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Discuss Maximum demand - please help. in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

1

12345aob

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Under "PARTICULARS OF INSTALLATION REFERRED TO IN THE CERTIFICATE" maximum demand (load)...................kva/Amps

Do I put in the whole load for the house or just the load for the new circuit I have added?

I have just put in an outside switched socket and two 150w swiched floods. less that 15 amps in total.

Also has the 17th been amended yet?
 
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1

12345aob

  • Thread Starter Thread Starter
  • #2
Under "PARTICULARS OF INSTALLATION REFERRED TO IN THE CERTIFICATE" maximum demand (load)...................kva/Amps

Do I put in the whole load for the house or just the load for the new circuit I have added?

I have just put in an outside switched socket and two 150w swiched floods. less that 15 amps in total.

Also has the 17th been amended yet?
17th has not been amended. maximim demand if for the whole house.
 
C

coxy

  • Thread Starter Thread Starter
  • #3
Hi,
What you need is a clamp meter get the house holders permission and turn all major appliancies and take a reading then apply diversity.
 
J

Jurassic Spark

  • Thread Starter Thread Starter
  • #4
You can ASSUME a max demand of 80amps - same as the cut-out fuse size.
 
M

maddfridge

  • Thread Starter Thread Starter
  • #6
I concur maybe 100amp though, only problem is finding out what the fuse size is without cuting the seal on cutout.[/quote


Hi all
diversity = maxiumum demand / connected load !

so someone may have designed the house with future additional loading allowed you need to assess the whole house loading first then see what amps you are theortically pulling. take the reading see what the difference is if it to much up the service cut out fuse assuming all circuits will meet bs7671 for volt drop etc.

fun this diversity and max demand is it not

doing 2391-20 its mind blowing !!!!!!!!!!

cheer all
 
E

ezzzekiel

  • Thread Starter Thread Starter
  • #7
as a quick guide adding up all breakers in use and taking 40% will give you an estimated demand
 

Des 56

-
Arms
Esteemed
The way I see it is that you ascertain wether the system can carry the additional load initially
You install the additional circuit The certificate is then applicable to that circuit only
So the max demand is then the load of that single addition
Otherwise you could have a single circuit certificate for say a 8Kw shower with a max demand of maybe 80amp on the cert
 
S

sparkymaz

  • Thread Starter Thread Starter
  • #9
Calculating max. demand, i am up to 145A with diversity taken into account. all notices on boards say max current should not exceed 100A. Demand with dirversity on 2x 40A (showers) 3x32A (ring main) and 1x16A Radial takes me to 114A already. Just seems high to me, am i calculating right? if so what are the implications? I heard someone say you can assume 100A as thats what the main incoming fuse/main switch is rated at. Is this right?
 
N

Nutter

  • Thread Starter Thread Starter
  • #10
Hi all. Maximum demand for domestic testing is the incoming fuse size, ie, 60A, 80A or 100A, depending on what is fitted. That’s what I put on the test results. The demand on the main fuse takes into account that every electrical item will not be switched on and used at exactly the same time. Most electrical items have a higher start current for a few seconds, and then drop back down to a lower than rated current when running. And don’t forget, you can’t pull more amps than the main fuse is rated at!
 
M

maddfridge

  • Thread Starter Thread Starter
  • #11
Hi all. Maximum demand for domestic testing is the incoming fuse size, ie, 60A, 80A or 100A, depending on what is fitted. That’s what I put on the test results. The demand on the main fuse takes into account that every electrical item will not be switched on and used at exactly the same time. Most electrical items have a higher start current for a few seconds, and then drop back down to a lower than rated current when running. And don’t forget, you can’t pull more amps than the main fuse is rated at!

on site guide gives guidance which really should be taken in to account then completely burnt and common sense used.

it is all down to what is needed for that installation with allowing for future growth or use but who know what it will be used for in a weeks time to a years time.

look at the number odf outlet points and make a calcualted judgement and think to add for future growth

wiil the sub mains take a 100 amp fuse ?? just basing it on fuse size is a good methos but will the supply cable carry the future load. in all installations whats coming in through the floor tells you what you can and cannot do anyone agree . if it aint big enough to cope make it bigger ??

