Capacitors have a voltage of 63V (not 60 or 65). Fuses have a value of 63A (not 60 or 65). And there exist more odd values in electronics...
Who knows the history of this convention?
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Often these values are from https://en.wikipedia.org/wiki/E_series_of_preferred_numbers. However, that doesn't seem to be the case for 63 in particular. It is exactly 5% over 60V though, maybe that's it? – Justin May 10 '21 at 18:22
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11Just an FYI link to a similar question (with good answers), so it may help readers searching for information in future - but it doesn't seem to answer the *specific* question here about 63: "[What is the reason that the value “47” is so popular in electrical engineering?](https://electronics.stackexchange.com/q/67975/101852)" – SamGibson May 10 '21 at 18:41
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13Unrelated to your examples, but 63 is the maximum number that can be represented using 6 bits... – Eugene Sh. May 10 '21 at 18:44
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6In the chart posted by Spike, note that each voltage down the left column is 25% greater than the one before it. -ish. Like many things in science, it is a logarithmic sequence. – AnalogKid May 10 '21 at 18:51
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1You're not the first to wonder! https://www.sent-trib.com/community/the-interesting-and-bizarre-significance-of-63/article_bef90a9e-7d6c-11e5-b70d-47bc820447e1.html#:~:text=As%20it%20turns%20out%2C%2063,horse%2C%20for%20crying%20out%20loud. – Kyle B May 10 '21 at 21:24
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These are numbers from the 10^0.1 series. I am trying to learn the numbers as well as I learnt my 'times tables', so I can use them as mental logarithms, for doing rapid mental multiplication, division, powers, roots and reciprocals. I'm getting there slowly. – Neil_UK May 11 '21 at 05:11
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[In the standard resistor and capacitor value](https://ecee.colorado.edu/~mcclurel/resistorsandcaps.pdf), there is no value like 6.3 ohm or 63 pF. The closest standard value for resistors is 6.2/62 ohm and 6.8/68 ohm .For capacitors, it is 68 pF/0.68 uF. – Shadow May 11 '21 at 05:49
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4I suppose we need a reference question about log scales instead of 1 question per value. – Dmitry Grigoryev May 11 '21 at 07:53
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1Does this answer your question? [What is the reason that the value "47" is so popular in electrical engineering?](https://electronics.stackexchange.com/questions/67975/what-is-the-reason-that-the-value-47-is-so-popular-in-electrical-engineering) – pipe May 11 '21 at 10:02
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4Also unrelated to your examples, but 63 comes up quite often in the context of exponential decay. 63% is the amount of loss in a quantity after one [e-folding](https://en.wikipedia.org/wiki/E-folding) event (stemming from the fact that 1/e is about 0.37, so 1-(1/e) = 0.63, where e is Euler's number). – Brian61354270 May 11 '21 at 19:12
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@KyleB Unfortunately, that link is unavailable in EU. – Mast May 13 '21 at 09:28
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@pipe That would make sense if the question was about 68, not 63, right? – Mast May 13 '21 at 09:28
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I don't know... all my geeky EE friends tell me that 42 is the answer to the universe and everything... – RockPaperLz- Mask it or Casket May 13 '21 at 22:01
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FWIW, in US/NEMA/NEC-land, 60A and 70A are the standard fuse sizes, not 63A. – Theodore May 21 '21 at 20:20
3 Answers
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Using the table in @VoltageSpike's answer we can see that the standard values follow a ratiometric step increase from one value to the next.
| V | Ratio to previous value |
|---|---|
| 10 | |
| 12.5 | 1.25 |
| 16 | 1.28 |
| 20 | 1.25 |
| 25 | 1.25 |
| 32 | 1.28 |
| 40 | 1.25 |
| 50 | 1.25 |
| 63 | 1.26 |
| 80 | 1.27 |
| 100 | 1.25 |
| 125 | 1.25 |
1.258910 = 10 so a series of ten steps gives a ten-fold increase in value. Note that geometric progression can continue indefinitely with equal ratios. (Doing something like 10, 20, 30, ... 90, 100, 200, 300, ... gives unequal ratios.)
63 happens to be one of the standard values.
This is similar to the E12 series used in resistors where the ratio is \$ 10^{1/12} \$.
As @Charles points out in the comments this is the R10 series which is one of the Renard series, a system of preferred numbers dividing an interval from 1 to 10 into 5, 10, 20, or 40 steps.
Figure 1. Two decades of the R10 series on a logarithmic axis show the even steps in the series.
devnull
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Transistor
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5+1 A search in https://product.tdk.com/en/search/capacitor/aluminum-electrolytic/characteristic for DC rated voltage shows that most values below 100 V match the table/ratio you mentioned: 10, 16, 25, 35, 40, 50, 55, 63, 75, 80. – devnull May 10 '21 at 22:03
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7This is the [R10 series](https://en.wikipedia.org/wiki/Renard_series), except that R10 has 31.5 in place of 32. – Charles May 11 '21 at 13:33
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Those "numbers" were popular in the days of vacuum tubes- 6.3V and 12.6V for heater voltages. For obvious reasons, incandescent pilot lamp bulbs tended to have similar voltage ratings.
I suspect that, in turn, was related to A battery voltages, which is probably from the electrochemical voltage of a lead-acid battery. 3 cells in series would be about 6.3V.
It would also have made sense to keep the voltages as integer multiples so that even numbers of windings could be used on filament transformers that had to feed multiple filament voltages.
As to why they are used for capacitor voltage ratings rather than E3 or E6 series, I suspect it simply worked out better for practical applications such as 48V telecom (again related to lead-acid cell voltages, but plus a reasonable voltage margin).
From memory, the 63V capacitor rating was more popular with Philips parts (now spun off) a company which was heavily into telecom applications, whereas the more military/commercial US suppliers such as CDE would tend to have 50V and 100V.
Spehro Pefhany
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I would imagine that this relates to clearance distances with IEC 61010 (which is where I have seen 63V come up in some tables). If the pins of the capacitor have the spacing of the IEC table then I suspect the clearance would have been taken into account for the design of those parts.
IEC 61010-1, Table K-13
Voltage Spike
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