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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.

I just finished wrestling with that very issue. I became the
unexpected owner of a 5 HP Single Phase Unisaw and had to figure out
how to wire it. In a phone call to Delta's technical people, they
told me:
1. Delta's 5 HP motor draws 100 Amps for about 1.5 seconds at
startup.
2. A 30 Amp "slow trip" breaker will hold it, but they are more
expensive and hard to find. (Home Depot never heard of 'em.)
3. A 40 Amp breaker will hold it fine, and that is what Delta
recommends.
4. The 5 HP saw comes without a power cord because they expect it to
be hard-wired by someone who knows what he's doing.

I sought advice from a couple of Electrical Engineers at work. They
assured me that:
1. The NEC allows a 40 Amp breaker on #10 wire IF the wire is only
feeding an electric motor that is hard-wired, AND there are no other
loads on the circuit.
2. It's not unsafe to run such a setup, because the motor's internal
overload switch will protect the motor itself. The only other
catastrophe would be a direct short, and the #10 wire will carry
enough amps to trip the 40 Amp breaker before the wire overheats.

So, you'll need to ditch the idea of twist-lock plugs for a 5 HP
motor. Plan to use either a 30 Amp slow trip or a 40 Amp breaker,
along with whatever wire size makes you happy (as long as it's #10 or
bigger).

The information presented above is hearsay. I am not a licensed
electrician, nor an electrical engineer. Check local codes before
installation. Void where prohibited. Your mileage may vary. Free
advice is worth what you pay for it.

DonkeyHody
Even an old blind hog finds an acorn every now and then.