design exam on tuesday helppppppppppppppppp!!!!

must be mad cheers all
 
T

trebor

  • Thread Starter Thread Starter
  • #12
Hi all. Maximum demand for domestic testing is the incoming fuse size, ie, 60A, 80A or 100A, depending on what is fitted. That’s what I put on the test results. The demand on the main fuse takes into account that every electrical item will not be switched on and used at exactly the same time. Most electrical items have a higher start current for a few seconds, and then drop back down to a lower than rated current when running. And don’t forget, you can’t pull more amps than the main fuse is rated at!
what utter tosh, you do not use the main fuse rating as md even if you think you know it, a carrier can be marked as 100 amps and have a 60 amp fuse fitted.

im afraid if i was assessing you for 2391 using the main fuse figure would be a fail

read your osg it has a clear tabulated guide to calculating md
 
M

maddfridge

  • Thread Starter Thread Starter
  • #13
hi trebor

when you do the design project the onsite stuff just does not work after all the on site guide is just that a guide.

you have to look at the oveall loads and allow a common sense diversity against known standards. Doing this now some showers tradinitonal are up to 10kw power showers are as low as 150 watts
trition
Voltage 230/240v Ac. Rating 150W nominal Temperature Control Manual
Plumbing System Compatibility Gravity hot & cold only Inlet Connections
15mm push fit Water Entry Points Rising, falling, back Outlet Connection
1/2” BSP Minimum Head Pressure 75mm Maximum Head Pressure 10m Maximum Open Outlet Flowrate 14l/min Exposed Wall Mounted Only

Approvals BEAB, CE, BKM guarantee year
AS2000x integral pump power shower

The reason is because the hot water is pulled off the super duper gas fired water heater thingy not the old combi type. stuff like that so just looking at loading is a good method against the fuse sizestarting point to see where the loading can be reduced or increased a drop like that from 30 amps to 45 amps. no immersion heater no tank so there another 13 etc get the general gist

cheers

ps the exam was a right Bast**D selv every where will have to wait for feb to find out meantime project
 
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N

Nutter

  • Thread Starter Thread Starter
  • #14
what utter tosh, you do not use the main fuse rating as md even if you think you know it, a carrier can be marked as 100 amps and have a 60 amp fuse fitted.

im afraid if i was assessing you for 2391 using the main fuse figure would be a fail

read your osg it has a clear tabulated guide to calculating md



Thank you Trebor.
Fortunately for me I have 2391 and thankfully, you didn’t assess me!
The regs do state that you make an assessment of the demand and use of the installation, and include a diversity factor, when making that your calculation for maximum demand (311.1 + 313.1)
My argument was purely for domestic properties. If for example your total demand of breakers in the consumer unit was 180A and you added a diversity factor that brought it down to 120A, that is then your maximum demand, but how can you pull that through an 80A fuse?
The other point that you made about the fuse carrier, in my opinion, anyone who is not prepared to pull out the main fuse and open it up to see what the fuse rating is should not be testing. First, you do not guess what is in the carrier. Second, how do you know the tails are the correct size for the fuse? And finally, you must have safe isolation of the system you are working on to comply with the Electricity at work regs (reg 13)
 
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D

Dave

  • Thread Starter Thread Starter
  • #15
what utter tosh, you do not use the main fuse rating as md even if you think you know it, a carrier can be marked as 100 amps and have a 60 amp fuse fitted.

im afraid if i was assessing you for 2391 using the main fuse figure would be a fail

read your osg it has a clear tabulated guide to calculating md
You can put your point forward without being offensive....please remember this next time before you hit your enter key...
 
H

hughesy

  • Thread Starter Thread Starter
  • #16
Thank you Trebor.
Fortunately for me I have 2391 and thankfully, you didn’t assess me!
The regs do state that you make an assessment of the demand and use of the installation, and include a diversity factor, when making that your calculation for maximum demand (311.1 + 313.1)
My argument was purely for domestic properties. If for example your total demand of breakers in the consumer unit was 180A and you added a diversity factor that brought it down to 120A, that is then your maximum demand, but how can you pull that through an 80A fuse?
The other point that you made about the fuse carrier, in my opinion, anyone who is not prepared to pull out the main fuse and open it up to see what the fuse rating is should not be testing. First, you do not guess what is in the carrier. Second, how do you know the tails are the correct size for the fuse? And finally, you must have safe isolation of the system you are working on to comply with the Electricity at work regs (reg 13)
excellent i totally agree.
 
