My latest addition - Radialva Groom

[Yorkie]

I think that when you get to the testing stage John, you'll find that the actual working voltages when you test the set will be way below the WV of the caps - often just a few volts. Same with most sets - the working voltage of the caps is often way over that dictated by the circuit. Cheaper for the makers to buy in large quantities of high voltage caps and gain the economies of scale, than to order caps in several voltages - say 150, 250, 300, 350 etc. It's for a similar reason that the keypads on self-service petrol pumps have Braille markings. The same keypads have other applications and it's easier to make all keypads with the Braille dots added than to make special ones. (Besides which some drivers seem to need Braille, judging by how they drive!).

I assume that the WV of a cap is dictated by the type and thickness of the dielectric material. If thicker material is used, to get the same value of capacitance will mean that the cap will have to be larger in diameter and longer. Having said that, I have some 1,000VW yellow polyester caps that are smaller than 630V ones of the same make.