Each size is about 60% larger than the previous. It is just a mathematical series rounded off. The same series is used for fuse sizes and a lot of other things. (10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100A)
There shouldn't be a 0.75 mm² size: To fit the series it should either be 0.6 mm² or 1.0 mm².
Why not just drop it?
[This message has been edited by C-H (edited 07-25-2003).]
C-H, I've often wondered that myself. Over here, we have 0.5mm too, but only in flexible cable sizes. Also, a 50A fuse is a common size, here too, for mid-size 3 phase installations, where the tariff is based on the size of fusing.
Let's face it, these days if you're not young, you're old - Red Green
0.5mm2 is used mostly for LV stuff here and not available at hardware stores. 2x0.75mm2 zip cord is available everywhere but hardly ever used for appliances (usually speaker wire or botched repairs). I've seen old lamps with 0,5mm2 zip cord though. And I've seen the odd scary installation with 0.5mm2 buried in plaster, spliced into a normal 10A circuit, feeding a receptacle. AAARGH!
On the fuse sizes, all the old British standard ratings were pretty much round numbers: 5, 15, 20, 30, 60A etc. [...]
The odd sizes of 6, 16, 32, 63 and so on only appeared in coimparatively recent years with the adoption of European standards.
If you free your mind from our decimal system for a second you will see that the latter numbers are less odd than the former.
Let me tell a little bedtime story for engineers:
(Read the AWG story in the AWG thread too, it shows how the Americans tackled the problem with wire sizes.)
Back in the 1870's the French military engineer Charles Reynard was given the job of improving military balloons. He discovered that 425 different sizes of cable were being used to moor the balloons, a logistical nightmare, and set about determining how best to reduce these to a smaller number of sizes.
Reynard succeeded in replacing the 425 sizes with 17 sizes that covered the same range. To do this he made the sizes a geometric series in which by every fifth step the mass per unit length of the cable increased by a factor of ten:
To users of a decimally-oriented system of units, such as SI, Reynard's series is much more useful than these other geometric series, because it begins on 10 and ends on 100. The ISO adopted Reynard's series as the basis of the preferred numbers for use in setting metric sizes. The designations of the series they have defined begin with R as a tribute to Reynard, and the series are called Reynard series.
It's actually quite interesting -- I'd not stopped to really think about the geometric progression of the series before. On the fuse sizes, we have an oddball size of 3.15A which is quite commonly specified in electronics work.
There are also similar related series such as for standard resistor values, e.g. for many years the preferred values for most applications were: