Motor rating and circuit

[Paul McGaha]

Oh I'm applying them correctly Art. Once you get used to using them they sort of become 2nd nature.

I'd advise you to put some time into learning how to do it yourself. The book "Guide to the National Electrical Code, 2005 Edition" by Thomas L. Harmon is a good book to learn from. Has numerous examples in it. Enough repetitions to get the hang of it. It's the book I've learned the most from:

http://www.buildersbook.com/01314800...830303232.html

By the way if you take a Unisaw for example, 3 HP, 230 Volt. If you installed the tool in a residential garage, a commercial warehouse, or an industrial plant the components of the branch circuit would all be sized exactly the same way. No difference based on the type of structure the tool is located in.

Good night creekers.

PHM

[Art Mann]

Sorry but I simply do not believe you and you have not yet proved your case sufficiently for me to change my mind. I know how to work the math to figure circuit capacity for industrial installations. At one time, it was my professional job. I just don't think the rules you site even apply to a hobby workshop. For example, so far you haven't acknowledged that there is a difference in requirements depending on whether the motor is being used at full load continuously or intermittently. It doesn't really matter because very few people, including electricians, will run 8 gauge wire to power a 5 hp motor in a home shop no matter what you or the NEC manual says. As I have said several times already, there is no safety risk here. You would have to be very knowledgeable or very unlucky to come up with a loading profile that would cause a fire using the normal rules regarding current loads and breaker capacities.

[Paul McGaha]

Art,

As for the duty cycle see the note under table 430.22E - Any motor application shall be considered as continuous duty unless the nature of the apparatus it drives is such that the motor will not operate continuously with load under any condition of use.

That's what it says word for word Art. The electrical inspectors will be expecting the wire to be sized 125% higher than the full load rating in the tables in the code book. Table 430.22E (or something similar to it) has been in the code book since at least as far back as 1977, which is when I got into the trade. It's not there by accident. My take on it is the NEC looks at almost all motors as possible continuous loads and wants 25% additional capacity for the wire. Like it or not that's how it is. Personally, I went along with it a long time ago.

The planer has a nameplate rating of 21.5 amps. 21.5 amps x 125% = 26.9 amps. To me, in my experience, a dedicated 30 amp circuit would really do. I think it would last and not overheat.

But that's not what the code book says to do. The code book says to add 25% from the full load amps from the motor tables (28 amps x 125% = 35 amps). So, a 40 amp circuit and #8 wire it is, and it's not because I said so it's because the NEC says so.

I've worked for electrical contractors the last 37 years. We have to install the work to wear it meets the NEC. The only way we can stay in business, is to do quality work.

To the original poster, Please do not run that planer on a 20 amp circuit. A 40 amp circuit would be NEC compliant.

PHM

[Duane Meadows]

Kinda of hate to stick my 2 cents worth into another one of SMC's famous electrical advice threads, but here goes...

Seems to me the operative part is "operate continuously with load". With load should be obvious... can you keep enough boards going thru that planer to operate continuously with load?

Then there is the question of the definition for continuous. To my thinking, an 8 hour shift is what comes to mind(ie commercial use). To try to apply that to home hobby shop seems rather extreme.

I fully agree with Art. This is overkill for most home shops, and seems only to benefit the electrician doing the install. There is a difference between "quality work" and "overkill", or "excessive cost without benefit".

As you say, Paul, the phrase "operate continuously with load" is in there for a reason! I rather think it is there to preclude this requirement for intermittent(not continuous) use cases. But then I am neither an attorney, nor a licensed electrician. I was however a licensed broadcast engineer/electronic technician for forty years.

[Paul McGaha]

Well boys,

To the NEC continuous means 3 hours - Article 100 (Definitions).

I'm a master electrician (Virginia and Maryland) Duane, but I quickly admit I don't know the 1st thing about being a licensed broadcast engineer/electronic technician.

As for the argument about should a motor be considered continuous or not, it was explained to me back in apprenticeship class that unless there is some kind of mechanical or electrical interlock which eliminates the possibility a motor can run for 3 hours under load under any conditions, then it will be considered a continuous load (no matter the motor application).

The result is the NEC requires an additional 25% capacity for the branch circuit for a motor. A 25% safety factor for motor circuits. Not that bad of a thing is it? I'm fine with it. I can't think of any electrician I've ever met that would disagree with having a 25% safety factor for motor circuits.

PHM

[Art Mann]

The electrical inspectors will be expecting the wire to be sized 125% higher than the full load rating in the tables in the code book.

The inspector never has to know what is going to be plugged in to the outlet. The OP can just tell him it is none of his business if he asks. Where I live, the inspectors never ask and don't care what is plugged in to an outlet as long as the wire and outlet are adequately protected by the breaker. Where you are, things may be different. Contrary to popular opinion, the NEC book is very much a book subject to interpretation.