Outbuilding

[Traineeboy]

when it comes to diversity, I prefer the Western Power calc, total load * 0.4  which suggests 25mm as suitable?

thanks Binky that is exactly what I was looking for. Is this an official Calculation ? Guess it’s not in the regs ? 

Would that be to times 57.9 by 0.4 or before diversity by 0.4 ? 

 

[poni]

Stick it on a 32A breaker 

 

[Traineeboy]

I’m beginning to think with the extra load required it might be worth the customer to get the electrical board out.  As with a 5 bedroom house , another outbuilding ( which has a garage and another DB of it) and him wanting to add an additional outbuilding they might need to put another supply in. 

 

[Traineeboy]

I suppose the best thing would be to measure the use of all the current being used at the moment via a clamp meter. When this lockdown ends probably should measure the current being used to know if there is capacity left on the main fuse. Is that something you guys would do ? 

 

[Traineeboy]

This has raised another question I’m curious. If you do end up the a 50mm2 SWA how would you even terminate that into the main supply. I would have thought into a metal unit with a switch that can terminate 50mm2 but from that you can’t go into a Henley block as surely the tails supplying that would only be 25 or 35mm2 ? Would that part of the job be western power ? 
 

 

[Evans Electric]

its the heating that you need to know Kettle, water heater, room heater, not all will be no at the same time (kettle 3mins ,water heater depends on how much water you need, room heater on depends on time of year heat loss of room and the length of time its occupied) the rest are low current and he's  unlikely to have the kittle on when he's on the treadmill 

Thats right ,  have you any idea ?  You are working in the dark without this .        As Ponl  says ,  forget the kettle ,  forget the stereo .

It looks like you have a room  heater  (power not known)  ...a Treadmill   , guess at   6A   ....  a light   1A   .... and an Immersion Heater  kicking in & out  on a stat .  

So on a cold day  the  guy feels the need to exercise  so he  turns on Treadmill   6A.....heater 12A....  stereo/ or TV /   2A  ...light  1A      total load    21A    .

Turn off treadmill  ,  Load  15A   ..has a wash , Immersion kicks in for 15mins  , load  26A      

As Poni said above  , a 30 A  supply  .   

Forget new supply from DNO.

Work out the Volt drop , your cable length was 100M    .       How come its so far from the house  . ?  Thats a football pitch .   

 

[binky]

thanks Binky that is exactly what I was looking for. Is this an official Calculation ? Guess it’s not in the regs ? 

Would that be to times 57.9 by 0.4 or before diversity by 0.4 ? 

total load before diversity. I gave up on the diversity calcs years ago. When I rewire houses I like to install more circuits than required, eg boiler on it's own supply, kithcen as a ring, remaining sockets as 2 radials up and down etc etc. The net result is that according to the 'official' diversity calc I would melt the main fuse every time, which was a total nonsense.  You need to consider loadings, but, you alos need to apply common sense too.

 

[Murdoch]

total load before diversity. I gave up on the diversity calcs years ago. When I rewire houses I like to install more circuits than required, eg boiler on it's own supply, kitchen as a ring, remaining sockets as 2 radials up and down etc etc. The net result is that according to the 'official' diversity calc I would melt the main fuse every time, which was a total nonsense.  You need to consider loadings, but, you also need to apply common sense too.

So did I - the guidance in the OSG is misleading at best, grossly out of date at worst

You have to look at the numbers of circuits and apply common sense - an older property with a BS 3036 board with 2 x 30A, 1 x 15A and 2 x 5A "may" work with the OSG guides but a property with 10 + circuits - absolutely no way - in these cases I tend to add up the circuit breakers and multiply by 0.25

What will affect these calcs is electric space heating and electric water heating / multiple showers

 

[SPECIAL LOCATION]

thanks Binky that is exactly what I was looking for. Is this an official Calculation ? Guess it’s not in the regs ? 

Would that be to times 57.9 by 0.4 or before diversity by 0.4 ? 

There is no official one-size fits all diversity calculation...

Read second paragraph of Appendix A (Max demand & Diversity ) from the One Site guide...

It basically says... " information is only for guidance as it is IMPOSSIBLE to specify appropriate allowances for diversity for every type of installation"

But for any half accurate estimation you must take account of the number of hours per day each load will be drawing full power,

and for how many of those hours multiple appliances will also be drawing full load.

