A small panic about maximum demand!

[HappyHippyDad]

Hello all,
I will be getting assessed very, very soon now! I’ve completed the consumer unit change and am just having a bit of a panic as I’ve worked out the maximum demand and it exceeds the supply fuse by quite a bit.
I realise the assessor is not likely to questions my figures so I could write anything in max demand on the Electrical Installation Certificate but I’d rather bring his attention to it so he can see that I am capable of working out max demand properly and questioning the safety of something rather than trying to hide it.
Here are my figures, in case i have got something wrong:
1. Cooker (2.5Kw). 2500/230 = 10.9A. Apply diversity 30% of 0.9 = 0.27. First 10A + 0.27 = 10.27(plus 5A for socket attached) = 15.27A

2. Shower (9.5Kw). 41.3A (but on a 40A mcb) = 40A (No diversity)
3. Ring 32A (No diversity)
4. Shed (max demand approx 16.4A but on a 16A mcb) Apply diversity 40% of 16A = 6.4A (in reality I use about 3-4A at any one time!!)

5. Water heater 3A (combi) (unsure of any diversity, but wouldn’t help much))
6. lights (900w). 900/230 = 3.9A Apply diversity 66% = 2.6A

7. loft lights (200w) after diversity = 0.6A

Total Maximum demand = 99.87

The supply fuse is 60A on 16mm tails as are all the other houses around here, which all have the same things in them.
The DNO chap was out recently to fit an isolator and he said there were no signs over overheating or deterioration. It is still safe as 16mm tails protected by 60A fuse.

Have I worked out my diversity correctly?

No time to change tails/supply fuse as the assessor is coming too soon.

Any suggestions greatly received!!

 

[plugsandsparks]

I would steer clear of this in assessment as you will end up in a circular argument as it depends how many people are in the house and whether all these things can be used at the same time BS7671 has a go at Max demand but its not actually a science as such as in reality its unique to the householders pattern of behaviour.
e.g. Whats the probablity you are having a shower, whilst working in the loft and clearing out the shed, whilst simulataneously loading your ring up.
Its really quite hard unless you have a house full.

 

[Rauer]

Just write it in in kva should make it a bit too complicated to work out in his head!!!

 

[HappyHippyDad]

Thanks both,

Any more suggestions?

Have I calculated diversity correctly?

Will he want to see a completed EIC?

Cheers...

 

[telectrix]

just put down 60A . that's the fuse rating so, theoretically, that is your max. demand.

 

[HappyHippyDad]

just put down 60A . that's the fuse rating so, theoretically, that is your max. demand.

Thats kind of what I thought Telectrix, just needed to hear it from experienced chaps. Thats what I will do, thanks..

 

[dagrat]

He will want to see the completed EIC but as far as maximum demand goes he will just want to see that it has been filled in. as plugsandsparks says it is a location specific value. There is the diversity methods outlined within the OSG and there are other permutations that can be used. However as this is an existing installation and you are not designing the installation I would switch all major loads on and clamp the live tail. This in my opinion would provide a reasonable assessment of the maximum demand. Even this would be on the pessimistic side as a situation where everything is on at the same time is almost impossible.
Hope this helps

 

[Rauer]

What is the job that you are showing him?

 

[HappyHippyDad]

What is the job that you are showing him?

A consumer unit change

 

[malcolmsanford]

Hello all,
I will be getting assessed very, very soon now! I’ve completed the consumer unit change and am just having a bit of a panic as I’ve worked out the maximum demand and it exceeds the supply fuse by quite a bit.
I realise the assessor is not likely to questions my figures so I could write anything in max demand on the Electrical Installation Certificate but I’d rather bring his attention to it so he can see that I am capable of working out max demand properly and questioning the safety of something rather than trying to hide it.
Here are my figures, in case i have got something wrong:
1. Cooker (2.5Kw). 2500/230 = 10.9A. Apply diversity 30% of 0.9 = 0.27. First 10A + 0.27 = 10.27(plus 5A for socket attached) = 15.27A
Above is an excellent example of why the OSG is flawed when it comes to Diversity, it is giving you a figure higher than the appliance rating !!!!!!

