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Table saw horsepower - how does my contractor saw get 4hp?

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Hi there, I’m looking around for a cabinet table saw and am going through the classic horsepower decision making. My question isn’t so much how much do I need, but rather, where in the spectrum is my current saw. 
 

I have a Bosch 4100 (latest generation, purchased 2017) portable contractor saw that is purportedly putting out 4hp. It is a 15A 110V-60Hz saw that does 3650rpm. I’m looking at either 1.75 or 3hp cabinet saws for increased infeed and rip capacity and everything I’ve read suggests that 3hp is at the upper end of hobbyist use saws, which makes sense with the pricing, but I’m left wondering what change in performance I can expect coming from this apparent 4hp. 
 

Is this an RPM thing? What else might I not be realizing here?

 

Thanks for any input!
 

 

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I've never seen a contractor saw put out 4 hp. Generally they only put out 1.5 to 1.75 hp, especially since they run on 120 volts.  A 3 hp saw will almost always require a 220 volt input. 

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Other manufacturers have also been guilty of the same thing. One way to quote HP is peak HP rather than effective HP.They measure the amp load just before it blows up and convert that to HP. That's why you sometimes see the "develops 4HP" or whatever. Rick is correct - rarely if ever does a contractors saw run at more than 1 ½ or so HP.

To your real question, do you have 220V service? If so, I recommend getting a 3 HP saw. You won't see many 110V saws rated at more than 1 ½ to 2HP. If you don't have 220V then I would look into having it run to my shop. You'll forever be limited without it.

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@RichardAis correct.  It's all marketing wank by the manufacturer, like my 6hp Rigid vac.  They are measuring the split second inrush current and probably not using rms readings.  Probably using peak to peak readings.

1 hp = 745 Watts.  Watts = Volts x Amps.  Full stop.

120V x 15A is 1800 Watts.  Divide that by 745 and you get 2.46 hp, which is not acheivable because there are safety margins involved.  If you are getting 2 hp from a 15A cct, you are right about the maximun capacity of the cct.

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1 minute ago, Mick S said:

To your real question, do you have 220V service? If so, I recommend getting a 3 HP saw. You won't see many 110V saws rated at more than 1 ½ to 2HP. If you don't have 220V then I would look into having it run to my shop. You'll forever be limited without it.

This is what I would recommend as well.

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Specs say it delivers max 4hp which, as mentioned earlier, is right before the motor blows. You won’t get anywhere near that in normal use. 

Typically the universal motors have ratings based on when they blow up, while induction motors like on cabinet saws have more realistic power ratings. 

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Having made a similar switch a few years ago, from a "3 hp" job site saw to a 1 1/2 hp hybrid saw, the difference is amazing in terms of how much more powerful the hybrid saw is. The other guys are right, the ratings on the job site saws are complete nonsense - it's probably really something closer to 3/4-1 hp, despite what it may say. You can also just see it in the physical size of the motor. 

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Okay, this sounds pretty clear, that makes sense. 
 

I’ll be running 220 to the saw. I would likely convert even the 2 (1.75)hp model to 220 as well, even though the gains seem limited. 
 

I’ll have the chance to test the 2hp in person so I can decide there whether 3hp is the way to go. I expect so. 
 

Thanks everyone! 

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Correct, 4hpin a job site saw is total baloney. But the electrical conversion is probably completely corrupted by them using "locked rotor" current, which will be sky high, and melt the windings.

While amps x volts ÷ 746 is a useful guide for estimating the equivalency between electrical power (energy to heat) and work (force applied over distance), true horsepower (work accomplished over time) can only be determined by using force (torque) × speed (rpm) ÷ 5252.

Power conversion rant over. We now return you to normal programming.

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50 minutes ago, Mick S said:

Not saying we know what we're talking about, but between us we have about 500+ years experience in such matters! Yes, I'm old...

You young'un's are always bragging.

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51 minutes ago, Mick S said:

Not saying we know what we're talking about, but between us we have about 500+ years experience in such matters! Yes, I'm old...

All right Mick careful now we're not quite that old, 499 maybe LOL

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8 hours ago, drzaius said:

That's true at the output shaft. Even a good induction motor takes a lot more than 745 watts to make 1 HP. Probably closer to 900 W due to motor inefficiency. A universal motor like that in a direct drive job site saw is probably a lot closer to 50% efficient, which means that a 15 A motor is really somewhere between 3/4 & 1 HP.

I can't understand, with all the laws & regulations on truth in labelling & advertising, how they can get away with putting outright lies about power right on the labels.

Thanks, I could not think of the proper term for the inefficiencies, still can't lol, and never thought of using the term inefficeincy also (aging gracefully is a myth).  That's why I mentioned that 2 hp is prbabably pushing the limit.

 

I think the European standard for motors is just to list wattage.  I will leave that open to be corrected also.

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10 hours ago, Mick S said:

Not saying we know what we're talking about, but between us we have about 500+ years experience in such matters! Yes, I'm old...

Hah, the only reason I’m entertaining it is I have a lead on a heavily discounted, lightly used demo model of the Laguna F2. The new generation has vastly superior dust control though, so I may go for a new 3 regardless of how the 2 feels. 

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1 hour ago, Jim DaddyO said:

hink the European standard for motors is just to list wattage.  I will leave that open to be corrected also.

Yes, European motors have the output rated in KW, and you can figure out the efficiency by calculating the input from the nameplate data. Much better system.

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