Maximum demand - please help.

[hughesy]

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.

 

[sparkymaz]

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?

 

[WarrenG]

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.

 

[sparkymaz]

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 ?

 

[WarrenG]

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

 

[sparkymaz]

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

 

[WarrenG]

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

 

[MJRElectrical]

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.

 

[mrloy99]

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

 

[WarrenG]

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?

 

[mrloy99]

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.

 

[Bearhands]

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.

 

[jools p]

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.

 

[maddfridge]

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.

now just who did i go with ah parents

cheers all

 

[Bearhands]

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!