Obvious question? What’s the use for 5A 4 pole junction boxes?
- Thread starter Malx
- Start date
Sponsored Links
Lighting circuits.
Historically they were 5 amps.
The ratings of junction boxes is more to do with the size and number of conductors that will fit, rather than the current it can deal with.
A 5A junction box will not fail if used on a 6A circuit.
Historically they were 5 amps.
The ratings of junction boxes is more to do with the size and number of conductors that will fit, rather than the current it can deal with.
A 5A junction box will not fail if used on a 6A circuit.
- Joined
- 27 Jan 2008
- Messages
- 23,739
- Reaction score
- 2,676
- Location
- Llanfair Caereinion, Nr Welshpool
- Country
As said it was 5 and 15 amp, today 6 and 16 amp, but a 5 amp can be used with 6 amp MCB or a 15 amp socket on a 16 amp MCB.
Afaict It's a traditional British Standards vs EN/IEC standards thing.
Products designed to traditional British standards tended to use ratings of 5A, 15A, 30A, 45A, 60A, 80A and 100A with 10A and 20A being less common intermediate values. BS1363/BS1362 are a bit of a strange exception with their 3A and 13A values,
Products designed to EN/IEC standatds OTOH tend to use ratings of 6A, 10A, 16A, 32A, 40A, 50A, 63A, 100A and 125A.
Products designed to traditional British standards tended to use ratings of 5A, 15A, 30A, 45A, 60A, 80A and 100A with 10A and 20A being less common intermediate values. BS1363/BS1362 are a bit of a strange exception with their 3A and 13A values,
Products designed to EN/IEC standatds OTOH tend to use ratings of 6A, 10A, 16A, 32A, 40A, 50A, 63A, 100A and 125A.
Sponsored Links
Indeed - they generally use the Renard R10 series which, rounded to whole numbers, gives:
6A, 8A, 10A, 13A, 16A, 20A, 25A, 32A, 40A, 50A, 80A, 100A, 125A ....
[ the mathematical basis being that each number is approximately 1.259 (the "tenth root" of 10) times the one before ]
Most of those numbers are familiar in relation to electrical items - whether as the In of MCBs/RCBOs/fuses and/or as 'maximum current ratings' of other things (RCDs, switches, JBs etc.), the two main exceptions (in those contexts) being 8A and 13A (although 8A and 13A MCBs do exist, although seemingly rarely used). The latter is, of course, very familiar in relation to BS1362/BS1363,and I can but suspect that 8A was considered to usually be too close to 6A and 10A to be 'useful'. Some manufactures do, of course, produce 45A MCBs, which does not exist in the R10 series (but does in the R20 and 'higher' Renard series), I guess because there was a 'perceived need'.
Kind Regards, John
Plus , most of those commonly sold "5A Junction boxes" were actually 20A for a long while, true 5A junction boxes tend to be cheap nasty things that cost a farthing less
Indeed - and, as has been said, far too small an 'fiddly' to use, at least for me!
Kind Regards, John
Just like connectors, they are rated for their likely use, rather a current limit.
Curiously wikipedia claims that the "most rounded" ISO renard series contains 12. Despite the fact that 13 is closer to value of 10^0.1 than 12 is.
I wonder where the british values came from. Were they also Renard values but just rounded differently.
Do not take a look at the strip connectors of old, Our old imperial 2Amp connectors now take 5 or 6 Metric amps easily anyway. Our old 240 Imperial Volts is more than a match for those new fangled 230V metric Volts too. Don`t worry.
I know I should not have said "Metric" I should have said SI , sorry!
I know I should not have said "Metric" I should have said SI , sorry!
Having just looked, yes, you're right - and, as you say, it's not really mathematically logical (aka 'correct'!). However, I think you'll find that it' the "least rounded" Renard series that is used for most purposes - and the figure then is 12.5 - which, at least in my book, definitely rounds to 13!
I would have guessed that it was probably based in perceptions of what 'round numbers' would be useful/practical figures to use, rather than any mathematical series. However, if one takes the 'British' figures (including 10A and 20A) which plugwash cited, those figures are (at least, from 15A onwards) very close to a hypothetical "R7" Renard series [ i.e. increments of "x 10^(1/7)" ], although it escapes me as to why anyone would have chosen that!
Kind Regards, John
2 thing come to mind, I was always taught it's multiplied by 1 1/6 but only some values used. I've not looked anything up or done any calculations and never given it a thought until now however just the glimmer of a thought and I think that has to be very incorrect.
Secondly doing some homework with my grandson yesterday he was doing areas of shapes involving π, answers to various sig figs or d.p's.
The homework is done on a school Ipad so instantly marked, they get 2 chances to get it right, then a second set of figures and 2 chances and a third. after 6 goes it will be classed as incorrect.
