Max Demand

[hutch6447]

Hi all just got quick question 60 A Main service fuse total house rewire worked out tat Max demand using x 0.4 will be 64.8 A just wondering if it might be worth giving DNO a ring to see if its possible to upgrade to 80 A main fuse is it worth ringing?

 

[claret73]

I've never used the x4...use the diversity checks though (even as a Mate to a Spark) I guess it would be sensible for future proofing? Would like to hear how you get on...

 

[telectrix]

not until it blows. don't hold your breath. a 60A fuse will happily take 100A for a short while, and i doubt if you'll ever get near that. diversity calculations for max. demand are about as accurate as the luftwaffe bombing. they missed the spitfire factory and hit our chippy, 20 miles away.

 

[hutch6447]

yeah well that's what I thought but then out the blue he said in future he might be installing a hot tub in the garage now I don't know if he will go through with it but with this in mind might be worth just seeing if I can get and upgrade.

 

[Engineer54]

Didn't think anyone used the diversity tables in BS7671 these days!! The only one that has any credibility is for cookers... The rest well..... lol!!!

 

[richy3333]

Customers say a lot, like 'I'll pay you tomorrow'...
I'd worry about it when he installs the hot tub, if it ever happens. At the moment he'll have little interest in paying for the work, but when he's bought the tub and wants the work doing he will be interested then?

 

[hutch6447]

And yes its a big garage and yes he is the type of customer that constantly changes his f*****g mind.

 

[claret73]

Didn't think anyone used the diversity tables in BS7671 these days!! The only one that has any credibility is for cookers... The rest well..... lol!!!

Without sounding too wet behind the ears, it's what has been drummed into me by 3 years of C&G...therefore in the 'Real' World of ignoring said instruction would you share what perhaps they should have taught me & I'm kinda waiting for the 'experience' part...All seems a contradiction though, to comply with BS7671 seems I need to rip a couple of pages out as it's worthless referring to?...

 

[telectrix]

depends on the hot tub. last one i worked on was to replace a low IR SWA feed to the tub. it was "professionally installed" --- 13A BS1363 plug "fitted" ( and i use the term loosely) on the SWA, no earthing of the armour. plugged into a RCD socket adaptor, even though the circuit was RCD protected anyway. once up and running, it only drew 8A. mind you, it did take 3 days to get warm.

 

[hutch6447]

Yeah no this is the business this hot tub apparently according to what he say's will have to put it on its own circuit will know for sure when I see manual but I am thinking of getting the main fuse upgraded or enquiring about upgrading it in case he goes for a steam room and sauna lol.

 

[telectrix]

and this is probably a customer that fits those goddamawful cfl bulbs to save the planet.

 

[KFH]

I used the diversity calcs for my own house and got 130A including a 9kW pool heater on for days at a time, I could afford it at one time:-( It has run on a 60A fuse for the last 30 years! It has got a bit warm though.

 

[Rockingit]

Didn't think anyone used the diversity tables in BS7671 these days!! The only one that has any credibility is for cookers... The rest well..... lol!!!

Agreed - I've currently got a new build that according to the official sums comes out at something like 250A after diversity, sitting happily on an 80A DNO fuse being fed from a 16kW pole transformer!

 

[Farmelectrics]

I wouldn't worry as said fuse can take up to 1 half times it's load highly unlikely everything will be on at once

 

[malcolmsanford]

Without sounding too wet behind the ears, it's what has been drummed into me by 3 years of C&G...therefore in the 'Real' World of ignoring said instruction would you share what perhaps they should have taught me & I'm kinda waiting for the 'experience' part...All seems a contradiction though, to comply with BS7671 seems I need to rip a couple of pages out as it's worthless referring to?...

No quite right Claret if that was how your were taught then you are right in questioning why ................ personally I would think diversity is something that is impossible to teach.

I assume your talking about Appendix A in the OSG when dealing with diversity, and if you read the the cover page to this appendix you will see the IET take pains to tell you that

'The information and values given in this appendix are intended for guidance because it is impossible to specify the appropriate allowances for diversity for every type of installation and such allowances call for special knowledge and experience'

I've highlighted the crucial part to any diversity calculations. The 0.4 rule of thumb for domestic is perhaps still the best and most accurate way of doing any rule of thumb calculation, I still believe that the DNO use this when sizing up their network requirements


View attachment Guidelines_network_design.pdf