Power Tool Amperage Ratings?

February 3, 2010 by admin · 3 Comments
Filed under: Engineering 
power tool
Harvey asked:

Ok, I was shopping for a Mitre saw and I noticed that most of the tools there (mitre saws, table saws, etc…) were apparently “15 amp” devices. But this isn’t possible because at 120v my new mitre saw would have to have something like a 5 HP motor. And that’s simply not the case.

My best guess is that they are trying to indicate you can use the saw on a standard 15 amp circuit.

What am I missing here?

P.S. I haven’t tried it yet, I will this weekend. If it does somehow draw 15 amps I’ll be really surprised, and I’ll be without a saw because I don’t have a higher amp circuit running outside.
Here’s a link to a cheap ryobi saw on homedepot.com. Even it’s labeled as 15 amp even though it’s marketed to the home user. And there’s no way that’s a 5 hp motor.

http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&catalogId=10053&productId=100634340&N=10000003+90401

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3 Responses to “Power Tool Amperage Ratings?”
  1. mechnginear says:
    February 5, 2010 at 4:34 pm

    Start-up current can be 6X running current. They’re probably indicating that you’ll need to be on a 15A circuit so as to eliminate the nuisance tripping of the breaker. mechnginear

  2. Technobuff says:
    February 6, 2010 at 12:33 am

    Where did you get the figure of 5HP from?
    W=EI, so 120 x 15 = 1800W.
    If you use 746W as 1HP, that’s just over 2.4HP….. INPUT.
    The motor will be less in output.
    15A will be the current the machine will draw under FULL (cutting) load.
    Startup current will be higher, but that’s a short term overload.
    Sure, you use it on a 15A circuit. Hopefully you don’t use all the power other than intermittently. Technobuff

  3. Violet W says:
    February 6, 2010 at 10:30 am

    A 115V single phase motor rated 14 amps is about a 3/4HP motor (meaning the motor will in fact deliver 3/4HP output shaft power if presented with that large of a mechanical load). Power tool manufacturers sometimes claim higher HP ratings to hype their tools, but the electrical amperage rating really tells the story about actual output power.

    The reference lists design currents vs motor HP, voltage, and phases. For example, the design value for 3/4HP at 115V single-phase is 13.8A.

    If the product is UL listed, you can be assured that the product will not overload its power cord. If it is a 15A plug, then it is suitable for use by plugging into a duplex receptacle on a standard circuit. Otherwise, UL would have required a larger plug (e.g. 20A which isn’t physically compatible with a standard 15A duplex receptacle). (receptacle = wall outlet). Violet W

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