Imagine you have five loads, (36A, 25A, 12A, 8A &  5A)  connected to 40,32,20,10 & 6amp MCBs...  And lets say they are on for 2 hours each day.

1/ If they always come on together clearly you will have 2 hours running 86Amps max demand.

2/ But if they are NEVER on simultaneously then the max demand will be the 36A load for 2hrs..

3/ Then other options of multiple loads running simultaneously could give various other combinations of max demand such as; 

41A, 44A, 48A, 49A, 53A, 56A, 61A, 66A, 69A,  73A, 74A, 78A, 81A

If you try fiddling around with the various guidance calculations using the MCB breaker values, you will see how hit & miss these calculations can be...

OR.. another example is consider two identical properties..  Both identically wired, same fuse-box, circuits, same appliances etc..

One home has a family with four teenage kids, and the elderly grandmother all living at home.  

The other home has a 60 year old single person, no children who is out for 8hours every day.

Clearly the probability of multiple loads running at the same time will be quite different due to the users of the installation NOT the composition of the circuits.

Bottom line is you have to apply an element of knowledge about how the installation is going to be used... 

in addition to your assessment of type of circuits and loads and expected durations of operation.

You will not find a single calculation in any guidance book that I know of that will accurately take account of ALL relevant factors.

Guinness  

 

[Evans Electric]

When my mate was  estimating they priced a new build block of flats  .   12  storey   X  4 flats per floor   I think it was .     

If you took the busy period of the day and said 30A  per flat  you'd get 1440 Amps  on the mains  .    

When they spoke to the (Then MEB)   apparently they just said no, a 500A supply  will be fine , like all the others .     :C   

Diversity is a very difficult thing to calculate when they're not certain fixed loads etc.   

Main fuses  will hold overloads at peak times ,   like the proverbial  cuppas in the Coronation Street ad breaks ,  or Christmas dinner cooking all at once . 

You cannot actually design  for folk arriving home from work , cooking , showering  at the same time every day .     

 

[Traineeboy]

Thanks to all who took the time to comment. It’s looking like it probably could go on a 32A breaker and a 25mm2 cable would be OK , although I understand I have to account for the wiring inside the new outbuilding. So for example it might be a radial circuit but using 4mm2 rather than 2.5 mum. And I presume if necessary the lighting maybe have to be 1.5 if not 2.5 . Presume this is the way to get round volt drop but increasing the CSA of the cable. 

 

[UNG]

When my mate was  estimating they priced a new build block of flats  .   12  storey   X  4 flats per floor   I think it was .     

If you took the busy period of the day and said 30A  per flat  you'd get 1440 Amps  on the mains  .    

When they spoke to the (Then MEB)   apparently they just said no, a 500A supply  will be fine , like all the others .     :C   

Diversity is a very difficult thing to calculate when they're not certain fixed loads etc.   

Main fuses  will hold overloads at peak times ,   like the proverbial  cuppas in the Coronation Street ad breaks ,  or Christmas dinner cooking all at once . 

You cannot actually design  for folk arriving home from work , cooking , showering  at the same time every day .     

I always find the DNO's diversity rather extreme although it has to be appreciated that a large amount of the network was installed in the 30's and 40's recently a network cable blew up close to a job I was on, talking to the DNO lads they said this was likely to happen more often as EV's become more common and loads increase as most of the supplies across the town are the imperial equivalent  of 4c 16mm² PILC with some 4c 25mm² PILC in places

 

[Andy™]

I always find the DNO's diversity rather extreme although it has to be appreciated that a large amount of the network was installed in the 30's and 40's recently a network cable blew up close to a job I was on, talking to the DNO lads they said this was likely to happen more often as EV's become more common and loads increase as most of the supplies across the town are the imperial equivalent  of 4c 16mm² PILC with some 4c 25mm² PILC in places

not sure what size the cable in the street is, but there's a few places round here were the cable from the steet to the house is 4mm²

 

[Murdoch]

Slightly off topic but on the subject of EV's - yup we can just go and install them - wonder if the records are being collated to watch for issues in the future? 

Seems lots of the bigger houses are having them fitted.