2. Shower (9.5Kw). 41.3A (but on a 40A mcb) = 40A (No diversity)
Yes again to the OSG correct but again how can you not give this a certain amount of diversity. A shower will be used maximum an hour a day, so you can not apply anything even though it is off for 23 hours?
Especially as your 60 amp fuse will chug away at nearly 2 times it's In for an hour

3. Ring 32A (No diversity)
In fairness they do give some guide in the OSG. It's 100% of the largest point of utilisation
(wow where did they get this, 100% a point on a kitchen ring could be a 3kw kettle, on an upstairs ring it could be a clock radio!! ) and then 40% blah blah

4. Shed (max demand approx 16.4A but on a 16A mcb) Apply diversity 40% of 16A = 6.4A (in reality I use about 3-4A at any one time!!)
Is there a diversity factor for a shed

5. Water heater 3A (combi) (unsure of any diversity, but wouldn’t help much))
This is a WAG but not really desperate

6. lights (900w). 900/230 = 3.9A Apply diversity 66% = 2.6A
That is most likely the nearest you will get in the OSG to being right, especially if you had daughters like mine who thought it was a challenge to leave as many lights on in an empty room as they could

7. loft lights (200w) after diversity = 0.6A
Yeah ok

Total Maximum demand = 99.87

The supply fuse is 60A on 16mm tails as are all the other houses around here, which all have the same things in them.
The DNO chap was out recently to fit an isolator and he said there were no signs over overheating or deterioration. It is still safe as 16mm tails protected by 60A fuse.

Have I worked out my diversity correctly?

No time to change tails/supply fuse as the assessor is coming too soon.

Any suggestions greatly received!!

So the bottom line is never use the OSG for diversity. A good rule of thumb for me is the DNO one of adding up protection devices and then multiply by 0.4

Cooker 16amp
Shower 45 amp
RFC 32amp
Shed 16amp
Water heater 5amp
Lights 5amp

Total 119* 0.4 = 47.6

Now that to me is nearer the mark

 

[fredrico]

Why would you not use the OSG to apply diversity, when it is a book produced from the IEE wiring regs, the same institution that we all take guidance from ????

 

[thebigcheese]

1. Cooker (2.5Kw). 2500/230 = 10.9A. Apply diversity 30% of 0.9 = 0.27. First 10A + 0.27 = 10.27(plus 5A for socket attached) = 15.27A

My line of work means I never have to calculate max demand so I'm more than happy to be corrected but I thought it was 30% of the full load and then you add 10A... not 30% of the difference

I also seem to remember my tutor at college being adament that you apply diversity to the rating of the ocpd and not that of the load... is this right?

 

[malcolmsanford]

Why would you not use the OSG to apply diversity, when it is a book produced from the IEE wiring regs, the same institution that we all take guidance from ????

Because it is a guide and it is wrong as highlighted in this case if you read the start of the appendix it clearly states that it is purely a guide and accepts that other ways of working out diversity are quite acceptable. My way is better and until someone tells me different that is what I will continue to use

My line of work means I never have to calculate max demand so I'm more than happy to be corrected but I thought it was 30% of the full load and then you add 10A... not 30% of the difference

You are right it should have been 10.9-10 then 30% of the load total load which is 10.9* 0.3 = 3.27 then + 10 + 5 which would have made it 18.7..................even worse

I also seem to remember my tutor at college being adament that you apply diversity to the rating of the ocpd and not that of the load... is this right?

.........................................

 

[HappyHippyDad]

So now my max demand has gone up to over 100A!! :shocked3:

I think I'm going to try my hand at plumbing.. :joker:

 

[fredrico]

I have just read that bit in the OSG, your way is a lot simpler Mr Malcolmsanford!!!!

 

[dagrat]

I must admit the X .4 method is the one I generally use. This does however, highlight the problem with applying diversity. there is no hard and fast rule and each situation must be evaluated on its own merit. I do think the OSG formula is oversimplified and and can be misleading, at the end of the day it's the experience of the electrician that provides any accuracy in applying diversity.

 

[malcolmsanford]

View attachment Guidelines_network_design.pdf

I've attached this a few times on diversity threads and wonder if we should make this a sticky to refer to

 

[smu_diddly]

Niceic assessors tell you to add up the total Number of amps and multiply by 0.4. Job done.

 

[Engineer54]

Why would you not use the OSG to apply diversity, when it is a book produced from the IEE wiring regs, the same institution that we all take guidance from ????

The ONLY formula that works for diversity/max demand in BS7671 or the on site guide, is the formula for Cookers up to 15KW. The rest are literary a waste of print on paper space, they don't and have never worked!!!

The standard formula that anyone with experience will use for max' demand, is that given by Malcolm...

 

[Amp David]

+1 for the x 0.4 calculation