Saw something for the first time...

[texie]

Wouldn't a breaker on a dedicated circuit do exactly the same thing? What is the advantage of the fuse if it's fed by a breaker?

I notice in many parts of the country SSUs are still pretty common. I think it stems from older days when furnaces did not have internal OL protection and a certain size fuse was required. And so in some parts it is commonly believed that a SSU is required based on urban legend. When I moved here years ago it was standard practice, and being the pain that I can be, of course I made an issue of it and dared anyone to explain how the code required this on any modern furnace. In recent years it seems to finally be a thing of the past.

 

[drcampbell]

I remember seeing like 6 and 10 amp fuses for ancient furnaces ... probably not needed for any newer replacement gas furnace.

That would be a bad idea today. A lot of furnaces have a hot-surface igniter that draws ~6 amps.

 

[kwired]

That would be a bad idea today. A lot of furnaces have a hot-surface igniter that draws ~6 amps.

There is normally very little other load running when ignitor is in operation, unless fan switch is in manual on position. But as long as it is a time delay fuse it 6 amp fuse possibly holds for no longer than ignitor is in operation, 10 amp fuse even more likely.

 

[kwired]

I remember seeing like 6 and 10 amp fuses for ancient furnaces not 15 and 20, probably not needed for any newer replacement gas furnace...

Probably goes back to when you commonly saw belt drive blower motors. Kind of guessing most direct drive designs likely had integral overload protection from day one.