S

sparkymaz

  • Thread Starter Thread Starter
  • #17
Hey everyone. Quick question about the fuse carrier eg a 100A main fuse. am i right in saying that this may not necesarilly blow at 100A, depending on the fusing factor. so therefore you could be drawing more than 100A continuously through mains tails only capable of a 100A load max.

I may be taking on a kitchen refit including a board change on a house that has 2 shower circuits 40A each. I will not be adding to the demand as all appliances are new and to be used in the new kitchen with no additions. This brings my max demand with diversity allowances above the 100 amps and am not sure how to go about this. Im sure someone has come across this problem before, in my opinion the previous sparky was wrong to add another shower to the demand, but can i get round this as no diversity allowances are allowed on a shower!!!!!! Should i avoid this job?
 
W

WarrenG

  • Thread Starter Thread Starter
  • #18
Hey everyone. Quick question about the fuse carrier eg a 100A main fuse. am i right in saying that this may not necesarilly blow at 100A, depending on the fusing factor. so therefore you could be drawing more than 100A continuously through mains tails only capable of a 100A load max.

I may be taking on a kitchen refit including a board change on a house that has 2 shower circuits 40A each. I will not be adding to the demand as all appliances are new and to be used in the new kitchen with no additions. This brings my max demand with diversity allowances above the 100 amps and am not sure how to go about this. Im sure someone has come across this problem before, in my opinion the previous sparky was wrong to add another shower to the demand, but can i get round this as no diversity allowances are allowed on a shower!!!!!! Should i avoid this job?
Don't avoid the job!

I would first calculate the maximum demand exactly with the given diversity and see what is required. If it exceeds the suppliers protective fuse then you should contact them.

It would also be wise to check the sizing of the tails required against the existing ones.
 
S

sparkymaz

  • Thread Starter Thread Starter
  • #19
Fuse is 100A, 25mm tails. Its the fact that there is 2 showers is throwing me. No diversity on the showers allowed so add the immersion and thats 96 amps already. The rest brings me to about 140A. what would you do ?
 
W

WarrenG

  • Thread Starter Thread Starter
  • #20
Fuse is 100A, 25mm tails. Its the fact that there is 2 showers is throwing me. No diversity on the showers allowed so add the immersion and thats 96 amps already. The rest brings me to about 140A. what would you do ?
Don't forget the diversity allowed is quite conservative and in that case you would probably find that lots of installations maximum demand would calculate to higher ampage than the protective device.

Not everything will be on at the same time (example ring circuit not every plug is used at the same time) - hence the diversity figures

You have already said that you are not adding to the maximum demand so the fuse must be holding out? Has the customer had any previous problems?

Just a suggestion but I would maybe split the loads with 2 RCD's, but put one shower circuit on the one RCD side and the other shower on the other RCD? (see attached)

http://i421.photobucket.com/albums/pp295/WarrenG_017/17thEditionBoard.jpg

If you are still concerned give the supplier a quick call and see what they would recommend before you start the job.

Warren
 
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S

sparkymaz

  • Thread Starter Thread Starter
  • #21
Thanks warren, i think im going to buy a clamp meter and see what the installation is pulling at the min. Would you recomend any?

Much appreciated
 
W

WarrenG

  • Thread Starter Thread Starter
  • #22
Thanks warren, i think im going to buy a clamp meter and see what the installation is pulling at the min. Would you recomend any?

Much appreciated
Fluke are good ones ;)
 
M

MJRElectrical

  • Thread Starter Thread Starter
  • #23
Thank you Trebor.

The regs do state that you make an assessment of the demand and use of the installation, and include a diversity factor, when making that your calculation for maximum demand (311.1 + 313.1)
My argument was purely for domestic properties. If for example your total demand of breakers in the consumer unit was 180A and you added a diversity factor that brought it down to 120A, that is then your maximum demand, but how can you pull that through an 80A fuse?


