The voltage rating of the fuse is a maximum only. You can use a 600 volt fuse on a 6 volt circuit, but you can't use a 150 volt fuse on a
600 volt circuit.
If you want to fuse the low voltage side, you want to fuse it at or above the load current, but below the transformer current rating. If you want to fuse and protect the primary, you will want to fuse it at about 1.5 amps.
I say "about" because I don't know exactly what transformer you have - and what the pawer (in Va) rating is.. You take the va rating, and devide by input voltage for the input fuse, or the output voltage for the output fuse.
fuses don't limit voltage, they limit current. so 250V fuses are appropriate (what that means is, they will reliably interrupt 250V, so they should be fine for 120V.)
Now the question is, how much current does the xformer draw when loaded to 15A at 12V? It ain't 15A. It probably isn't 1.5A either (that would imply that it is 100% efficient - remember, P = IV) probably something around 2A or 2.5A would be appropriate
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