Max Demand (Help)

[1strangefish]

HI All

First Off Merry Xmas to you all!

Recently doing a friend a favour and didn't think much of it until, in the cold light of day I sit down to do the paper work, set up as follows:

TNCS feeding a town centre commercial coffee cafe.  Two 12 way dual rcd boards already in place, asked to change two radials to rings as one of the radials is popping the 20amp MCB.

Board set ups as follows

DB1

RCD 1

2 rings 3 lighting circuits  Total MCB rating: 82a

RCD 2

2 Rings 2 radials 2 ighting circuits  Total MCB rating: 116a

DB2

RCD 1

3 rings 1 radial  2 lighting circuits   Total MCB rating 128a

RCD2

2 rings 2 radial  1 lighting circuits   Total MCB rating 102a

Main fuse still holding but the best Max demand I can get is 173 amps and I have only added 24 amps to the circuit layout.

What to do now?  After some kind and friendly advice from the old and wise, please be gently.

Thanks

Fish

 

[SPECIAL LOCATION]

Well you cant work out max demand properly with just MCB ratings...

There are zillions of threads in the bowels of the forum about it if you search a bit...

It is the number of light fittings, plugged in accessories & appliances fixed loads etc..

and then duration they will be on for..

once you have collated that info you can do the calcs....

If your max demand if still too high..

then you need to apply for a bigger incoming supply..

OR

disconnect some load off!

I think its reg 132.16 (verifying incoming supply & earthing are adequate for any alterations Temp or Permanent) you missed off...

But then again...

If I was doing a favour for a friend I would have done the proper calc's first as you don't want to leave a friend in the lurch with a dangerous install.

The question that should have been asked to start off with is who designed the original 20A circuit that kept tripping the MCB...

AND WHY had they only done it as a 20A circuit to start off with??????

 

[ProDave]

Well the work you did has not actually increased the load in use. If I'm reading it correctly by converting the 20A radial to a 32A ring, you have just eliminated nuisance tripping, The load was still being used, just that it was too much for the 20A MCB but the 32A one is happy.

Have you measured the total current being used? Try monitoring that for a while to see how close you are getting to 100A. It could be the other rings are very lightly loaded (in which case you might consider downrating them to make the diversity calculations work)

But if the actual load in use is close to or exceeding 100A then you will need to apply for a larger capacity supply, or find a way to reduce their loads (not easy if most of them are cooking applainces. Low energy light bulbs is unlikely to make much of a difference)

 

[1strangefish]

Have you measured the total current being used? Try monitoring that for a while to see how close you are getting to 100A. It could be the other rings are very lightly loaded (in which case you might consider downrating them to make the diversity calculations work)

Hi ProDave

Thanks for the reply, havent been able to measure usage as I have not been able to get to site when in use.  My first commercial and came to the job without prior, so a lesson learnt. 

Keen to rectify the issue, any ideas of the costs associated with an increase in supply to a commercial property (how long is a piece of string I suppose)?

 

[kerching]

Keen to rectify the issue, any ideas of the costs associated with an increase in supply to a commercial property (how long is a piece of string I suppose)?

A piece of string is EXACTLY twice as long as it is from one end to the middle....simples!

The costs levied by the DNO to,install a new service is as fixed as a Politicians expense claims.....unfathomable

Just sleighing

 

[Evans Electric]

Fishy ,  just go back and measure the current at their busy time ,  you can do calcs until the cows  come home ,  the ammeter does not lie.

If you don't have one you can get a cheapo Silverline for the price of two packs of fags .  We have one on the van in case of theft or breakage and a quality one in the office ........cheapo is around 2 amps different to the better one . 

 

[Manator]

As it it Christmas I will offer some really useful advise. for your next short course try plumbing.

 

[1strangefish]

As it it Christmas I will offer some really useful advise. for your next short course try plumbing.

I don't suppose you have every found yourself in such a position, Be it electrical or otherwise. It must really make your day to make ignorant snipes from "on high"after being asked for help. Well done you.

Merry Christmas

Fishy , just go back and measure the current at their busy time , you can do calcs until the cows come home , the ammeter does not lie.

Thanks Mr E

 

[SPECIAL LOCATION]

Whist Deke is correct that ammeter doesn't lie and calculations can be time consuming and a bit of a burden...

