Given that a breaker exists only to protect the wire connected to it and
that is fact:

1) a 30A breaker connected to and protecting #10 wire will give (by your NEC
code) 30 x 80% = 24 amps of continues current

2) You must get the FLA off the nameplate of the motor. Motors are ALL built
with different efficiciencies and housing structures - therefore the only
accurate current draw is that which was tested at the factory on their
design model. Motors, at startup typically drew about 6 x FLA at startup,
but today motors can easily hit 12 x FLA at startup because of lighter
housing, thinner internal wiring etc - in otherwords the motors have been
cheapened up.

3) If you motor nameplate, and it read 22 amps FLA - then you'd have 2 amps
left over and you are good to go. If the motor nameplate on the other hand
said 26 amps FLA you need to revisit the breaker/wire combination. It will
still work but likely it will trip, maybe not for hours (only the breaker
trip curve could answer that question) - the code can't stop you from
attaching a 35 amp load to a 30 amp breaker combo - it just won't stay live
very long.

The code derates by 20% to give you the easily calculated values that allow
you to enjoy continuous power on a circuit. The derating comes from the fact
that circuit breakers are thermal devices (BTW so are fuses) in that they
have as part of their protection system, a bimetallic element that works on
heat principles. Once you put a breaker in an enclosure, heat can build up
and trip the breaker. Enclosure are build with minimum cubic inches of
"cooling space" and in conjunction with the 20% derating rule offer you a
product that can supply current on a continuous basis. Put that product in
the boiler room of a ship, where the ambient temp is high and the breaker
will trip sooner. Same applies to a "hot" woodworking shop - this is why the
20% derating. If you want to spend the money, you can buy 100% rated
breakers which are certified as such and you can draw 100% of the current.
These breakers start at 400Amp and go to 6000Amp.

Anyway, I digress - don't try to re-engineer some that has been designed for
you and enforced by the NEC:

12A circuit = 15A breaker + #14 wire
16A circuit = 20A breaker + #12 wire
24A circuit = 30A breaker + #10 wire
32A circuit = 40A breaker + #8 wire

Once you pick the right size "service" - make sure you have the proper
device to protect your motor - the upstream breaker has nothing to do with
that - but that a lot more typing and I'm not good at that.

Regards
John

wrote in message
news:1078204646.761648@smirk...
What amperage breaker does one need for a 5HP table saw on a 240V
single phase circuit?

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

--
The address in the header is invalid for obvious reasons. Please
reconstruct the address from the information below (look for _).
Ralph Becker-Szendy _firstname_@lr _dot_ los-gatos _dot_ ca.us