In article 1078204646.761648@smirk,
wrote:
What amperage breaker does one need for a 5HP table saw on a 240V
single phase circuit?

5 HP is 3725 watts. At 240v, this is a 15.52A draw, assuming 100% efficiency.
at 80% efficiency, you've got a 19.4A draw @ 240V.

My logic: Typically, single phase 5HP 240V (or 230V) motors are rated
at 19.8 or 20 A. Given that a circuit is supposed to be loaded 80%
(look it up in the NEC sometime),

"No more than 80%", for permanently attached loads, yup.

this means a 25 A breaker would be
borderline sufficient, and that a 30A breaker should be generous.

Yup. 30A is "more than sufficient".

With a slow breaker (not a fuse), there should be no problems with the
startup surge of the motor tripping the breaker.

The reason I'm asking is that I've recently read a few posts claiming
that a 5HP tool needs a 40A or even 50A breaker.

At 120V, yup. not at 240.

There are a few
reasons not to do that. First, it requires 8 gauge wire (for 40A) or
6 gauge wire (for 50A), which is considerably harder to work with.
Second, it requires a different connector, which means less
flexibility in moving tools around. For this reason, I had been
planning to make all the 240 V outlets be twist-look 30A outlets in my
soon-to-be-built shop. Third, it is unsafe to use too large a
breaker: If something goes wrong, there is more current around to fry
things or start a fire.

While we are it it. I have converted on of our waterpumps (a 1.5HP
pump) from single-phase to three-phase motor, with a VFD (or inverter)
driving it. This gives me a really nice slow start, with no current
surges (which is vital when running on a generator). I'm planning to
convert my drill press similarly, one of these days. This would give
me the ability to run at variable speed (within a range of maybe a
factor of two), without having to change belts, and it would again
remove the startup surge and the mechanical stress associated with it.
This brings up the following question:

Is there a point using a 3-phase tablesaw motor on an inverter? There
is no point running a tablesaw at reduced speed, is there? So the
only point would be slow start (which is not worth the extra couple
hundred $$$ for the inverter). Also, in principle, one can buy 3 HP
inverters that run on single-phase 240V, but if the single-phase motor
on the table saw has a service factor 1, a nominal 3 HP inverter will
not be quite sufficient. Does anyone know whether electronic VFD
inverters with single-phase input and 5HP capacity even exist? I
haven't seen one yet.

They, and even bigger ones, do exist. You *don't* want to see the price tag,
however.