Wonder when the grid will start asking installers to consult them for approval and they then they get turned down?

And as for new blocks of flats, then surely EV charge points should be fitted as part of the build - assuming there is parking - so how would this work?

 

[Geoff1946]

It seems to me that no-one really believes EVs will ever be available in the way petrol vehicles are now.

If the intention to replace traditionally powered vehicles with EVs within a couple of decades was sincere it would have to be supported with massive investment  in power generation and infrastructure development, which just isn't happening.  Deduce from that,  the number of EVs on the road will only ever be a fraction of the present fleet.

I believe that the best hope for the freedom of mass motoring we presently enjoy is the emergence of hydrogen fuelled technology. Who knows?  -  But I can't see it being in EVs.

 

[Murdoch]

Re EV's - the issue is cost, followed by EV charge points (which can be a red herring) as people think they drive further than they actually do. I recall that the average journey in the UK is under 10 miles - so range isn't the issue

If Governments around the world were truthful - changing all our minds on petrol isn't going to happen and most modern diesels can be cleaner than a lot of petrol cars.

Pardon the pun but to drive down the emissions, they should legislate against gas guzzlers .......... more directly than they are doing now and small isn't always good as a small car with a very powerful engine can burn a lot of fuel.

 

[Zeespark]

Traineeboy, have a look on Youtube for a chap named SparkyNinja, He has just done a webinar on this exact subject, very well worth a watch, I would keep a notepad and a pen handy for it.

Andy Guinness

 

[UNG]

Re EV's - the issue is cost, followed by EV charge points (which can be a red herring) as people think they drive further than they actually do. I recall that the average journey in the UK is under 10 miles - so range isn't the issue

If Governments around the world were truthful - changing all our minds on petrol isn't going to happen and most modern diesels can be cleaner than a lot of petrol cars.

Pardon the pun but to drive down the emissions, they should legislate against gas guzzlers .......... more directly than they are doing now and small isn't always good as a small car with a very powerful engine can burn a lot of fuel.

While the cost of an EV makes it very much a considered purchase the issue will always be that an EV's range is a bit of an unknown with some having a range of as little 60 - 80 miles in some adverse conditions, while the average journey may be under 10 miles travelling between 5 -6 jobs in a day could be multiple journeys of 5 - 10 miles, add into that the possible need to have a top up charge during that day when a fuel top up isn't a quick splash and dash on the forecourt means more planning is needed

It will be difficult for a lot of people to move away from the flexiblity the ICE gives us as the limitations of the EV become even more apparent over it's petrol and diesel equivalents

Never quite understand the term "gas guzzlers" while some people don't need a big vehicle there are others who pick a vehicle to meet all their needs throughout the year rather than have a number of vehicles on the drive. The bit I find hard to work out is how they arrive at the vehicle emissions I have a Seat Alhambra it's a big car but it returns 35 - 40 mpg round town on a long run I get up to 60 mpg the road tax is £240 there are plenty of small less fuel efficient vehicles out there that pay a small fraction of what I pay in road tax because they have lower emissions. It really needs some clarity how can a gallon of fuel in one vehicle produce less emissions than a gallon of fuel in another given the same mpg

 

[Evans Electric]

Wonder when the grid will start asking installers to consult them for approval and they then they get turned down

I read somewhere about putting a notification in when installing  an EV  charger .    

 

[Murdoch]

Never quite understand the term "gas guzzlers" while some people don't need a big vehicle there are others who pick a vehicle to meet all their needs throughout the year rather than have a number of vehicles on the drive. The bit I find hard to work out is how they arrive at the vehicle emissions I have a Seat Alhambra it's a big car but it returns 35 - 40 mpg round town on a long run I get up to 60 mpg the road tax is £240 there are plenty of small less fuel efficient vehicles out there that pay a small fraction of what I pay in road tax because they have lower emissions. It really needs some clarity how can a gallon of fuel in one vehicle produce less emissions than a gallon of fuel in another given the same mpg

My misses used to have an "ecotech" 1 litre Focus - road tax £20 / year. Economy - about 35 around town, 42 on a motorway if you drove carefully - no where near the claimed figures

My 2l T6 - will give around 40 on a motorway and being diesel will pull up any hill far better than the Focus!