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HP ratings for routers and some other handheld power tools

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Forum topic by richgreer posted 1316 days ago 1572 views 3 times favorited 16 replies Add to Favorites Watch
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richgreer

4522 posts in 1671 days


1316 days ago

I tripped onto a Q & A in the latest edition of Fine Woodworking that cleared up a mystery for me. Maybe everyone else knew this but in case someone else does not understand horse power ratings, I will share my discovery.

First the formula HP = (volts x amperage draw x efficiency factor)/746

The efficiency factor reflects how efficient a motor is at turning electricity into motion. On most electrical motors it is about .7.

The number “746” converts watts to horsepower.

Assuming a .7 efficiency factor, a router that draws 11 amps using a 120 volt circuit will produce 1.25 hp. A 15 amp draw would produce 1.7 hp. I have one router with a maximum of 20 amps of draw so it produces 2.25 hp (if my circuit breaker does not trip).

So how do we get these routers that allegedly produce 3hp or more? The answer is “they cheat”. They run a router motor at full speed and then throw a “stopper” mechanical load at it. There will be an instantaneous surge in amperage draw before the circuit breaker trips or the motor burns up. They use this amperage draw from the surge to calculate horsepower. It’s a meaningless number because the router motor cannot sustain that power.

The maximum amperage draw from your router (it’s usually printed on a label) is the better indication of actual usable hp.

Frankly, I think we are seeing some misrepresentation from companies that claim hp ratings based on surge amperage. Yes, they all (except festool) do it. That does not make it right.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.


16 replies so far

View Jim Bertelson's profile

Jim Bertelson

3622 posts in 1761 days


#1 posted 1316 days ago

The HP ratings of various tools and household appliances has always been a source of amusement to me, and I usually look at the amperage as being the only reasonable measure. There ought to be some accepted way to determine the usable power of a tool, but I suspect this will continue to be an area of unethical and deceptive advertising.

-- Jim, Anchorage Alaska

View Paul2274's profile

Paul2274

324 posts in 1709 days


#2 posted 1316 days ago

Rich,
So basically when I go to look at buying a new power tool (be it router, circular saw, jig saw…) I really should be looking at the label for the amps that the tool is rated for on a constant and not for the so-called “HP” rating that the manufacturers have fudged on the numbers. Right?

It really does make sense and I did not know this. Just goes to show that the marketing people are doing their jobs and I, the little obedient sheep, am being led to the dollar slaughter.

I think I’ll go jump off a cliff now.

Paul

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richgreer

4522 posts in 1671 days


#3 posted 1316 days ago

Paul,

Please don’t jump off a cliff. I would forever feel like I contributed to a suicide.

I agree with you that focusing on the amps (not the stated hp) is the better way to judge power.

FWIW – Festool sells 4 routers and they are labeled: 700, 1000, 1400 and 2200. They never state an hp rating (that I have seen). Now get this – - Their maximum amperage draw is 7, 10, 14 and 22 respectively. Isn’t that interesting?

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

View Jim Bertelson's profile

Jim Bertelson

3622 posts in 1761 days


#4 posted 1316 days ago

Armed with that knowledge, if Festool doesn’t succumb to the distortion of the facts rampant in the rest of the industry, one can get a pretty good idea of what the router can do from it’s model number. Neat. Shall we make a wager on whether they become corrupted as well?

I remember my Dad complaining about furniture stores that always had everything on sale, or had been going out of business for the last 10 years, that was about 50 years ago. He worked for a small retailing company that owned a furniture store, and tried to advertise ethically. You can still see the inflated prices in Amazon’s statements about how much you are saving. They quote the normal price for an item that seems about 50% higher than anything I have ever seen. Meaning, they claim they have a discounted, or sale price, but it is obviously malarky, much of the time. I am talking about quoted prices that are 50% higher than Lowes and HD, as well as the local industrial hardware store that sells pretty much at list, and this is Alaska. I have no idea where they find these inflated “usual” prices. I have never seen them.

Oh well, buyer beware….......

Jim

-- Jim, Anchorage Alaska

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crank49

3336 posts in 1567 days


#5 posted 1316 days ago

What ! You mean that 12amp shop vac I bought with the 14ga power cord is not really good for 6 hp. Seriously, I just pointed this out in a post a couple of days ago. The 70% efficency is not a hard and fast rule, but whether a motor is 70% or 85% is not going to account for the difference between 1.5hp and 6hp. It’s just a flat out lie. It comes about due to the way a series wound motor, the type in most portable tools, developes power. These motors are the ones that have brushes. They regularly generate way more torque for instantaneous surges than they can sustain. They get by with this because they are rarely fully loaded for a continuous long sustained run at max power draw. Take a router motor running at 20,000 RPM and drawing 15amps. It’s generating, as Rich pointed out, about 1.7hp. Load that same motor down with a big bit and shove it hard into some very hard wood and it might pull 25 to 30amps for a few seconds. For those few seconds it is generating 3.25hp. The point is that it won’t do this long before something is fryed.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

View poopiekat's profile

poopiekat

3556 posts in 2331 days


#6 posted 1316 days ago

wow, I’m amazed… I always figured 1500 watts equals 1 horsepower, based on some practical-arts teacher when I was a lad. Perhaps I’m thinking of DC current instead of AC. Yeah, I’ve always rolled my eyes at the wild horsepower claims, Sears is famous for it especially. Wattage consumption really is the true indicator of a power tool’s power.
A bit off-subject, but these days has anybody noticed that lawnmowers and snowblowers with gasoline engines no longer indicate HP? You might get a displacement value, but never a HP indicator any more.Again, it was Sears who would claim 5.5 or 6 HP on their tinyest lawn mower engines, as if…. There is a relative formula for calculating HP on small gas engines too, for those willing to get a true comparison.

