[gelagasse]

I don't very much about capacitors but I have a supplier that insists that a 40 mfd 370 VAC rated running capacitor can adequately replace a 40 mfd 450 VAC rated one. Based on what I read, a starting capacitor can vary by up to 10% but that running capacitors must be matched perfectly.

[KISS]

They are both 40 uf and operating off a 240 volt source. Just fine. Give him a beer for giving him such a hard time. He's right.

[gelagasse]

So that I can advise customers in the future, where on this capacitor does it say 240 volts? I see 450 VAC, 50/60 Hz

[KISS]

Well, since this is a DIY board, and domestic voltages are 240 VAC, so I assumed 240.
The voltage rating of the capacitor must be greater than the line voltage. Hence 370 and 450 V are greater than 240, so your OK. The higher voltage introduces a higher safety and likely the lower voltage has a lower ESR (Effective series resistance). This will not likely matter.

The start cap uf value is more critical than a run cap, if used. The run cap may change the speed.

Extension cords, for instance, come in 300V and 600V ratings.

[Ken66220]

Many 450VAC capacitors are used on the secondary side of a transformer (lighting ballast) where the voltage is 400+VAC. While the supply voltage is from 120 to 277VAC a 370VAC capacitor is not adequate. While in this situation a 370 VAC capacitor may work, it is really not adequate protection for the real voltage incurred. I would use caution when the substitution is not up to OEM specs.