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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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    \$\begingroup\$ Just 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?" \$\endgroup\$ – SamGibson May 10 at 18:41
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    \$\begingroup\$ Unrelated to your examples, but 63 is the maximum number that can be represented using 6 bits... \$\endgroup\$ – Eugene Sh. May 10 at 18:44
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    \$\begingroup\$ In 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. \$\endgroup\$ – AnalogKid May 10 at 18:51
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    \$\begingroup\$ I suppose we need a reference question about log scales instead of 1 question per value. \$\endgroup\$ – Dmitry Grigoryev May 11 at 7:53
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    \$\begingroup\$ Also 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 event (stemming from the fact that 1/e is about 0.37, so 1-(1/e) = 0.63, where e is Euler's number). \$\endgroup\$ – Brian May 11 at 19:12
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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.

enter image description here

Figure 1. Two decades of the R10 series on a logarithmic axis show the even steps in the series.

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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.

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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 enter image description here

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