The unit is a Trane and is 22 years old and blew the 35amp dual element, time delay (current limiting) fuses that are in the little metal box on the side of my house next to the unit yesterday while running the AC during a hot day. I replaced them this morning with exact replacements and the new ones blew today after using the unit for about 3 hours during the hottest (100F) part of the day. Does this usually mean the outdoor condenser unit should be replaced?

Also: I notice that on the fuses, there's a groove at one end of the copper ends. What does the groove do or indicate?

The unit is a Trane and is 22 years old and blew the 35amp dual element, time delay (current limiting) fuses that are in the little metal box on the side of my house next to the unit yesterday while running the AC during a hot day. I replaced them this morning with exact replacements and the new ones blew today after using the unit for about 3 hours during the hottest (100F) part of the day. Does this usually mean the outdoor condenser unit should be replaced?

Also: I notice that on the fuses, there's a groove at one end of the copper ends. What does the groove do or indicate?
Don't count it out yet... Something is definitely getting hot enough to blow fuses... check for a loose wire somewhere, bad connection, capacitor, contactor.or possibly clogged condenser coil, that would be the first things I would check... but 22 years is longer than average... so replacing might not be a bad idea

If that unit is 22 years old, it probably uses enough electricity to light up every home in the neighborhood! I would replace it with an Energy Star rated unit. After 22 years, it has served it's useful life.

Thanks guys, I figured it's given me more than I could realistically expect from it in service life. The point that Handyman made about its electrical footprint on my monthly PG&E bill has been on my mind for years now.
My house is a two story, 2000 cubic foot home (3 bedrm/2.5 bath). I believe the Trane I currently have is a "4 ton" unit (model BTD748A100B0)? How much do the new energy star units cost?

Most of the good products today are energy star rates. Trane is among the better brands. Your best bet is find a Trane dealer. I believe that The Home Depot even sells Trane Products. 4 ton is a little small for the application. If you were to increase the capacity, with the new energy technology, you will still save on that electric bill. FYI, the evaporator/exchanger (in the attic) should also be checked and probably replaced to match the compressor unit. It will not be a cgeap job but in the long run, the savings and better coooling will be worth upgrading.

Check to make sure the outside condenser fan is turning. If not, try and unstick it for a temporary fix or replace it for a more permanent fix. 22 years worth of service is very good and you would be doing yourself a favor by replacing it and your inside coil to something more economically to run. $4-5K is normal in the Northeast.