-- Einstein: "The intuitive mind is a sacred gift, and the rational mind is a faithful servant. We have created a society that honors the servant and has forgotten the gift." I'm Poopiekat!!

View richgreer's profile

richgreer

4522 posts in 1671 days


#7 posted 1316 days ago

Regarding lawn mower horse power claims.

I purchased a new lawn mower about 3 years ago. About 2 years ago I was notified of a settlement from a class action law suit. A class action law suit had been filed and won against manufactures of lawn mowers that made false claims about horsepower. If I had bothered to fill out the paperwork I would have gotten $25.

Maybe that is why we are not seeing hp ratings for lawn mowers. However, some manufactures show their engines torque rating and to one who is not very observant they make think that is an HP rating.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

View b2rtch's profile

b2rtch

4286 posts in 1645 days


#8 posted 1316 days ago

It is not directly related but not far: I have been puzzled by wattage and BTU rating on electric heaters lately.
Most of you know that I built a shop in which I have a wood stove.
In addition to it I wanted to install an electric heater to keep the temperature from falling to freezing level when I am not in the shop.
I first bought an inexpensive 4000 watts electric heater, the very first one was not working at all but its replacement was working but given, it seemed to me , very little heat compared with my 1500 watts heater in my bathroom. SO I got a third heater ( same model) which was giving no more heat than the first replacement.
So I returned everything for a full refund and I bought an “expensive” 5000 watts, 22 000 BTU electric space heater and again I am disappointed by the heat output compared with my cheap bathroom heater.
The heater gives heat but compared with my 1500 watts bathroom heater it certainly does not seems to give 4 times as much heat ( i cannot keep my hand in front of the bathroom heater, I can do it with no problem in front of my space heater), I am puzzled and disappointed

-- Bert

View Greedo's profile

Greedo

465 posts in 1557 days


#9 posted 1316 days ago

in europe you never see HP ratings on powertools, thats probably why festool doesn’t have an hp rating in the states. only Watts are used to indicate power

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b2rtch

4286 posts in 1645 days


#10 posted 1316 days ago

“in europe you never see HP ratings on powertools ”,
may I add or on engines

-- Bert

View richgreer's profile

richgreer

4522 posts in 1671 days


#11 posted 1316 days ago

I don’t know this for a fact but I think I am right. Perhaps someone can correct me if I am wrong.

I think the watts that are noted on european tools is referring to the power consumption (how much electrical power you are using) and not power output. In theory, the output power would equal the power consumption if the motor was 100% efficient. Of course, no motor is 100% efficient (not even festool).

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

View Lochlainn1066's profile

Lochlainn1066

138 posts in 1374 days


#12 posted 1316 days ago

I have an older Craftsman shop vac that draws 12 amps and has a 3.0hp rating! Wow, what will they come up with next? Maybe ratings that don’t lie?

Conversely, I have “big boned” motors from the 50’s in my tablesaw and bandsaw that sure feel more powerful than the 1hp they advertise.

I wish they would just give us the dynamo ratings like are done on car engines. Show the whole curve, from stall to no load. We’d get much more accurate info that way.

-- Nate, thegaragestudio.etsy.com

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dbhost

5378 posts in 1828 days


#13 posted 1316 days ago

Sort of like my 110V 6HP Ridgid Wet / Dry vac? Or the 3 or so HP Craftsman 110V table saws. Ratings trickery, not honesty… And is it any wonder why people are pessimistic about tool manufacturers anymore?

I’m good with the BHP ratings like automobiles. How do we get the manufacturers to be honest though?

-- My workshop blog can be found at http://daves-workshop.blogspot.com

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b2rtch

4286 posts in 1645 days


#14 posted 1316 days ago

In Europe on engine the KW indicates developed power, the power the engine actually gives at the fly wheel.

-- Bert

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8iowa

1489 posts in 2358 days


#15 posted 1316 days ago

Rich:

Indusrial motors, ie: AC induction, DC, AC variable frequency, are all standardized by The National Electric Manufacturers Association (NEMA). These motors have HP ratings that apply to continuous operation within a specified maximum temperature rise. Actually, the efficiency of three phase AC induction motors is frequently higher than 90 %.

Universal electric motors are not standardized by NEMA. Often the only information that is provided is an operating voltage and amperage rating. Incidently, your assumption of 70% efficiency may be “generous”. I have no doubt that my routers, if put on a dynamometer, and loaded to their HP “rating”, would continuously rise in temperature until they burned up.

Sometimes manufacturers use the term “peak HP”. An AC induction motor has a very flat speed torque curve and can be loaded beyond it’s continuous HP rating with only a slight drop in speed. Obviously the motor can’t operate continuously at this load. Such ratings have no legitimacy.

-- "Heaven is North of the Bridge"

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