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Thread: breaker, receptacle, plug for 3 hp dust collector

  1. #1
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    breaker, receptacle, plug for 3 hp dust collector

    Hello and happy holidays!

    My new (to me) Woodtek 3 hp dust collector comes with 14 awg pre-wired to the switch box, but without a plug installed. The previous owner had installed a 6-20 plug, which is same as I have had on my past two 3 hp table saws.

    As a somewhat electrical-confused guy, I'm wondering....

    My table saw (and now this dust collector) both have the same 6-20 plug, on the same size wire from switch box on the tool. Both plug into a 20 amp receptacle. Both receptacles use 10 gauge wire running back to the panel (about 25 feet away). Both have their own dedicated 2-pole 30 amp breakers. The run for the dust collector was done last week by our licensed electrician who reviewed the entire thing and recommended repeating the same approach as seen on the table saw. Both tools fire up every time without ever tripping the breaker, and the plugs are cool to touch even after hours of use (yes, I know this ain't very scientific).

    My question is... I'm having trouble understanding 30 amp breaker vs. 20 amp receptacle and plug. Seems like everything would be 30 amp across the board... remote switches recommended for this unit accept the same 20 amp plug that I have. The manual and calls to manufacture provide vague answers at best. Forum discussions of this quickly evolve to a level that's above my head.

    The reasons I care are... I want a remote switch for the DC, and the ones that fit the plug I have state "up to 2.5 HP"... so my plug will fit the remote receptacle, but my motor is over the stated HP rating. Huh? Also, I always worry about unsafe conditions.

    Can someone help me understand the dynamics here in terms that my 12 year old daughter could understand? Seriously, keep it really basic please, I'm still recovering from too much egg nog
    Last edited by Bob Riefer; 12-25-2019 at 11:21 AM.
    - Bob R.
    Collegeville PA (30 minutes west of Philly)

  2. #2
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    The breaker protects the wire not the tool. Since you end device requires less than 30 amps, you can use 20 amp terminations with no issue. Since the circuit is setup for 30 amps and with 10 gage wire in the wall, should you need to use it for a 30 amp tool in the future, you can just change out the receptacle to a 30 amp version at that point and be good to go. 30 amp receptacles and plugs are more expensive and since you already have 20 amp plugs on the tools, it makes sense to have the receptacles match for now.

    (What you shouldn't do is put 30 amp terminations on a 20 amp circuit)
    --

    The most expensive tool is the one you buy "cheaply" and often...

  3. #3
    breakers are normally sized to fit the wire as yours are. But it is also normal to make the switches and outlets also rated for the same current. In 110V, 20 amp outlets are different from 15 amp outlets, for instance.

    The risk you run with a smaller outlet on the circuit would only occur if something overloaded the circuit. Your tools should not. But a bigger tool plugged into the outlet could. Breakers are meant to protect during unusual conditions - like a tool malfunction resulting in overload - and your breaker will not protect your outlet from an overload. Outlets can fail and cause fires.

    I did not dig out the NEC, it is possible this is OK on a 220V circuit but I am sure it is not on 110V. I doubt it is OK on a 220V. Simplest solution would be a smaller breaker. They are pretty cheap. A breaker smaller than the wire can handles is no problem at all. Breaker should really reflect the smallest rated part of the system. That way is should trip before anything overheats and causes a fire.

    Here is an old post on essentially the same subject: https://sawmillcreek.org/showthread....legal&p=860199

    I learned that 20A and 30 amp plugs are different, I had forgotten that. But it also references a section of the NEC that reportedly says not to do it. But lots of people do not see a problem. I still think a 20 amp breaker is a smart thing to do.
    Last edited by Jim Dwight; 12-25-2019 at 12:35 PM.

  4. #4
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    You stated a licensed electrician reviewed and installed the new circuit that would be good enough for me.
    As to the remote switch you probably are on or above the line where the switch will destruct with use.
    I used current transformers on every circuit (table saw, wide belt sander, jointer, etc) I want to monitor to turn the dust collectors on via a contactor.
    By the way my dust collectors are 20 amp receptacles on 30 amp contactor and breaker. Due to inrush current which is only at startup.
    I do use a 30 amp receptacle and cord end on table saw, planer, and wide belt sander on 30 amp breakers.
    Due to actual power draw during use, ends up being a judgement call based on past experiences.
    I would trust your licensed electrician on this one.

  5. #5
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    Hmmm...

    So Jim's reply above had me nodding my head, seemed sensible.

    Then later, I see Ron saying that my electrician steered me straight, but that a remote switch that will work with my 20 amp plug / receptacle is borderline.

