Amperage for 5HP table saw?

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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.
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Hello Ralph, the big point in using an inverter on the TS is not the speed (except you want to cut steel on your saw), but the inherent motor break in the inverter. My saw was spinning for close to a minute with a heavy dado stack on after switching it off. With the inverter, it comes to an instant stop.
Regards Matthias

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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.

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

Yup. 30A is "more than sufficient".

At 120V, yup. not at 240.

They, and even bigger ones, do exist. You *don't* want to see the price tag, however.
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Using Ohm's law this is a relatively simple question to answer...
1 horsepower = 745.7 watts
Power (watts) = I (amperage) x E (voltage)
When you have two values for the variables in the equation you can figure out the third.
P 3728.5 (watts 1 HP x 5) Therefore I = --- or I = ------ E 230 volts
230 volts = 16.21 amps 240 volts = 15.53 amps
Using the "80% rule", a 20 amp breaker should not exceed a continuous load above 16 amps. It seems to me, that with the proper type of "slow" breaker, you should be OK with a 20 amp breaker.
Joe
PS
I am not an electrician or an electrical engineer. In fact, I have never even played one on TV.
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That only works for an ideal motor. Unfortunately, they do not exist in real life. You need to adjust for power factor and efficiency.
P = (HP * 746) / (pf * eff)
So with an efficiency of 80% (.8) and a Power factor of 90% (.9), 1 horsepower = 1036 Watts
For the 5 HP example I = (5 * 746) / (.9* .8) / 230 = 22.5. amps
Using the 80 % rule on breaker sizing, 22.52/.8 = 28.23, so a 30 amp breaker will be required.
-- Al Reid
"It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so." --- Mark Twain
wrote:

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Alas, this assumes 100% efficiency, which saws are not. For example, my 2HP saw is rated to draw 9A @ 220 or 1980W, considerably more then the 1492W ideal rating. It is this that makes me shy from 2HP+ tools; don't wanna run 220 all over the shop. Though a 2HP DC is calling me...
Paul K
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wrote:

you guys are starting to get there, but whats so hard about looking on the motor for the current rating or calling the manufacturer and getting a power factor curve for it if you insist on going that route?
randy
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i didnt mean to come off as an ass, but now that i read it, i kind of do..
all i mean is that if you want to start doing the math, there are many factors involved. the load conditions on startup. the load conditions during running. and none of these are simple to calculate but most motor manufactures will provide charts.
but in any case, if you cant run your 5horse motor on a 30 amp circuit, something is wrong.
randy
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Delta states a requirement of a 40 amp slow blow fuse for protection on their 5 HP Unisaw. It may run on a 30 amp circuit, but I would not bet on it! Greg
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do..
motor
well, i cant vouch for that particular motor, but imho a 40 amp slow blow fuse on a 5hp table saw motor is kind of like saying, ok, once the thing catches on fire and explodes and the blade has spun out of control and the power wires are actually touching and welding together, we better stop supplying current to it <g>
i would bet that saw has internal protection that blows long before that fuse does.
randy
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A 30 Amp breaker should be O.K. for a table saw, I had real problems when I was trying to run a 5HP 24" planer/thicknesser because the mass is so large it takes that much longer to get up to speed. Your 20 Amp running load is right, but the startup Amps are round 2 to 2 1/2 times that.
Bernard R
_dot_ los-gatos _dot_ ca.us
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firstname_@lr_dot_los-gatos_dot_ca.us wrote in message

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.
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Are you sure about that one. All of the references I am aware of limit the overcurrent device to 30A on a #10 wire.
Anyone know for sure?
-- Al Reid
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Somebody wrote:

"Al Reid" asks:

the overcurrent device to 30A on a #10 wire.

It is all a function of the insulation used.
The function of the c'bkr is to protect the insulation on the wire, not the wire itself and most definitely not the motor.
Don't have my code book handy to verify; however, the use of a #10 conductor on a 5HP/240V/1PH motor even if legal, would at best be considered a bad installation.
The difference between the I^2*R heat losses of that #10 vs the lower heat losses of a #8 would soon consume any material savings made during installation.
After that, the power company would be laughing all the way to the bank.
Personally, I'd use #6 wire /w/ a 2P-40A c'bkr for a 5HP/240V/1PH motor.
SFWIW, have designed and sold the equipment to build more than a few electrical distribution systems during my career.
HTH
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Lew

