Is 30 amps enough for a 3hp cabinet saw?

[LarryG]

A table saw motor in a hobbyist wood shop is not a continuous-duty motor. That would be for things like air conditioners, pumps, air compressors in auto repair shops, etc.

I think these provisions are generally applied to machinery that is hardwired, with a disconnect switch. The code has no way to regulate what lay people plug into a given circuit -- that's the whole point of breaker sizes, wire gauges, etc. If someones does something dumb, the breaker trips before the wire melts and starts a fire.

That said, I agree with the conclusion. My 3HP cabinet saw is on a 20A circuit with 10ga wire.

[DaveStL]

II. Motor Circuit Conductors
430.22 Single Motor
(A) General, Conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor's full-load current as determined by 430.6(A)(1).

Note, this is for conductors, meaning a motor that draws 18A needs wires sized for at least 22.5A.

III. Motor and Branch-Circuit Overload Protection
430.32 Continuous-Duty Motors
(A) More Than 1 Horsepower. Each motor used in a continuous duty application and rated more than 1 hp shall be protected against overload by one of the means in 430.32(A)(1) through (A)(4).
...
(2) Thermal Protector. A thermal protector integral with the motor...
...

The 80% rule, you might be confusing from non-motor household appliance tables. In particular, table 220.55 specifying that no fixed kitchen appliaance (i.e electric range) should present more than 80% load on the circuit it is on.

So basically, an external motor overload protector cannot exceed 125% of motor amp rating, and in case of a 3hp (about 18A) motor, that's 22-23 amps tops. I maintain - that 30 amp breaker can cost OP his saw.

Thanks. I appreciate the discussion, and think we're pretty much on the same page now. I didn't realize _all_ motors >1 hp had to have 125% of full load current.

430-6(a)(1) says use a table for the full-load current, rather than the motor nameplate. 3 hp, 230V is 17A (Table 430-148). This is significant for the Jet because it has a 14.5A nameplate; code says you still use 10-ga wire. The table lists 28A for a 5 hp saw, so it _does_ need 8-ga wire! I won't have to worry about that.

And I see now that the 80% rule for 20A circuits, 210-23(a), doesn't apply, because Article 210 doesn't cover branch circuits that supply _only_ motor loads.

The only difference is that the Jet already has thermal overload protection, so I don't think you have to rely on the breaker in the panel to protect the saw. But it makes sense to use a 20A breaker-- maybe extra protection, and maybe a little cheaper.

[dramey]

Wow have things changed. It used to be that 15A circuits was wired with 14 ga, 20A with 12 ga, 30A with 10 ga, etc.

[LarryG]

I didn't realize _all_ motors >1 hp had to have 125% of full load current.

They don't. Note the section heading of the passage Scorrpio quoted: Continuous-Duty Motors. The first sentence reads:

"Each motor used in a continuous duty application AND [emphasis added] rated more than 1 hp shall be protected against overload ..."

Again, a table saw in a home shop would not be considered continuous-duty, even if it is larger than 1HP. IOW, a motor used in this particular application fails the "and" test in the foregoing.

[SARGE..g-47]

The Jet only needs a 20 A circuit to give it and the field line true protection. I ran two 220 v in my shop. One for 30 A to run a 5 HP planer and the other with a 20 A breaker to run a 3 HP Uni-saw.. 8" jointer and 18" BS.

Opps... I added a cyclone which needs 20 A with one of the 3 machines running on the existing 20 A. Do I run another 220 V to get the two I need?

Nope........ If I use the 5 HP planer.. the 20 A covers the cyclone and the 30 A covers the planer. But with the cyclone running on the existing 20 A, how do I get the 3 HP and under 220 V's plugged in and running at the same time without having to plu them into my 30 A. And BTW.. the 220 V all have the appropriate twist plugs which have different configurations for 20 A and 30 A to avoid plugging one into the other.

The answer is: The 220 V 30 A is wired with the proper 10 gauge to the outlet. I come off the other side of the outlet with 12 gauge (proper for 20 A) and drop a foot down to a single in-line breaker box with a 20 A rating. From there drop 12 gauge out of it to another 20 outlet just below and that gives me 2 20 A 220 V circuits when needed and the original 30 outlet is there for the larger draw planer.

Illustration below.. 30 A on top outlet.. 20 A single in-line breaker.. 20 A outlet below with another 20 A outlet from another line higher up the pole you don't see from the 2nd 220 line I mentioned... 30 A and two 20 A's from only 2 lines. Your compressor needs a 30 A and your Jet a 20 A.

Regards...

[bluzcat]

Thanks Sarge. I think this is what I'll do. My 30A circuit is close to the ceiling anyway, so I was planning on running EMT down the wall (by the way, I love that industrial look) and putting the receptical about 12" from the floor. Installing a 20A breaker inline would be a piece of cake.

[SARGE..g-47]

Afternoon Bluzcat...

It would probably be the easiest route to kill two birds with one stone. If you ever add a cyclone, plan on a 2nd 220V as you will have the ground-work for the 1st 20 A and 30 A already in place.

BTW.. good look with that "industrial look".

Regards...

[DaveStL]

They don't. Note the section heading of the passage Scorrpio quoted: Continuous-Duty Motors. The first sentence reads:

"Each motor used in a continuous duty application AND [emphasis added] rated more than 1 hp shall be protected against overload ..."

Again, a table saw in a home shop would not be considered continuous-duty, even if it is larger than 1HP. IOW, a motor used in this particular application fails the "and" test in the foregoing.

I'll have to get a newer copy; the language in 430-32 has been clarified. Perhaps 430-33 as well?

From what I have, "intermittent duty" on a continuous-rated motor requires a 140% factor for ampacity (Table 430-22(a) Exception); still 10-ga for a 3 hp saw. "Varying duty" requires a 200% factor, which puts you into 8-ga! From the definitions in 100-A, you could make a case for either to apply.

Overload protection for intermittent duty motors can be the panel breakers, rather than a separate device as specified in 430-22, and they can trip at significantly higher than 125% of the motor nameplate (430-33 and Table 430-152).

But is it what you're doing with the device, or what the device can do that determines the rating? 430-33 also says, "Any motor application shall be considered to be for continuous duty unless the nature of the apparatus it drives is such that the motor cannot operate continuously with load under any condition of use." I suspect this is the answer-- that for purposes of the NEC, the saw is counted as continuous duty.

Again, I'm looking at an older version; perhaps these sections have been updated. It's good that we're working through this.

[scorrpio]

Correct. From NEC point of view, a saw is continuous duty. It will keep running until you turn it off. It does not shut itself off after a certain rated maximum.