The OSG gives ways of calculating maximum demand for differnt circuits, how is it possible to apply diversity to all MCB's when for example a shower ciruit must be taken at 100%. And i think 100A would flow through an 80A 1361 fuse for a while if nt a long time before it would trip.
 
regarding your 2 shower situation,you can get and indeed are recommended,a shower priority switch which works on current demand which effectively reduces your maximum demand.We used this on a B&B.It can also be usefull in other situations.They come in 2 types priority and non priority.One type you can nominate who has prioity,the other type is 1st come 1st served.We had to wire 4 of them thru this changeover switch.Don't know what they cost,or have a link,but I know that ABB do one and also GE.Can result in the odd cold shower but,can also pull you out of a hole regarding your demand problem
 
W

WarrenG

  • Thread Starter Thread Starter
  • #25
regarding your 2 shower situation,you can get and indeed are recommended,a shower priority switch which works on current demand which effectively reduces your maximum demand.We used this on a B&B.It can also be usefull in other situations.They come in 2 types priority and non priority.One type you can nominate who has prioity,the other type is 1st come 1st served.We had to wire 4 of them thru this changeover switch.Don't know what they cost,or have a link,but I know that ABB do one and also GE.Can result in the odd cold shower but,can also pull you out of a hole regarding your demand problem
Be interested in seeing these. Are they actually called priority switches?
 
Be interested in seeing these. Are they actually called priority switches?
Yes,warrenG,I think they are in fact known as shower prioity switches.The B&B we did was in Ireland.This wholesaler lists them www.alphaslam.ie but everything is a lot dearer in Ireland even when you allow for euro/pound,but it will maybe give you an idea.I don't see why they cant be used for other applications though.
 
B

Bearhands

  • Thread Starter Thread Starter
  • #27
Just 'Googled' it as 'shower priority switch' several links came up (the first few were Irish domains.
Incidentally, I recently installed a 'Body Drier' (Triton, I think). The manufacturer's instructions said that this could be connected to the same service as the shower. This confused me for a while until I realised that this had a built-in switch so that the two devices could not be running at the same time - first time either of us had seen such.
 
J

jools p

  • Thread Starter Thread Starter
  • #28
Adding all the MCB's up and then taking 40% does not work on a house like mine. It has 15 circuits. Its not a big house but has three floors and lots of small circuits. Add them all up and it goes way over. I want a quick reliable rule of thumb that I can put down on the forms.

Who the hell is going to read this **** anyway.
 
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M

maddfridge

  • Thread Starter Thread Starter
  • #29
regarding your 2 shower situation,you can get and indeed are recommended,a shower priority switch which works on current demand which effectively reduces your maximum demand.We used this on a B&B.It can also be usefull in other situations.They come in 2 types priority and non priority.One type you can nominate who has prioity,the other type is 1st come 1st served.We had to wire 4 of them thru this changeover switch.Don't know what they cost,or have a link,but I know that ABB do one and also GE.Can result in the odd cold shower but,can also pull you out of a hole regarding your demand problem

hi all

i remember these switches from the 1979 we went to pontins for a week and every time you wanted a shower you switched over from all things the cooker circuit so a butty and a shower certainly was not a thing to have.:D

now just who did i go with :rolleyes: ah parents :p

cheers all
 
B

Bearhands

  • Thread Starter Thread Starter
  • #30
A bit like the Hitch-Hikers Guide to the Galaxy the NICEIC book refers you to IEE Guidance Note 1. I'll quote anyway: "This is not, as is sometimes assumed, the rating of the electricity distributor's cut-out fuse(s). the maximum demand is a value, expressed in amperes per phase, evaluated on the basis of the connected load with an aloowance for diversity.' They say there are other methods but don't say what! Arguably, you could switch everything on and put a clamp meter on the tails. This of course would not work if the tails burn out in the process! Am I being a cowboy if I say we often just take a long hard look at what's connected, and knock 5a off the value of the cut-out? Seriously, though, if a customer asked us to connect a couple of 8 kw showers and a 6kw hob where there was a 60amp cut-out we'd say no. I was once told (on reliable authority) that the distributors rate a property at 1kw for the purposes of the network!
 

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