What is the correct formula for fuse rating volts x ohms equals watts volts x amps equals ohms?

Answer 1

The fuse rating is inherently tied to the current flow through the fuse, not necessarily the voltage.

The fuse has a small internal "wire" that has a specific resistance; when enough current flows through it, the power dissipated by this "wire" will melt. This power can be expressed P = I^2 x R.

For isolation purposes, fuses are also specified with a voltage rating. This rating is spec'd based on the distance between the (internal) terminals of the fuse that don't melt. For higher voltages, this distance must be increased to prevent power from "jumping across" the small area where the "wire" has melted (this is called arcing).

So when picking a fuse, you must pick a fuse that is rated for the voltage you are using (or higher - you can use 250 volt fuses on 120 volt equipment), and rated for the current you want to allow to flow (specified in amps).

The equations you list in your question are not inherently tied to fusing, but are power fomulas:

P = watts = voltage * current; ohm's law states V = I / R, so

P = V * I = V^2 / R = I^2 * R

V*R does not equal power.