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Posted on Is it possible to run a 2hp Dust Collector and Saw on a single 240v/30amp dryer circuit?

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22783 posts in 4956 days

#1 posted 03-30-2021 07:35 PM

Good summary CaptK, well done. Minor correction, manual motor starters with overload protection meet code requirements. Magnetic starters not required in all circumstances.

In my experience troubleshooting commercial and industrial systems, most motor overloads are oversized. The excuse made by those who knowingly do it is to prevent nuisance tripping. Properly sized overloads do not nuisance trip, they indicate a problem, overloaded motor. Unfortunately, most do not take the time to properly size the overloads. Many motor starters used to be shipped with NEC-sized OL heaters based on the NEC generic circuit requirements if the specific motor amps are not available. They are supposed to be changed when the motor is connected. Most motor’s full load amp will be 5 to 10% lower than the NEC generic values. When the systems are connected, the oversized overload heaters are left because the installer either doesn’t know or doesn’t make the effort to make it correct. Electrical inspectors cannot check every detail, they approve if overload protection is provided. Bottom-line, most of the motors I found burned up were because they did not have proper OL protection when the bearings went out or other issues overloaded the motor. Puget Power used to have issues in the Green River Valley constantly during the conversion from rural to industrial. Lots of motors could have been saved with proper overload protection ;-))

A computer tech wrote a program error that did not turn off a fan motor after it started. It burned up running all night. The electrical contractor blamed him. He asked me about overloads. If the electrical contractor had the proper overload heaters installed in the motor starter, it would have tripped and saved their motor.

The reason that motor was so fragile running all night is it was not a continuous duty rated motor. Motors have a service factor of 1.0, 1.15, or 1.25. 1.25 is rare. 1.15 was normal for most of my career. 1.0 is nearly all motors I saw in my last 15 years in the trade. The 0, 15, and 25 are the percentages of overload the motor is designed to handle.

That programming error mention above reminds me of another when energy management was in its infancy. The job was adding the boilers in an elementary to their computer-based energy management system. Programming error would not allow the boiler to turn off after it started. The engineer’s interface wiring design bypassed the high temp cutout and other safety devices. Fortunately, the pressure relief valve worked. Hot water under pressure expands at a ratio of 1600 to 1 when released to the atmosphere ;-( There isn‘t enough room for that expansion in a school full of kids!

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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