.

 

[Des 56]

Yes I have to agree with you tel,his proposed loading just looking at his circuits is to an extent bog standard for a Domestic supply

I agree also with frightening them with the supply upgrade cost,but its better than just turning away


On a lighter note,I am supposed to be filling out quite a few certificates, but I have a problem :sad_smile:

I find it so boring that in the last half hour its been
Test certificates 0 minutes
Forum watching 25 minutes
Making tea and a smoke 5 minutes

At least the Mrs thinks I am knuckling down to the task :tounge_smile:

 

[telectrix]

i'd just crack on and get the job done. switch on as much as i possibly could , then clamp the incomer tail. if it looked as if it could be excessive, i'd then advise the client accordingly. ( fdull steam ahead and damn the torpedoes. even if it's durham at the periscope).

editr should i say , especially if it's durham at the periscope.

 

[wade88]

"1 Nº Wise Box wireless remote control distribution box, rated at 40A but on a 20A MCB"

As Telectrix states you will be hard pushed to go beyond 100A unless you are going to hold regular Arc welding parties at the bottom of the garden.

As I remember it, the Wisebox RF controller has 4 outputs rated at 10A fused per output for the 866MHz and the older one 434MHz was 16A unfused outputs. I assume that you are using 1* 2.1kW heater per output? I've never attempted to run 4 *10A loads through them as the small output relays do not look as though they could cope with switching that amount of current.

I seriously thought i was the only one who had Arc welding parties....im so relieved. I just love all the slags........

 

[ackbarthestar]

How true!

 

[ackbarthestar]

I seriously thought i was the only one who had Arc welding parties....im so relieved. I just love all the slags........

Get a '******* file' ( I wonder whether that will be accepted by the moderators since it is a real file !) and you can clean them off.

 

[wade88]

I use an angle grinder and wire brush for the real tough slags