The (Simplified) Truth About Motors and Horsepower

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Forum topic by jonah posted 12-10-2013 02:50 PM 2261 views 6 times favorited 28 replies Add to Favorites Watch
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1722 posts in 3323 days

12-10-2013 02:50 PM

People have linked various explanations of electric motors and horsepower numbers, but I still see people throwing motor horsepower ratings around as if they are accurate. See here for some basics:

What I wanted to address is some basic truths about motor operations at 120VAC. 95+% of 120VAC motors are designed to function on a 15amp household circuit. That circuit allows the motor to exceed 15amps for fractions of a second on startup (all do), but allows a maximum of 15amps under load. Some physics equations:

Voltage (V, measured in volts) = Current (I, measured in amps) x Resistance (R, measured in ohms)
Power (P, measured in watts or alternatively, in horsepower) = Current (I) x Voltage (V)
1 horsepower = 745.7 watts
745.7 = I x V
745.7 = I x 120
745.7/120 = I
6.21 = I

So at 120VAC, 1 horsepower equals 6.21amps. So in physics fantasy world where motors are 100% efficient at converting electricity into power, a 1hp motor would draw 6.21amps. In reality, most motors are 50-70% efficient, so a motor developing 1hp of power would actually draw ~9-12amps. This is equally true of the kinds of induction and universal motors commonly used in tools. Also, keep in mind that many tools are designed for nominal operation at 110VAC, not 120VAC, and also that lots of power grids don’t even provide 120VAC. Stick a multi-meter into an outlet in your house. There’s at least a decent chance, depending on where you live, that your house has 110-115V coming out of those outlets.

Realize that there are some complicated vagaries of alternating current that make the equations much more complicated, but for our purposes we can ignore them. They don’t change the basic facts.

If your “3 1/4 horsepower” router actually developed that much power, it’d draw 27-36amps – which it clearly doesn’t. That router probably draws around 12amps (most manufacturers don’t even approach 15amps, let alone 20). Let’s say it actually is rated for 15amps. Your 15amp router is probably around 1.5hp.

What the manufacturers are doing, as has been mentioned before, are taking “peak hp” numbers from the milliseconds when the tool starts (and is drawing dozens and dozens of amps) and sticking them on the box. Shady? Sure, but it’s not illegal and considering how few people actually understand electricity, they can sell a lot more shop vacs by claiming they are 6hp than by actually telling the truth.

I hope this helps people make intelligent choices about what they’re buying.

28 replies so far

View Tedstor's profile


1643 posts in 2657 days

#1 posted 12-10-2013 03:40 PM

Yep. Good article.
I have a “3 hp” Craftsman table saw that works on 120v.

View WhoMe's profile


1564 posts in 3267 days

#2 posted 12-29-2013 05:54 AM

Thanks for this posting. My “2+” hp craftsman is in the same league of hyped HP numbers. I’ll have to look at the amperage noted on the plate to get me my actual HP rating.
Mine runs fine on a 20A 120V circuit.

-- I'm not clumsy.. It's just the floor hates me, the tables and chairs are bullies, the wall gets in the way AAANNNDDD table saws BITE my fingers!!!.. - Mike -

View NormG's profile


6137 posts in 3028 days

#3 posted 12-29-2013 06:57 AM

So true, it is amazing what the manufacturers are able to get by with

-- Norman - I never never make a mistake, I just change the design.

View exelectrician's profile


2327 posts in 2451 days

#4 posted 12-29-2013 07:06 AM

A very good explanation of the truth.

-- Love thy neighbour as thyself

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11774 posts in 2404 days

#5 posted 12-29-2013 07:45 AM

Universal motors are rated by their peak HP while AC induction motors are (usually) rated by continuous HP; DC motors often have both ratings and if they have a flywheel the HP rating may include the effective benefit of that wheel. It’s pretty much a waste of time trying to 2nd guess HP ratings, a cheap AC motor will probably have an inflated number while a quality AC motor will probably be close; the amp rating means little more than the HP rating. I’ve seen a few machine comparison reviews where the magazine tested the motors in various ways and generally the cheaper machines underperformed the more expensive machines with the same HP rating.

-- Rick M,

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Shawn Masterson

1325 posts in 1972 days

#6 posted 12-29-2013 02:14 PM

great read. You seem very knowledgeable. my question for you is a common one on here. If I switch motor from 110 – 220 does it draw less amps, or just draw it from 2 places. My understanding of ohm’s law is that its the same draw, its just split from each leg.

View bigblockyeti's profile (online now)


5137 posts in 1745 days

#7 posted 12-29-2013 02:48 PM

Many small engine manufacturers are now identifying displacement and torque ratings only due to a class action suit brought due to the inflated hp numbers many manufacturers were stating. Honda for example used to have their engines rated at 5.5, 6.5, 9, 11, 13 and so on hp. Now they only identify the displacement in cubic centimeters in the model number.

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8207 posts in 2601 days

#8 posted 12-29-2013 02:53 PM

A definite favorite, thanks for posting Jonah!

View wb8nbs's profile


164 posts in 2716 days

#9 posted 12-29-2013 03:08 PM

My opinion is that the trend to inflated horsepower ratings ws started by Sears/Craftsman in the late 1980,s. Once they started calling a half horse motor a two horse motor all the other brands had to follow suit to compete. One of the rasons I seldom shop ar Sears anymore.

-- The only difference between men and boys is the price of their toys.

View oldnovice's profile


6896 posts in 3392 days

#10 posted 12-29-2013 08:58 PM

Rick you provided the information that everyone should be able to use to make the proper selection of HP and power supply requirements IF the manufacturers would tell the truth about the motors.

-- "I never met a board I didn't like!"

View CharlesA's profile


3329 posts in 1822 days

#11 posted 12-29-2013 09:06 PM

My 6.5 hp shop-vac begs to differ!
(Btw, by your calcs it is a little over a 1hp—9.7 amps.).

-- "Man is the only animal which devours his own, for I can apply no milder term to the general prey of the rich on the poor." ~Thomas Jefferson

View darthford's profile


608 posts in 1948 days

#12 posted 12-29-2013 09:09 PM

I have long suspected something fishy with the manufacturers HP ratings, like how a 3hp and 5hp motor both 220 draw exactly the same number of amps (darthford raised eyebrow something fishy eh look)

View oldnovice's profile


6896 posts in 3392 days

#13 posted 12-29-2013 09:25 PM

It’s a lot like MPG rating and who is telling the truth and who is stretching it to the limit!

-- "I never met a board I didn't like!"

View WibblyPig's profile


172 posts in 3298 days

#14 posted 12-30-2013 03:17 AM

great read. You seem very knowledgeable. my question for you is a common one on here. If I switch motor from 110 – 220 does it draw less amps, or just draw it from 2 places. My understanding of ohm’s law is that its the same draw, its just split from each leg.

It draws less amps at more voltage so the electrical usage is exactly the same (amps x volts = watts). However, since it’s fewer amps, you can get by with a smaller wire when using 220 which may or may not save you money depending upon your wire runs (the longer the run, the heavier gauge wire you need due to voltage drop)

One exception was a Delta saw that came out maybe in the 90’s(?) It was 1-1/2 hp at 110 and 2hp at 220. Something to do with capacitors and things that are way above my pay grade.

-- Steve, Webster Groves, MO "A society grows great when old men plant trees whose shade they know they shall never sit in."

View Hinge's profile


79 posts in 1712 days

#15 posted 12-30-2013 04:07 AM

It’s like speaker ratings…rms vs peak.

-- The Jesus is my Savior

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