Am i wrongly calculating my maximum demand?

[iwantmyNVQ]

Hi guys i calculating my expected maximum demand for an installation to see if the 60A cut out will handle it.

heres what will go in and calculations incl diversity

2X 32A Rings, so 32A(first ring) + 12.8A (40% all others)
1X 6A lighting, so 4A (66%)
1X10A lighting, so 7A
1X32A Shower, so 32A
1X45A cooker, 10A+ 40%of 35A= 24A
1X32A Under floor heating, 32A

so this equals 144A????

am i missing something this isnt right!!

 

[Jay Sparks]

Ive always taken the biggest mcb away, then add the others togehter, then divide them by 4, then add back the biggest one. Which is about 81A but I doubt you will get anywere near this as it is unlikly you will have everything on all at once.



Jay

 

[iwantmyNVQ]

so if i do that it comes to 81A, but ive only got a 60A cut out! what can i do client wants to do away with gas heating and oven so everythings going electric!

 

[Jay Sparks]

Its very unlikley you will have everything on all at once. So IMO the 60A should be ok. But if hes adding more stuff then this will need recalculating and maybe then the cut out will need upgrading.

Jay

 

[morph]

is this a TT system ??

 

[iwantmyNVQ]

thanks for the advice jay, its in a block of flats and the suppliers armoured looks about 10mm so is upgrading the cut out an option, surely 80/100 A would require a 25mm, considering its a good 80M run.....

no its a TNS morph...

i understand scientifically that load wont be reached but what happens when it comes to writing out the certs and my demand is higher than the cut out>?:crazy:

 

[iwantmyNVQ]

i want to point out im using this as my 2nd example for the niceic to come out and NEED everything to be bang on!!

 

[Jay Sparks]

When I first started doing testing for a company a few years ago, I asked what should I put in the max demand if it calculated over the cut out rating and they said put half the cut out rating. But now, if its more I just put the acctual size of the cut out because in theory the installation cant demand more that the cut out anyway. A standard 2/3 bedroom house will pull roughly 45A max.

Jay

 

[morph]

hi
if tns you could get a 100 A put in , however i would just jot done 60A as max demand they wont say owt, its all a bit finger in the air really anyway to be honest
i have all leccy and a 6o A main fuse and all is cool

 

[Jay Sparks]

hi
if tns you could get a 100 A put in , however i would just jot done 60A as max demand they wont say owt, its all a bit finger in the air really anyway to be honest
i have all leccy and a 6o A main fuse and all is cool

If its all cool maybe your ALL leccy heating is broken,lol

Jay

 

[iwantmyNVQ]

yeah that makes sense obv the max USED demand cant be higher than the cut out as itl blow but from the assesors point of view he will notice my design current is higher than my supply will allow so im worried about being pulled up on it. would an explanation that even though there are two rings for functionality of the computer room niether of the rings will be pulling anywhere near 10A!!

thanks jay this is reassuring and well needed meeting with the client tomorrow to tell them if we can go electric UFH or wether we will have to go for a wet system

 

[Top Cat]

I was pulled up on my NIC inspection for putting 60 Amps, i said yes i know with gas central heating, no shower and gas hob it probably should be near 40 Amps.

No he replied, because i had put in a lot of circuits he worked it out for me. The answer 160 Amps!!!!!

I just tell them the method i use to work it out now is common sense! Doesn't always go down well though.

 

[Jay Sparks]

I think the underfloor heating should be ok. I dont think your assessor will have a problem with it. To be on the safe side you coud ring the DNO and tell them what circuits you have and what circuits you will be adding and see what they say. But if they need to upgrade the main cut out it may cost you loads.

Good luck with the assessment

Jay

As TC says, the assesor could be a burke.

 

[iwantmyNVQ]

I was pulled up on my NIC inspection for putting 60 Amps, i said yes i know with gas central heating, no shower and gas hob it probably should be near 40 Amps.

No he replied, because i had put in a lot of circuits he worked it out for me. The answer 160 Amps!!!!!

I just tell them the method i use to work it out now is common sense! Doesn't always go down well though.

so he was telling you to put 160A thats ridiculous!!