S/A: Challenge, The Bullet Proof Boat, (Under Construction in the Southland)
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No, it's not. Theampacity may be higher than 30 A, but code limits the evercurrent device to 30A. See the tables in section 310. Section 310 states that #14 = 15A, #12 A and #100A. I agree that the rated ampacities may, however, be greater than those limits.
My question is, can anyone cite the section of the NEC that allows a larger overprotection that is stated in section 310?
I have never ran across that exception and I have been an Electrical Engineer for 24 years working in, among other areas, electrical construction engineering. I am still looking at the code book.

the
conductor
Southland)
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Well shucks. Y'all made me get out of my chair and go look it up. Check out Section 430, Part D - Motor Banch-Circuit Short-Circuit and Ground-Fault Protection. Paragraph 430-52 (b) All Motors. "The motor branch-circuit and ground-fault protective device shall be capable of carrying the starting current of the motor." (c) Rating or Setting. (1) "A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430-152 shall be used." EXCEPTION NO. 1 (Paraphrased) Exception 1 allows you to go to the next size up if you are between sizes. EXCEPTION NO. 2 (Quote) "Where the rating specified in Table 430-152 as modified by Exception 1, is not sufficient for the starting current of the motor:" "(a) The rating of a nontime-delay fuse or a time-delay Class CC fuse shall be permitted to be increased, but shall in no case exceed 400% of the full-load current. "(b) The rating of a time-delay (dual element)fuse shall be permitted to be increased but shall in no case exceed 225% of the full-load current. "(c) The rating of an inverse-time circuit breaker shall be permitted to be increased but shall in no case exceed 400 percent for full-load currents of 100 amperes or less or 300 percent for full-load currents greater than 100 amperes."
The commentary in the handbook states: (Quote) "It should be noted that it is not necessary to size the branch-circuit conductors to the percentages (150 to 300) permitted for the branch-circuit short circuit and ground-fault protective devices." . . . "During a short-circuit or phase-to-ground condition, the extremely high current causes the protective fuses or circuit breakers to open the circuit. Excess current flow caused by an overload condition passes through the overload protective device at the motor controller, thereby causing the device to open . . ."
DonkeyHody Even a dumb old country boy gets it right once in a while.
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Well, now you caught me without my code book but that seems to imply that it is a dedicated, hardwired motor, being supplied by a motor controller that incorporates both instantaneous (like an MCP) and overload protection AND that it pertains to the wiring between the motor controller and the motor.
I could be wrong, but I doubt you could convince a residential electrical inspector to pass an installation with #10 wire being protected by a 40A breaker.
-- Al Reid

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Al, That section is specifically NOT about the factory installed wire between the factory installed motor controller and the motor. I went to the trouble to look up the reference for you and even took the time to quote chaper and verse. I'm not going to type the whole chapter for you. If you want to argue context, at least go to the trouble to read the chapter so you'll know what your're talking about.
Now I remember why I mostly just lurk here.
DonkeyHody We should be careful to get out of an experience only the wisdom that is in it - and stop there; lest we be like the cat that sits down on a hot stove-lid. She will never sit down on a hot stove-lid again---and that is well; but also she will never sit down on a cold one anymore. Mark Twain
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Hey DonkeyHody, don't get your drawers in a knot!! I thought we were having a discussion. As anyone that has dealt with the NEC or other NFPA publications, interpretation is everything. And I have found that there is no universal interpretation of all aspects of the code.
I was merely expressing my doubts as to the wisdom of violating the 30A on a #10 wire rule in section 310. Most inspectors get hung up un that one and usually don't budge. Perhaps, my designs are too conservative, however, I have never had one rejected by a PE review before installation or by an inspector during/after installation.
Also, I started my comment by stating that I did not have my code book in front of me.
-- Al Reid
"It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so." --- Mark Twain

a
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"> Don't have my code book handy to verify; however, the use of a #10 conductor

Lew, The plate on my 5 HP motor indicates that it draws 20 Amps at full load. If it was a motor that operated for extended periods at full load, or on a long wire, I'd agree with your thinking on the I squared R loss. But my table saw will rarely load the 5 HP motor, and the run of #10 wire is less than 20 feet. Since I have a pretty good feel for the tradeoffs involved, I elected to use the smaller wire, not because of cost, but because it's easier to pull. Others may certainly choose another path.
DonkeyHody Even an old blind hog finds an acorn every now and then.
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