# Is 3 HP possible really on a 120 volt circuit?



## Duane Bledsoe (Oct 18, 2012)

I once read that a 3 HP motor was not possible to run on 120 volt circuit. Well, there are 15 and 20 amp circuits of 110 - 125 volts. By the way 110 through 125 is all the same, it's a standard wall receptacle. The average is usually stated as 120 volts, or so it is around here. Doing the math, 120 volts x 20 amps = 2400 watts. 2400/746 (1 HP) = 3.217 HP roughly, so yes a 3 HP motor is possible for a 120 volt circuit. Of course there will be losses, as no motor is 100% efficient, but how much, I don't know. At 85% efficiency, the output is just 2.73 HP, but you also sometimes get more than just 120 volts too, which will add to the overall number. I regularly meter 123 volts in my receptacles here at home, so I could see a possible 2 3/4 HP motor being ran on the output side of things. Also, this scenario assumes the 20 amp breaker is ran at maximum capacity, too. But there are also 30 amp breakers available as single pole breakers. Just use 10 gauge wire. So 3 HP is possible on single phase wiring, no 240 volts is needed. I didn't know where else to post this but in power tools so here it is. I'm just posting it because of what I've read in the past about 3 HP 120 volt tools not being true, and some of the various routers offered as 3 1/4 HP supposedly not able to be done. I'm not sure how manufacturers rate their products, either on input power or output, no load or under load, or what, but for those who don't know the math involved in figuring volts, amps, watts, and horse power, there's a bit of info for you. After seeing this, it makes me wonder why some people think 3 HP cannot be achieved on 120 volt circuits.

I also know someone will say this scenario isn't realistic, so here's one that is. Using 30 amp single pole breaker on 10 gauge wire, and loading the breaker to no more than 80% which is a recommended level, I get 24 amps x 120 volts = 2880 watts/746 (1 HP) = 3.86 HP. Assuming only 80% efficiency of a motor, which is awful, I'm still at 3.088 HP on the output side.


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## Carvel Loafer (Dec 31, 2011)

The way I understand it.....

3 HP single phase on 110 volt is possible. However, I don't think it is practical. It's a matter of how many amps will the motor draw on that voltage then the wire size and breaker size can be determined. Lower voltage, higher amperage, larger wire and breaker size. Increase the voltage and you reduce the wire and breaker size requirements. So it isn't a matter of voltage necessarily.


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## rrich (Jun 24, 2009)

The math is correct however everything in the real world don't agree with the math.

So, 3 HP at 746 watts (In a perfect world) is 2238 watts. 

Nominal voltage is probably 107 (Or what you have to design for) says almost 21 amps. 

Now things get ugly. The length of the wire from the panel contributes to voltage loss, a.k.a IR drop. So pick a number, 80, 85 or 88 volts. And now you're looking at almost 25-1/2 amperes.

Normally we allow for a 1/3 to 1/2 increase in amperes for the starting current. Now we're looking at 35 plus amperes. To insure that the breaker won't trip, a 40 ampere breaker is needed. 

Now code steps in. The breaker at 40 amperes may require #8 wire. Maybe code will allow #10 but I'm not sure. Now you need an outlet and plug that can carry 40 amperes. I don't think that you'll find the plug and outlet that will support 120 volts at that high of a current. 

Assuming that you could find all the NEMA equipment that would support 40 amperes at 120 volts, the building inspector wouldn't allow it. Not because it is unsafe but because the inspector didn't understand it. 

Taking the Mythbusters approach, yes it is possible but not realistic in the real world.


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## jigs-n-fixtures (Apr 28, 2012)

As to how manufacturers rate universal motors:

They misapply the torque-rpm equation. Hp= RPM X Torque / 5252, which is valid only if the RPM and Torque are measured simultaneously. 

They use the free spinning, no load RPM, and the locked rotor torque, just before the motor lets all it's smoke out and dyes. Which results in horsepower ratings that would only be possible if the universal motor they are using is capable of over 200% efficiency. 

A motor drawing 12-amps at 120-Volts is 1440-Amps. Which if the motor is 100% efficient is 1.93-hp, at a realistic 80% efficiency it is only 1.5-hp. The router doing this is rated by the manufacturer as a 3.25-hp router.


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## knotscott (Nov 8, 2007)

My understanding is the most commonly used motors are closer to 65-70% efficiency....some even less. A true 3hp motor on a standard 120v circuit is at risk of being seriously starved for power, which will likely shorten it's working life and won't run very well. It could also wreak havoc on your circuit (or worse), potentially putting your household at risk. 

The fact that I've seen several 1.5hp motors run better on 220v suggests that there are real world circuitry shortfalls that are difficult to measure....if a 1.5hp motor is starved for power on a typical 120v circuit, I can only imagine that a true 3hp motor would be severely starved. 

Not a good idea IMO.


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## rrbrown (Feb 15, 2009)

You bunch of Party poopers!!!! :laughing:


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## woodnthings (Jan 24, 2009)

*there ought to be a motor dyno*



jigs-n-fixtures said:


> As to how manufacturers rate universal motors:
> 
> They misapply the torque-rpm equation. Hp= RPM X Torque / 5252, which is valid only if the RPM and Torque are measured simultaneously.
> 
> They use the free spinning, no load RPM, and the locked rotor torque, just before the motor lets all it's smoke out and dyes. Which results in horsepower ratings that would only be possible if the universal motor they are using is capable of over 200% efficiency.


