Horsepower ratings for tools
As an electrical engineer, I was taught that 1 hp equals 746w. At 100% efficiency, the most a 15-amp, 120v outlet in a home circuit can supply is 15 x 120 or 1800w; 1800 divided by 746 equals 2.41 hp. So how is it possible that router manufacturers can claim horsepower ratings of 3 hp, 3.25 hp and even 3.5 hp when none of these tools draws more than 15 amps? For an electric tool operating at 120v, it would have to draw 21.76 amps to develop 3.5 hp (at 100% efficiency). No household outlet fused at 15 amps could supply enough power without tripping. Where do these ratings come from?
Al Comello, Camlachie, None
Barry Wixey, an independent power-tool designer and professional engineer, replies: This great question has been confusing woodworkers and homebuilders for years. You are absolutely correct: The most a tool that is drawing 15 amps from a wall outlet can deliver is 2.41 hp. What you are witnessing here is a rating system that was started many years ago by a certain major tool manufacturer. This system has since been adopted by other power-tool companies so that their tools don’t appear underpowered in comparison.
Although these ratings are a little misleading and I don’t agree with them, there is some logic behind them. First, the highest rating on most power tools is 15 amps. Rating a tool any higher would require that it be equipped with a nonstandard plug, such as a 20-amp plug, which would prevent the tool from being used in a normal house outlet.
It’s possible to draw more than 15 amps from a 15-amp circuit. (If you do so for too long or draw too much amperage, the circuit breaker trips.) And some people do run their tools on 20-amp circuits. It is possible to take a big cut with your router and draw 21.76 amps (3.5 hp at 100% efficiency) or even more for a brief period of time. Continuous operation at this level eventually will trip a breaker, and it may burn out the tool. In a nutshell, these inflated ratings are peak ratings and not ratings for continuous operation.
When approving a power tool, Underwriters Laboratories (UL) does not consider horsepower. UL ratings are based on either amps or watts, and these ratings are continuous ratings. In other words, the tool must be able to run continuously drawing the rated watts or amps without failure. If a power-tool company puts an amp rating or a watt rating on a UL-approved tool, you can rest assured that the tool can put out that much power continuously.
However, the horsepower claims (again, not considered by UL) a company makes can be derived by any testing method the company chooses. If all companies adopted the same testing methods to determine peak-power output, then maximumhorsepower ratings would be a good way to compare the robustness of a particular motor over another. But they don’t.
The bottom line is that when you’re comparing power among different power tools, it’s best to look at the amp or watt rating. The maximum-output horsepower ratings probably can be used to compare tools when they are coming from the same manufacturer, but these ratings are probably not an accurate way to compare tools from different companies.