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Thread: Motor rating and circuit

  1. #31
    Quote Originally Posted by John Lanciani View Post
    I guess you didn't look at the Square D PDF I posted the link to, you're not even close. at 150% of rated load a thermal/magnetic breaker will take upwards of 5 minutes to trip. Even at 500% load it can take as long 12 seconds to trip.
    And I guess you didnt read my next post. I will just go back to my hand tools. Try to thwart electrical protection all you want.

  2. #32
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    Quote Originally Posted by Cody Kemble View Post
    And I guess you didnt read my next post. I will just go back to my hand tools. Try to thwart electrical protection all you want.
    Not trying to thwart anything, just posting hard facts as published by one of the largest electrical equipment manufacturers in the world instead of hearsay and misguided belief, something that there is much too much of in electrical posts on this forum.

  3. #33
    You shouldn't overload circuits. I was looking at an older (2003) UL white book that I had laying around I was not aware that was hearsay.

  4. #34
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    Quote Originally Posted by Cody Kemble View Post
    You shouldn't overload circuits. I was looking at an older (2003) UL white book that I had laying around I was not aware that was hearsay.
    So when you said "Breakers are set to trip at around 80% of rated load. So a 20 amp will get you about 16 amps" you got that information from a UL publication? Care to give us the citation so we can look it up?

  5. #35
    Quote Originally Posted by John Lanciani View Post
    So when you said "Breakers are set to trip at around 80% of rated load. So a 20 amp will get you about 16 amps" you got that information from a UL publication? Care to give us the citation so we can look it up?

    I was incorrect in that statement. That was the reason for the next post saying "sorry" after the polite correction of Art Mann. I have deleted said post so it will no longer confound you. Good day sir.

  6. #36
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    From the National Electrical Code:

    1) Full load current for a 5 horsepower, 230 volt, single phase motor is 28 amps (Table 430.248).

    2) Breaker size is a maximum amperage of 28 amps x 250% for an inverse time circuit breaker (Table 430.52) = 70 amps. So you must use a breaker with an amperage rating of 70 amps or smaller for this saw. Personally, I'd use maybe a 40 or a 50 amp inverse time breaker.

    3) Wire Size is 28 amps x 125% (For a continuous load see NEC 430.22E) = 35 amps, Use #8 minimum.

    Please find the following link for more on the subject, the author is Mike Holt, a person that is widely known and respected for his electrical expertise:

    http://ecmweb.com/design/motor-calcu...uit-conductors

    To my knowledge, and I've been in the electrical construction field since 1977, that is how the circuit components should be sized and why. That is what I would expect an electrical inspector to be looking for when inspecting the installation of the wiring for a 5 hp saw.

    Good luck with it.

    PHM

  7. #37
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    The original poster is installing a 240VAC wall outlet. How he installs that outlet is the inspector's business. What he plugs into it is not. The articles of the NEC you are quoting apply to continuously run motors in an industrial environment. I know because I worked as an electrical engineer in that kind of environment. What you are quoting may not be applicable in the original poster's situation. Anyway, there will be no inspector there to say one way or the other what can be plugged in and the house wiring will be well protected if he follows conventional rules for breakers and wire sizes. There is a very large measure of over protection built into those standards. The only problem I would anticipate is startup current tripping the breaker and that apparently didn't happen.

  8. #38
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    That's actually how you do it Art. It's a motor circuit and you're supposed to follow motor circuit rules. In a 2 car garage residential shop for a hobbyist or an industrial application.

    Just my $.02 but it seems to me that the NEC is trying to accomplish 2 things with motor circuits that are a little different from regular branch circuits:

    1) They seem to want to build in at least an additional 25% capacity for the wire. Pretty much everything is to be considered continuous - This is an additional 25% capacity for the wire. Also use the code tables for the full load amp rating instead of the motor nameplate to size the wire and the breaker - this is more additional capacity for the wire.

    2) They are very generous on the amp setting of the over-current protection (short circuit protection) to allow the motors to start. In the case of the planer in this thread you could use an inverse time breaker up to 70 amps and be code compliant.

    Maybe the NEC people just want the additional capacity for the wire for motor circuits because they're concerned that the allowable rating of the over-current protection on a motor circuit is much higher than the rating of the over-current protection on a regular branch circuit. I don't know.

    I think people could make all sorts of arguments on why the code book has these particular rules for motor circuits (starting with why are pretty much all motors considered continuous loads) but I don't see any room at all for arguing if these code rules are to be followed. They are. Electrical inspectors will be looking for an NEC code compliant installation.

