97 mercury sable blows alternator fuse after a few hours. Not right away. Alternator & battery are new & test good when change fuse. Do you know what causes this? Thank you for your time. . .

Usually that fuse powers the alternator and the fuse box.
It would have to be a big short to ground to blow that fuse.
Its possible the main wire to the alternator is touching a metal spot, or the alternator is shorting out inside when it is running.
What is draining the battery may be something else, or the battery could be discharging after the fuse blows if the alternator is not charging it.

its your starter relay

The new alternator is probably ok.
I would say you burned a fuseable link in the wiring harness.
The alternator has a fuse wire between it and the battery to protect it from overload.
Have someone check the wires coming out of the back of the alternator. The big wire should have battery voltage, the small wires energize the alternator when you start the engine.

Try test the Mega alternator/ battery fuse, should have power across both circuits.

that's becasue the new abattery went dead because it was not charging. The battery light must be OFF to charge the battery. Take it back to whoever fixed it. May be a bad wire betwen battery & alternator too.

check the 175 amp mega fuse in the engine compartment.

cehck this ground for a good connection - G104
Left front fender near air cleaner

check the 20 amp fuse for the charging system. Fuse 26 in teh engine compartment fuse box.

The VR is integral to the alternator. Check the fusible link and the 10 amp fuse in the battery fuse box.

May have blown the fuse or connections to alternator may be poor.

this is caused by a failing alternator. Kragen auto and Autozone tests these for free.

check the fuses and make sure they are all good. they will be located in fuse box under hood.

Regulator in alternator obviously faulty, have it bench tested & fixed or replace alternator, and cure your problem.