There should be a dynamometer just like motorcycles and cars use to measure HP by an increasing load on the motor. This way the load on the motor can be accurately measured as well as current draw with a constant voltage supplied. Probably the motor manufacturers wouldn't like this idea as it would be an industry standard. 
The present system of watts, amps, volts, rpms, can be too easily "mis-managed" to suit the application. Come on, a 6.5 HP shop vac with a 16 Ga. cord....? No Way. Maybe they throw a little noise factor in to get a higher rating :thumbdown:


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## rrbrown (Feb 15, 2009)

Good idea Bill but your probably correct the manufacturers would not like it. 

The shop vac is the best example yet of how out of control the max developed rating system is.


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## Fred Hargis (Apr 28, 2012)

It was those phony HP ratings that got Campbell Hausfeld in trouble with their air compressors some years back, Out of that lawsuit I got a free pneumatic grease gun, the lawyers got a couple hundred million of dollars. Anyway, the above replies are correct. If you're pragmatic, you can't get 3 HP out of a 120V outlet.


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## Maylar (Sep 3, 2013)

Fred Hargis said:


> It was those phony HP ratings that got Campbell Hausfeld in trouble with their air compressors some years back, Out of that lawsuit I got a free pneumatic grease gun, the lawyers got a couple hundred million of dollars. Anyway, the above replies are correct. If you're pragmatic, you can't get 3 HP out of a 120V outlet.


Other brands were included in that, like Husky, Craftsman and even IR. I think I got 5 bucks as my part of it. Look at the ratings now and they specify "peak" hp (in big letters) and the "actual" hp. My Craftsman compressor says it's 5.5 hp and it's actually 1.5.


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## WarnerConstInc. (Nov 25, 2008)

It can be done. A real 3hp motor too.


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## Toller (Dec 30, 2007)

I had a reputable brand 2hp motor that drew 18a, so 3hp on 30a is certainly possible. It might not be available because there would be little use for it, but possible.


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## WarnerConstInc. (Nov 25, 2008)

Toller said:


> I had a reputable brand 2hp motor that drew 18a, so 3hp on 30a is certainly possible. It might not be available because there would be little use for it, but possible.


I have had a couple, but they were over 100 years old.


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## rrich (Jun 24, 2009)

rrbrown said:


> You bunch of Party poopers!!!! :laughing:


ROTFLMAO :laughing:


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## Toolman2 (Jun 15, 2010)

How hard would it be for you to install a 240v outlet and rewire the saw? I just did both this last weekend and the saw (1 3/4hp) runs a lot stronger and starts up faster and also is less noisy.:thumbsup:


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## TimPa (Jan 27, 2010)

Duane Bledsoe said:


> I once read that a 3 HP motor was not possible to run on 120 volt circuit. Well, there are 15 and 20 amp circuits of 110 - 125 volts. By the way 110 through 125 is all the same, it's a standard wall receptacle. The average is usually stated as 120 volts, or so it is around here. Doing the math, 120 volts x 20 amps = 2400 watts. 2400/746 (1 HP) = 3.217 HP roughly, so yes a 3 HP motor is possible for a 120 volt circuit. Of course there will be losses, as no motor is 100% efficient, but how much, I don't know. At 85% efficiency, the output is just 2.73 HP, but you also sometimes get more than just 120 volts too, which will add to the overall number. I regularly meter 123 volts in my receptacles here at home, so I could see a possible 2 3/4 HP motor being ran on the output side of things. Also, this scenario assumes the 20 amp breaker is ran at maximum capacity, too. But there are also 30 amp breakers available as single pole breakers. Just use 10 gauge wire. So 3 HP is possible on single phase wiring, no 240 volts is needed. I didn't know where else to post this but in power tools so here it is. I'm just posting it because of what I've read in the past about 3 HP 120 volt tools not being true, and some of the various routers offered as 3 1/4 HP supposedly not able to be done. I'm not sure how manufacturers rate their products, either on input power or output, no load or under load, or what, but for those who don't know the math involved in figuring volts, amps, watts, and horse power, there's a bit of info for you. After seeing this, it makes me wonder why some people think 3 HP cannot be achieved on 120 volt circuits.
> 
> I also know someone will say this scenario isn't realistic, so here's one that is. Using 30 amp single pole breaker on 10 gauge wire, and loading the breaker to no more than 80% which is a recommended level, I get 24 amps x 120 volts = 2880 watts/746 (1 HP) = 3.86 HP. Assuming only 80% efficiency of a motor, which is awful, I'm still at 3.088 HP on the output side.


I believe that you are confusing ELECTRICAL horse power with mechanical horsepower. Electrical horsepower (watts/746) is a value often used for EE calculations, and little more. The horsepower values related to electrical motors is mechanical horsepower (550 lb.ft/second) found on the motor information plate. 

Although motor horsepower is not universally rated (e.g. 6 hp shop vac??), an industry standard used over the years is 18 amps/1 hp motor. And i've seen that slip to 16 amps/1hp over the last decade. 

jmho.


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## Tman1 (Jan 14, 2013)

As a motor engineer, I could spend all day correcting the incorrect statements in this thread. But, I'm not going to even try. I will say that 3hp peak is feasible on a 120V, 15A circuit. 3hp continuous is not. The key is that a 15A circuit can handle higher current for very short periods of time. (If you have a gas dryer, it draws around 40-50A until the motor gets up to speed on a 15 or 20 A breaker.) This means that if you have a temporary load spike on your power tool due to a knot, binding or some other reason, the tool has more power to get through it. But, if you try to use that power for very long, such as by taking too aggressive of a cut, you will trip the breaker. 

Now, you still may not get the 3hp, because when they do their dynamometer testing, they test it with 120V at the tool. As has been stated, when you draw high current, the voltage drops and that drops (which is why your lights dim). 

Compressor companies got in trouble because their compressors never run at the peak horse power, except for a fraction of a second as the motor starts. So, they were advertising compressors as better when they knew they provided no benefit to the consumer.


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