Theory against real world

[androiduk2002]

Exactly what I am trying to get across Mr Flash , as I am a studying we are being taught the approved method of doing the calculations and how to find the information in the BRB , the real world against the theory .

My next point is if we designed the circuit then there is an issue and others got involved would we have a leg to stand on if we put in a smaller size than the calc says ??

 

[ADS]

Does it not strike anyone as madness that to feed a socket radial, 4 lights and a heater, you need a supply cable the same size as your main incommers but on a 40 A breaker - all because it passes through half a metre of insulation!!

 

[androiduk2002]

Its not just the insulation ADS its the buried underground that makes a huge differance.

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Why will this "Average " garage be pulling 94 A ? Where are you getting that from ? Heater ....12.5A

Lts........... 0.5A

1 Socket ... ?????? 4 A

17A ???

Deke if you refer to post 9 I have listed the figures, for the design current

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Does it not strike anyone as madness that to feed a socket radial, 4 lights and a heater, you need a supply cable the same size as your main incommers but on a 40 A breaker - all because it passes through half a metre of insulation!!

YES Hence why i brought the topic up

 

[fletchflash]

well on this one you'd have to use common sense. You dont always have to do what a calc says. You have to make judgements yourself, course you'd have a leg to stand on!

You have to use common sense with theory.

Its not theory against real world,

Its theory WITH real world

 

[kme]

Excellent O/P - shows the madness of some of the theory.

I would probably apply my diversity to the whole garage install; based on conversation with customer as to expected usage.

i.e. your typical garage would supply the 3KW heater from the radial; there aren`t many people who`d have 20A of load they`d be using at any given time.

So, theoretically, if you were going to install this, in a typical property with a 60 or 80A incomer; would you not have to assess the house loading (which isn`t likely to be less), and recommend a DNO upgrade to either 100A SP or more ( summation, TP etc.)? Suddenly, this garage with 3 circuits is going to cost ten times more than it "should"!

Hell fire.

Save matches.

**** a duck.

And see what hatches!

 

[Doc Hudson]

If the garage needs 25mm, then the rest of the house would have probably burnt down by now unless its off 50mm or 70mm maybe? Madness! I can't believe a 3kw heater is always running at 3kw unless their grandmother is living in the garage 24/7? A bit of common sense needed here surely?

Doc H.

 

[androiduk2002]

Is applying diversity to the whole installation allowed ? I am sure it is after a respected member says so , but if so what are the guide lines to doing so ?

 

[Sidewinder]

The DNO will allow around 60% diversity on every domestic connection they make, that is they will size for around 40% full load current!

I would nicely say to you "go figure"!



Have a think and come back to us.

Oh, and please don't take my flippant comments as offensive, it just seemed a good way to say it in my small mind!

 

[Doc Hudson]

Excellent thread Android, I have moved it to the student & learning zone as it relates to a college theory type question not a real job. Thank you.

Doc H.

 

[androiduk2002]

If the garage needs 25mm, then the rest of the house would have probably burnt down by now unless its off 50mm or 70mm maybe? Madness! I can't believe a 3kw heater is always running at 3kw unless their grandmother is living in the garage 24/7? A bit of common sense needed here surely?Doc H.

Doc I fully agree with you and everyone else , we also had the common sense discussion and could probably make do quite happily with a 10mm. But it goes to show that theory is not always upto the practical.

 

[Voltimax]

Exactly that. Guidelines, nothing is written in stone. The OSG states that the designer should use his experience coupled with guidelines to decide what diversity to allow. The reason these cables come out at ridiculous sizes is just down to insulation and the fact that correction factors are applied to the protective device rating.

 

[Doc Hudson]

Exactly that. Guidelines, nothing is written in stone. The OSG states that the designer should use his experience coupled with guidelines to decide what diversity to allow. The reason these cables come out at ridiculous sizes is just down to insulation and the fact that correction factors are applied to the protective device rating.

Very true Volitimax, the designer should always be the one with the experience using the guidance in the regs and OSG. It's Appendix 1 about diversity in the OSG isn't it? Some aspects of electrical design are set in stone due to the laws of physics, others are hints and guides for assistance to experienced and qualified people to use when designing circuits. But none of the regs can cover every possible real world situation!

Doc H.

 

[androiduk2002]

Yep and in reality passing through 500mm of insulation would be very rare , if just coming out back of CU and not all the run is underground only 5mtrs is the rest is clipped to the wall so alot of extra factors need to be taken into account.

But raised a few eyebrows and given pause for thought , my work here is done :Salute

 

[steptoe]

TBH, I wouldn't even have calc'd anything, would have stuck in a 6mm on a 20a MCB and would have been done,

Done my mates garage that way and absolutely no issues with readings, ok, so he has no insulation to contend with,

And as you say, common sense needs to prevail sometimes.

 

[Evans Electric]

Just to apply some "Reality" to this , I do actually have a garage (also the EE stores/workshop) its 15m from the house .

3.........58w Fluoros

1........ 2KW fan heater

2......... twin sockets

1.......... 300W flood

2.5mm x 3core SWA . 16A MCB Heater winter only for 1 hour tops. Lts quite often ...0.5A Floodlight for 2mins when fox strolls by. Plugs .inspect light under van occasionally..batt charger occasionally. Test stuff on bench occasionally .

No problem .

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Some of this lagging stuff is overrated I think .

Average semi, upstairs lights wired in 1.0 mm under loft lagging, current with all upstaiirs lights on ....1.6A ........rating of 1.0mm ..... 11A without looking. I just think all this worrying is unnecessary .

 

[androiduk2002]

Alot of it would come from actually talking to the customer I am guessing , if he does the odd bit of welding in the winter with his heater and lights on spends several hours a day up there then up the size , but if its joe bloggs who goes up there turns light and heater on looks for something for 10 minutes then turns it all off down rate is ok.

Twas just interesting to get in the open and have a good old fashioned discussion about such.

 

[Theorysparky]

' Theory against real world'

Firstly i am not against the real world but i do have a few issues.

just done a workshop at the end of someones garden

6 x double sockets 2.5mm ring in trunking 32amp mcb

3 x 58w florries and pir 6amp mcb

2 x 1200w tubular heaters

40amp mcb in consumer unit

19m of 6mm swa clipped and buried

all his tools lathes,drills grinders etc are on 13amp plugs

how much can one guy have working at anyone time ????

not phoned me yet with any problems

i dont doubt that following the book may come to 25mm but it does not take into account common sense

some of us have seen really poor diy electrical wiring and wondered....

how did that work,why is there no tripping,no heat damage or a fire ???

oh well nearly the weekend

 

[m4tty]

My dads got a 200 ft garden with garage at end. It has 2.5mm singles run in conduit under the concrete path (most probably water cooled lol) and he runs a mig welder, lathes, plasma cutters, big pressure washers, a compressor which is 3hp I think. He's never had a problem apart from the lights dimming when the compressor kicks in or when welding. The regs are well over the top

 

[Theorysparky]

Not bad Matty

For your assessment job !!!!!!!!!!!!

:run

 

[m4tty]

I changed the board there for my assessment last year .... The garage was left disconnected. Think me dad reconnected it afterwards tho