    If electrician looked at wire from tool and decided 20 amp plug and receptacle are in order.... then why would a remote switch that accepts a 20 amp plug and plugs into a 20 amp receptacle be a potential issue? That is... if there's an issue for the remote switch (which seemingly has the same anatomy as the tool plug and wall receptacle... just combined into one unit) why no issue with plug/receptacle?
    - Bob R.
    Collegeville PA (30 minutes west of Philly)

  6. #6
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    plug and receptacle will take more than rated load momentarily as long as it is not way over
    electronics in the remote switch are going to be ruined if it goes over rated load momentarily (this is based on observations and experiences of over 40 years in construction and maintenance work.) plug and receptacles of both are going to survive, they are thick enough. The electronic traces and contacts are typically sized just big enough for rated load and will not survive startup overcurrents. Go ahead and try, then report back how long that remote start lasts. It might make it for multiple starts, maybe even multiple years all depending on actual startup current draw, how often started and how it dissipates the heat from each startup.
    Very typical for a piece of equipment to require on the nameplate a breaker sized heavier than the rated wire ampacity to eliminate nuisance tripping on startup. Breaker still will trip if wire shorts.

  7. #7
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    Ahhh ok this helps me understand. Thank you!
    - Bob R.
    Collegeville PA (30 minutes west of Philly)

  8. #8
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    In Canada, the receptacle must have a current and voltage rating that matches the feeder.

    The only exception to that is multiple 15 ampere receptacles on a 20 ampere circuit.

    Motor circuits are allowed to have a larger rating if the motor will not start on the 100% rated breaker. I have only seen this allowed on dedicated motor circuits, not circuits with a receptacle where another device could be used.

    As for #14AWG flexible cord, it can be rated at 18 amperes with 2 current carrying conductors.

    As always, check with the AHJ.....Rod

  9. #9
    It is not code compliant to put a 20 amp rec on a 30 amp circuit

  10. #10
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    Well, I’m confused as ever now.....
    - Bob R.
    Collegeville PA (30 minutes west of Philly)

  11. #11
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    I agree with Ray. 30A circuits should be connected to a 30A rated receptacle. Buy your plug and receptacle from Amazon and save some money over the big box stores. Alternately, you could replace the 30A breaker with a 20A breaker and call it a 20A circuit. In my new shop, I standardized on 30A circuits and twist lock receptacles for all 240VAC, whether 20A or 30A. I currently have no equipment that requires more than 30A so I can plug in any machine anywhere I have a 240VAC outlet.

  12. #12
    If you plug a smaller motor into a 30 amp circuit and do not have overload protection on the machine you run the risk of burning out the motor because it will not trip the breaker as it should normally. For example wlth a dull blade on a table saw that should draw 12 amps it will not trip the breaker till you hit about 25 or so for a while

  13. #13
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    I talked to another electrician last night at a new years party. He thought that swapping out the breakers for 2-pole 20 amp breakers was the best bet at this time.

    His reasoning was that if both tools (previous to me owning them) were happily working with 20 amp plugs on 20 amp receptacles, then let's match the breaker to that setup. If the breakers start tripping, we'll quickly learn that upgrading to 30 amp breaker, receptacle, and plug are in order.
    - Bob R.
    Collegeville PA (30 minutes west of Philly)

  14. #14
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    No harm in that at all, Bob.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  15. #15
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    This is an illustration of how intuitive thinking can be very believable but not correct. This opinion is coming from an electrical engineer with over 30 years experience, part of it in overcurrent failure analysis. The purpose for limiting the current in house circuitry is to prevent fire. Nothing else. If you take a look at the behavior of a normal household breaker and compare it to the typical failure modes of a motor, you will see why. Breakers will sustain twice the rated current for a minute or so and 10 times the rated current for several seconds. They are designed that way to accommodate starting current. If you don't believe me, just download a trip curve from any manufacturer and study it a little while. Then read up on how long an electric motor will survive a locked rotor condition. By the time the breaker opens up, a motor with a locked rotor is probably already lost. On the other hand, if a motor develops an internal short, the breaker will open up just in time to let you know your motor has been destroyed. Modern motors have resettable internal thermal overload circuits that will protect a stalled motor by opening up long before the breaker opens. Of course, it is possible to destroy some saw motors by continuous overloading but that is a case of it being impossible to protect someone from being a total idiot.
    Quote Originally Posted by ray grundhoefer View Post
    If you plug a smaller motor into a 30 amp circuit and do not have overload protection on the machine you run the risk of burning out the motor because it will not trip the breaker as it should normally. For example wlth a dull blade on a table saw that should draw 12 amps it will not trip the breaker till you hit about 25 or so for a while

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