    PHM

  9. #39
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    Hi Paul, in Canada the CEC considers a circuit with a receptacle in a garage a general purpose circuit and the motor rules don't apply.

    In Canada a motor circuit is only used for a motor, for example a machine with a disconnect switch that only feeds the motor starter is a motor circuit.

    Obviously the US rules may be different...............Regards, Rod.

  10. #40
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    Hi Rod,

    That's interesting. In the US, Branch circuits for motor loads are covered by article 430 (Motors) in the NEC. Soup to nuts.

    PHM

  11. #41
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    Hi Paul,

    Are you sure that 430 covers cord-and-plug installations? I was under the impression that it was only for permenantly installed equipment and not gp circuits.

  12. #42
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    I only have a copy of the 2011 NEC but that document makes a distinction between the required current capacity of motor circuits that are 100% duty cycle and those that are used in intermittent duty applications. They give elevator motors and pumps as possible examples. Of course, a planer will seldom be operated at full load at even a 50% duty cycle, although it might happen sometimes. The bottom line is it doesn't matter what the NEC says. I have either been in charge of or worked on the wiring of many different houses and I have never even heard of an inspector asking what is going to be plugged into a particular outlet. If the OP is using his planer in this sort of environment, the inspector will never know or care what is plugged in. There are only 2 issues. (1) Will the service provide adequate power to the machine so that it doesn't trip breakers? This question can easily be answered by the OP. (2) Will the use of this circuit present a fire hazard? As I have already said, there is a lot of safety margin built into the NEC requirements for house wiring. You would have to be very clever or very unlucky to create a situation in which a properly sized wire will get hot enough to ignite when using an appropriately sized breaker.
    Last edited by Art Mann; 07-18-2014 at 8:04 PM.

  13. #43
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    Quote Originally Posted by John Lanciani View Post
    Hi Paul,

    Are you sure that 430 covers cord-and-plug installations? I was under the impression that it was only for permenantly installed equipment and not gp circuits.
    John,

    See 210.1, Article 430 (Motors) covers branch circuits that supply only motor loads, not article 210 (Branch Circuits).

    PHM

  14. #44
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    Quote Originally Posted by Art Mann View Post
    I only have a copy of the 2011 NEC but that document makes a distinction between the required current capacity of motor circuits that are 100% duty cycle and those that are used in intermittent duty applications. They give elevator motors and pumps as possible examples. Of course, a planer will seldom be operated at full load at even a 50% duty cycle, although it might happen sometimes. The bottom line is it doesn't matter what the NEC says. I have either been in charge of or worked on the wiring of many different houses and I have never even heard of an inspector asking what is going to be plugged into a particular outlet. If the OP is using his planer in this sort of environment, the inspector will never know or care what is plugged in. There are only 2 issues. (1) Will the service provide adequate power to the machine so that it doesn't trip breakers? This question can easily be answered by the OP. (2) Will the use of this circuit present a fire hazard? As I have already said, there is a lot of safety margin built into the NEC requirements for house wiring. You would have to be very clever or very unlucky to create a situation in which a properly sized wire will get hot enough to ignite when using an appropriately sized breaker.
    Art,

    Well, I think it might be useful to creekers to know how to properly size the components of a motor circuit for a tool in a woodworking shop.

    I think I've explained how to do it about the best I know how. I've provided code references and a link to more examples of how to do it in post #36.

    Now Art if you, or anybody else, doesn't think it's important for the electrical work to be NEC compliant, well, I'm just not in your camp.

    If you'll do the work per the NEC rules you'll wind up with a circuit that's safe and dependable for decades of service. What's wrong with that? That's the idea. That's why people hire electrical contractors.

    PHM
    Last edited by Paul McGaha; 07-18-2014 at 9:56 PM.

  15. #45
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    Paul, I believe you are not correctly applying the NEC. I don't think those requirements apply to plug-in type tools in a home or hobby shop environment. Such tools do not operate on a continuous duty cycle - just like the elevator example in the NEC book. If the paragraphs you sited apply, then a majority of hobby shops in the US would not meet NEC requirements. I just hate to see someone recommend expensive and unnecessary wiring just because it is required (and sensible) in an industrial environment. There just isn't a safety issue. As a matter of interest, why not call your wiring inspector and ask if the rules you are quoting apply to hobby shops. I will try to remember to do the same next time I work on a Habitat for Humanity house.

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