He made a calculation and rounded the answer 78.498... to 2sf as 79.0. I stopped him entering 79.0 but 78.0 was marked as incorrect as was 78
A different set of figures gave another answer marginally les than a 0.5 and same thing happened.
For every question there is a tutorial available.
After going through the calculation several times and several ways (which involved finding the total surface area of a shape consisting of a cone on a cylindrical pole on a larger pole:
I gave up and looked at the tutorial, one of my methods was bang on the nail, right up to the point they rounded 78.4989.. to 79.0 their method showed:
78.4989 rounded to 78.498+0.001 = 78.499, to 78.49+0.01 = 78.50, to 79.0
Last edited:
Are you talking about the 'British' figures? If so, a 1 1/6 ratio is 1.1666..... but that is appreciably less than all of the 'differences' in the common 'British' figures, so I don't think that what you suggest would be a credibl;e explanation.
I still think the most likely explanation is that they (particularly 5, 15, 30) were simply 'very round numbers' which were 'pulled out of the air' as being appropriate for purpose.
You were right, and they were plain wrong! What they have done, by repeatedly rounding, is to invoke what is known in the trade as the concept of the 'propagation of errors'.
To avoid such errors, one must round 'in one go' on the basis of the entire figure to be rounded - that way, one only gets a 'rounding error' once, rather than multiple times. In other words, ANY number less than 78.50000000000000000..... rounds (to 2 signif figures) to 78 - and that includes 78.4989 and even 78.499999999999999999999999...
Kind Regards, John
Quite frankly I don't know which system, it's just something from the mists of time which I hadn't given any thought to until this thread, my initial intent was to correct/augment your information but even without making any calculations I realised it was way out of kilter with both, however seeing your table everything apart from the 80-100 step is based on multiples of 1/6; ie 12/6, 9/6, 8/6. I'm not trying to clutch at straws with this by any stretch of the imagination.
Maybe the explanation to me was wrong or I remember it wrong but I don't even recall were I got it from.
I'm sure that was the original case and not only in UK, I think your list, or something relatively similar, was fairly universal.
I completely agree but that is what is curently being taught in schools.
.
Stupidly though 78.499 does round to 78.5 to 3sf and then using that and rounding to 2sf does go to 79 rather than 78
That's mathematically true, but probably a 'co-incidence' and/or 'red herring', not the least because, as one moves through the values, the number of multiples of 1/6 goes repeatedly 'up and down'
It certainly seems like the most probable original explanation (of the 'British' system). I don't think there is any doubt that what we now have is based on a Renard R10 series.
Hopefully not in all schools! Was this a primary school, where individuals teachers may well not have any 'specialist' further education in maths? In a secondary school, most maths teachers would have maths degrees, and I would hope that not many of them would make such an elementary mathematical mistake!
Indeed, but that is exactly the same mistake (rounding 'in stages', rather than in one go) that I described before.
I recall a good few years ago when a 'clever' (in some senses!) IT guy in a bank got away for quite a long time (until he was 'caught') with a fraud which was based on exploitation of a variant of this mathematical concept. In some (maybe many/most) banking systems, the convention is to always 'round down' interest charges to the nearest 1p. In some situations (I think including mortgages), interest is calculated daily but with the daily charges totted up' and only applied to the account once per year (or whatever).
Interest rates these days are often expressed to 4 or more significant figures, so daily-calculated interest charges will often (probably usually) include fractions of 1p. If (as one should, if one wants an accurate answer) one simply tots up those daily figures (including their full number of DP) over, say, a year, the greatest possible 'error' due to that one 'rounding down' (always resulting in a figure which is slightly lower than the 'true value) will be 1p, and (assuming things are 'random') the average error ('downwards') would be 0.5p - hardly anything for the bank to lose much sleep over.
This guy apparently did precisely that, with just one 'rounding' (of the annual total) per year for calculation of the (correct) amount of interest the customer was due to pay. However, when it came to calculating what the bank was told was the 'interest due', he apparently programmed the system to round (down) the daily interest figures, and then tot up those rounded figures at the end of the year. That resulted in an annual figure with a maximum (low) error of £3.65 and an average (low) error (assuming random) of about £1.82
The bank was therefore expecting an average of about £1.82 per customer less than the customers were being told to pay (and were paying), and the guy apparently found a way of diverting that 'difference amount' into his own account. Peanuts 'per customer', but with tens of thousands of customers, over a long period of time, it amounted to a very substantial fraud!
I seem to recall that, at the trial, an attempt was made to argue that what he had done was not erroneous, because he had simply been told to 'round the figures', without any indication as to whether that should be daily or yearly and, when that seemed to be failing, to attempt to argue that it was a 'victimless' happening (not a 'crime'), since the bank had received the amount of interest that they had been told was due to them. However, quite probably mainly because the ';difference' ended up in his own bank account, the jury didn't seem to buy into those arguments
Kind Regards, John
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links
