Dust collector power question

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"Tom Veatch" wrote:

---------------------------------------------- Tom brings up some very good points; however, must take exception to his comment about a 30A c'bkr being able to handle a 23FLA motor.
You will never get past the inrush of the magnetic portion of the thermal-magnetic c'bkr.
Buried in the NEC some place long forgotten, is a paragraph allowing thermal-magnetic c'bkrs used as motor protectors to be sized up to at least 250% of the FLA of the motor, simply to overcome the inrush problem which in the above example would mean at least a 50A c'bkr.
It's one of the reasons time delay fuses for motor loads have been so popular over the years.
Lew
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On Mon, 21 Sep 2009 16:18:32 -0700, "Lew Hodgett"

23 amps is 76% of 30 amps. I run a 21 FLA motor on a 30 amp circuit (70%) and a 14 FLA motor on a 20 amp circuit (70%) with multiple, frequent starts every day. Not a single breaker trip yet. During testing of the 21 FLA motor, I had it on a 20 amp circuit. Multiple starts, no trips and 105% load to capacity.. The only time it did trip the breaker was in an overloaded condition when it was drawing 44 measured amps (on a 20 amp breaker) and the breaker still didn't trip for about 30-45 seconds.
If his motor is tripping a 30 amp breaker within a few seconds of startup, there's something definitely wrong. But startup surge is not it. Residential type breakers, unlike the old fusible link "fuses", are "time-delay" devices and have a overload/time to trip curve. They will accept very significant overloads (see above - 44 on a 20) for varying durations. Very high overloads for short durations, low overloads for long duration. Unless the breaker is defective, a 30 amp residential service circuit breaker will accept the start surge from a 23 FLA motor for a much longer time than that necessary to get the motor up to rated RPM.
Even on motor circuits, and this isn't one of those, the NEC calls for a 25% headroom (if I remember correctly) on the circuit. That is essentially what the OP has. The oversizing breakers, again, if I remember correctly, is to accommodate very hard starting systems, for example, conveyor belt systems that may have to start under load and take an extended time to come up to rated RPM, during which time the motor(s) draw much higher current than normal - the "inrush" current. And, again, IIRC, motors on such circuits have to either have internal thermal protection or overload protected starters.
Although a Dust Collector motor has a harder job bringing a high inertia impeller up to speed than does, say, a table saw bringing a 10" blade disk up to speed, it still doesn't qualify as "hard to start". Incidentally, the 21 FLA motor mentioned above is the motor on my Dust Collector and was driving the DC impeller during all the testing - including the time it spent on the 20 amp circuit.
Also consider this. I know the breaker is there to protect the wiring, not the load, but if the motor is not equipped with an external starter with overload protection (most 2HP motors aren't) and doesn't have internal thermal protection breakers (some do, many don't) the breaker in the panel is the only overload protection the motor has. If whatever is wrong is something that will trip the 30 but wouldn't trip the 50 amp breaker (unlikely, but possible), then plugging it into a 50 amp circuit will essentially impose a death sentence on the motor - assuming it's not already shot.
Tom Veatch Wichita, KS USA
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"Tom Veatch" wrote:

---------------------------------------------------- The above would cause me considerable concern about whether the above c'bkrs are still functioning as designed. (See comments below about service life) ----------------------------------------------------

---------------------------------------------------- The above refers to the thermal and ignores the magnetic portion of a thermal-magnetic c'bkr.
Tripping on start up is definitely an infrequent problem. ----------------------------------------------------

---------------------------------------------------- If not a motor circuit, what would you call it? ----------------------------------------------------

---------------------------------------------------- Inrush is independant of motor type; however starting current of a function of motor design and connected load. ----------------------------------------------------

---------------------------------------------------- The ONLY way to protect a motor is with an overload relay which usually is, but not limited to thermal devices. ----------------------------------------------------

----------------------------------------------------
As you state above, a c'bkr NEVER provides protection for the load, in this a case a motor.
An overload relay is required to protect a motor(load). ----------------------------------------------------
Some molded case, thermal-magnetic, circuit breaker basics.
1) The only purpose of a circuit breaker is to protect the integrity of the distribution system insulation. If the load destroys itself , but the distribution system insulation is saved, the breaker has done it's job.
2) A circuit breaker only has to clear a fault one time to have done its designed task. If the circuit breaker fails or destroys itself in the process of clearing that fault, it has still done it's job.
3) Circuit breakers are not switches. Switching a breaker on-off on a regular basis is outside the design parameters and can lead to premature failure.
4) Thermal-magnetic breakers contain a magnetic portion which reacts immediately in the case of high over current flow such as a short circuit or motor start up. The thermal portion contains the time delay portion which is inversely proportional to the current being carried. This allows the breaker to monitor overloads within range for short periods without tripping.
5) Circuit breakers operating under load on a continuous basis have a service life. I have been away from the business for far too long to remember all the specifics, but those breakers that see a lot of service either as mechanical switching or electrical devices are candidates for replacement at least every 10 years.
HTH
Lew
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