 

[iwantmyNVQ]

sorry whats the DNO im a bit tired!?? all these numbers!!

im re-wiring the lot

 

[Jay Sparks]

sorry whats the DNO im a bit tired!?? all these numbers!!

im re-wiring the lot

The electricity supplier for that area. Like here its YEDL/NEDL.

Jay

 

[Top Cat]

so he was telling you to put 160A thats ridiculous!!

Yes, but i should put a note saying that it was not likely to achieve this.

He failed to mention that if this was the case i should be putting in a 200Amp main switch.

He got fired a year later!

 

[trev]

Distribution Network Operator

 

[iwantmyNVQ]

pahaha sounds about right!!

so can i take the understanding that even tho through applying diversity and the relevant calculations my design current can be higher than the cut out as the likelyhood of it reaching that load is slim?

 

[morph]

pahaha sounds about right!!

so can i take the understanding that even tho through applying diversity and the relevant calculations my design current can be higher than the cut out as the likelyhood of it reaching that load is slim?

technically no because design current is what you will really draw,
diversity is different in real terms , also i think its 30 % of full load not 40 % as you mentioned earlier
just put 60 A i would be amazed if the assessor says anything, dont stress it out

 

[Engineer54]

Your customer will rue the day they went all electric, ...unless of course they have plenty of money in the old tommy tank!! lol!! ...And totally buggered, if and when they have power cuts for any length of time!!

 

[D Skelton]

Hi guys i calculating my expected maximum demand for an installation to see if the 60A cut out will handle it.

This is how I would work it out:

1X 32A ring, - 32A
1x 32A ring - 32A x 0.4 = 12.8A
1X 6A lighting, - 6A x 0.66 = 3.96A
1X10A lighting, - 10A x 0.66 = 6.6A
1X32A shower, - 32A
1X45A cooker, - 10A + (35A x 0.3) = 20.5A (It's 30% of what is left over after the 10A (plus socket if applicable), not 40%)
1X32A underfloor heating, - 32A

Then leave out the largest load (in this case one of the 32A circuits where diversity was not allowed), while you work out 40% of what is left, then add together to get a figure: (This is the part you were missing out)

(32A + 32A + 20.5A + 12.8A + 6.6A + 3.96A x 0.4) + 32A = 75.144A

Then add up all circuits pre-diversity and work out 40% of that total:

45A + 32A + 32A + 32A + 32A + 10A + 6A x 0.4 = 75.6

Providing there is no difference greater than 5A between those two numbers, you can take whichever is the lowest of those numbers to be your maximum demand, otherwise, take the highest. In this case, your maximum demand is 75.144A

This is how I was taught in college anyway.

Seems to me you'll need to shove an 80A in to replace the existing 60A. But don't worry about upgrading the 16mm tails to 25mm ones as 16mm tails are rated at 87A anyway, so you'll be fine there.

Hope that helps

 

[Octopus]

If you're doing a re-wire why don't you fit fewer circuits and then the "old" diversity calculation will probably work.

But seriously as more and more homes are getting 8 or 10 way boards the approach to calculating loads needs to be revised.

Personally I add up all the breakers and multiply by 0.4!

 

[Farmelectrics]

dont you mean divide by 0.4

 

[Farmelectrics]

stand corrected multiply it is just done calcs

 

[Octopus]

dont you mean divide by 0.4

100 divided by 0.4 = 250

100 multiplied by 0.4 = 40

oops - sorry just seen your next post!

 

[Farmelectrics]

murdoch can you apply this 0.4 calculation to 3 phase boards aswell

 

[Rockingit]

Agree here totally with Murdochs thoughts on how the traditional methods (whichever one you use) are increasingly misleading with more circuits on wider boards. A method I'm starting to use more and more is the principal of starting with your max incoming capacity (20kW, say) and then 'spending it' around the install. You almost always have lots of spare to save.

 

[telectrix]

also bear in mind that a 60A BS1361 fuse will run all day up to 100A without blowing.

 

[Farmelectrics]

is that because all fuses breakers rcbos etc take up to one half times its load

 

[Farmelectrics]

not ideal though as the cables them selves are taking more current than they should

 

[telectrix]

agreed, but such currents would only be short duration.