When you are installing new circuits.. Or new installtions..

To supply equipment that is physically not installed yet.

Then calcs DO have to be made.. as you have no circuits or loads to stick an ammeter on to.

So it should be a basic skill you have attained just like skills to terminate a cable...

I think Manator could be getting frustrated by people omitting the basic first steps before doing any alterations

temporary or permanent   as per 132.16

If you check the supply and the loading first..

then you wouldn't be worrying about your max demand.

When we are working with a product that can kill, injure or cause major property damage..

and someone is a bit complacent or about doing the basic checks first...

Can you not understand why some do get concerned about the competence of the questioning?

To be quite honest..

to say you sat down and did a few calcs after doing the work..

is a bit 'Cart' before the 'Horse'..

Hopefully you will learn from this error and take this post in the manner it is intended

Have a good Christmas...

 

[Robin Spark]

Funnily enough, if Manator said to me take a short plumbing course, then i would think great idea!

Im useless at even taking a radiator off the wall or changing a tap without it leaking afterwards!

However, i wouldnt be advertising myself as a plumber afterwards!

 

[NozSpark]

I think I need a short plumbing course...

After carefully draining down and removing a radiator I proceded to cut through a couple of radiator pipes with my circular saw while attempting to lift some floor boards..... Water everywhere!!!!!

 

[Sharpend]

I'd say a carpentry course so you understood how to use the saw! Depth gauge and all that?

 

[NozSpark]

I'd say a carpentry course so you understood how to use the saw! Depth gauge and all that?

Ahhhhh,, I see where I'm going wrong, incorrect course for the type of work being carried out

 

[Sharpend]

Now where have I heard that before???

 

[Manator]

I don't suppose you have every found yourself in such a position, Be it electrical or otherwise. It must really make your day to make ignorant snipes from "on high"after being asked for help. Well done you.

Merry Christmas

Fishy , just go back and measure the current at their busy time , you can do calcs until the cows come home , the ammeter does not lie.

Thanks Mr E

I only suggested it because there is less maths, and no real design criteria.

I have found myself in the same position, however I was never allowed to work unsupervised for years, and I installed to designs and calculations provided by others. In fact I remember in the late 1970's when I did my basic training!!! Maximum demand and diversity was included in nearly every exam. How things change.  

It is good that you asked though, there are many out there who are totally ignorant.

 

[apprentice87]

I do hope Mr Fish comes back; It is good to talk...

john...

 

[Evans Electric]

I always found Maximum Demand an elusive & ever changing minefield to enter .  

I note that The Fish is adding up the size of breakers in his OP  ., which only works for fixed loads .   

The guide in the OSG  / Regs is a help but  for domestic only .      Commercial depends on the business really .  

A few examples from back in the day, as they say :

Theres a  printers I still look after after 20 years ,  first time there to look at their distribution as they were buying their first 5 colour presses  (2 off)  

The original set up was running on a  100A TPN  supply,  domestic size meter with 16mm meter tails and 10,000 different fuseboards added over the years.   Loads of fluorescent lighing ,  machines everwhere ,  1 colour Heidlebergs... Cutters, crimpers, folders, booklet makers, guillotines, gluers , load of Macs in the art studio  .30 staff.

16mm tails ... (OK  only 2ft long)   just a little warm  ..fuses never blown .   You'd think at first , this place can't run on that supply but it did .  Much like the scientists who did all the aerodynamic calcs and announced that the  bumblebee is incapable of flight .

And things never remain the same ,  we made alterations to take a 300A  supply ( largest avaliable for area)  with original presses running at 60A  plus IR driers . 

Those presses lasted 10 years  , latest ones , Sakuri  only run at 35A .

You can see the unpainted bit after 20 years where the original meter was.

I have a pic of the new mains job , looking knocked about now after 20 years .

 

[m4tty]

Is that brown parcel tape ip rated sandra

 

[Doc Hudson]

Is that brown parcel tape ip rated sandra

I am sure you could never suspect Deke would use sub standard brown tape Matty? But I am not convinced that the spacing between strips is totally compliant. May be done to 16th edition brown tape spacing?

Doc H.

 

[ProDave]

I've never seen brown parcel tape used as an approved means of lock out before.