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Circuit Breaker Tutorial-why you should not use a 20A breaker.

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  • Circuit Breaker Tutorial-why you should not use a 20A breaker.

    Seems to me like Circuit breakers are commonly used by everyone and little understood by most. I will explain Circuit breakers and why you should not use a 20A breaker with your BT3x saw.

    Technical basis for this tutorial comes from the Square-D company that makes circuit breakers used in their panels for industrial and residential uses. These are designed to the same guidelines of the National Electrical code as other manufacturers' breakers so the operation should be the same.

    I think the typical person thinks circuit breakers are black and white devices. That is, for a 15A breaker that if the current is 14.9 amps it will continue to conduct and if the current rises to 15.1 Amps it will open up and disconnect the circuit.

    But in reality, circuit breakers are both time and current sensitive. This is a very useful thing because it allows us to use devices which vary in current pull so that the average is 15 amps and have peaks (say when your saw encounters a knot, or when a motor or light bulb turns on) very much in excess of 15 amps.

    If you look at the end of the reference document, it has trip-point curves for the breakers , time vs. normalized current (e.g. the rated, or handle current is considered to be "1"). The curves and the text say that the circuit breaker should conduct the rated "handle" current (e.g. the value marked on the handle) pretty much forever if the ambient temperature is less than 40°C (~104°F). If the current is over about 1.5 times the rated current for longer than 10-100 seconds the breaker will eventually trip.

    The curves approach the rated current when the time exceeds 10-100 seconds, this is because one trip mechanism is a bimetal element that is heated by the current flow. The element takes time to heat up to the trip point, allowing brief overcurrent surges since the heating takes time. And, devices shifting quickly from 10-20 amps and back but averaging 15 will also work forever.

    The other feature of the curve is that around 9-10 times the rated current (about 135 Amps in this 15A breaker example), the breaker trips within one AC cycle (a few milliseconds) due to the electromagnetic trip element based solely on current. This is sufficient to let motor and light bulb starting curges through.

    Between the two portions of the curve, like from 1.5x to 9x the rated current, the time it takes to trip decreases from 10 seconds to less than .3 of a second.

    Thus you see that circuit breakers will trip immediately on gross overcurrent levels starting around 9x the rating, but allow brief, surges in the 1-10 second or more range. this is consistent with motors which can stand brief surges without overheating due to the thermal mass of the motor.

    If you understand how these work, the people who put a 20A breaker on their dedicated circuit for their BT3100/BT3000 are just fooling themselves and taking a risk that the saw will melt down when overloaded. A 15A breaker for the BT3100/BT3000 is more than sufficient, given the leeway designed into the breakers. A 15A breaker is more than sufficient, allowing current surges consistent with startup and brief surges in operation associated with knots and feeding inconsistencies and other short term irregularities which do not exceed 15A on average. It will also protect the 15A-rated motor properly since the saw itself does not use any current overload protection.


    Reference:
    http://ecatalog.squared.com/techlib/...01&action=view
    Last edited by LCHIEN; 12-30-2006, 10:12 PM.
    Loring in Katy, TX USA
    If your only tool is a hammer, you tend to treat all problems as if they were nails.
    BT3 FAQ - http://www.sawdustzone.org/forum/dis...sked-questions

  • #2
    Valuable information Loring. Thanks for taking the time.

    Greg

    Comment


    • #3
      OK you got my interest Loring. So then with that knowledge am I correct in thinking all my circuits for my tools like my DeWalt chop saw, band saw, drill press, etc should be on a 15 amp breaker? How about the HF DC? It's suppose to be rated at 20 amps so I assume that's really the only tool where I should use a 20 amp breaker then right?
      May you die and go to heaven before the Devil knows you're dead. My Best, Mac

      Comment


      • #4
        Good post, Loring. Thank you.

        Ed
        Do you know about kickback? Ray has a good writeup here... http://www.bt3central.com/articles/l...p?ArticleId=85

        For a kickback demonstration video http://www.metacafe.com/watch/910584...demonstration/

        Comment


        • #5
          A bit of additiional info for optimizing your dedicated circuits.

          Although Loring's info on breakers is quite correct there is a couple of things that can help support a high draw device on a circuit:

          Increase the gauge of wire you use to run your circuit. Even though a 15 amp circuit calls for 14 awg cables using 12 or 10 awg lowers circuit resistance. Likewise running conduit and using stranded thhn wire instead of solid wire will lower resistance. Using the combination of a larger gauge and stranded wire will insure you never choke your tool.
          Opinions are like gas;
          I don't mind hearing it, but keep it to yourself if it stinks.

          Comment


          • #6
            I had my saw plugged into a 15 amp circuit. It was always tripping, especially in hot weather. I used no extension cord either. Even if I had other devices plugged into the same circuit (the circuit powered additional plugs in my garage) but did not turn them on, the circuit still tripped when I ran my BT. I added a 20 amp circuit because of advise on this forum. I now plug my BT and miter saw (both with 15 amp motors) into this outlet. I have not had any problems. Will I really damage my tools? If so, how do I avoid tripping the breaker all the time?

            Comment


            • #7
              Thanks

              Hi!
              Thanks.
              Your posts are always informatives.

              Andre

              Comment


              • #8
                Originally posted by Sawatzky View Post
                I had my saw plugged into a 15 amp circuit. It was always tripping, especially in hot weather. I used no extension cord either. Even if I had other devices plugged into the same circuit (the circuit powered additional plugs in my garage) but did not turn them on, the circuit still tripped when I ran my BT. I added a 20 amp circuit because of advise on this forum. I now plug my BT and miter saw (both with 15 amp motors) into this outlet. I have not had any problems. Will I really damage my tools? If so, how do I avoid tripping the breaker all the time?
                Did you try replacing the original 15a breaker before adding the 20a circuit?

                Comment


                • #9
                  Originally posted by cgallery View Post
                  Did you try replacing the original 15a breaker before adding the 20a circuit?
                  That would be my question - perhaps your 15A breaker was bad.

                  My point is that a 15 Amp breaker will allow at least 15amps average to be delivered to those motors for a long time. A 20A breaker will allow 20 Amps average to be delivered to those devices for a long time, it has no conscience.

                  If you push a 15A motor to draw 19Amps it will burn out quickly. Because the magnetics of the motor will be saturated and the current will just end up heating the motor rather than doing mechanical work. The 20Amp breaker will allow it to self destruct.
                  Last edited by LCHIEN; 12-31-2006, 03:29 AM.
                  Loring in Katy, TX USA
                  If your only tool is a hammer, you tend to treat all problems as if they were nails.
                  BT3 FAQ - http://www.sawdustzone.org/forum/dis...sked-questions

                  Comment


                  • #10
                    Originally posted by lcm1947 View Post
                    OK you got my interest Loring. So then with that knowledge am I correct in thinking all my circuits for my tools like my DeWalt chop saw, band saw, drill press, etc should be on a 15 amp breaker? How about the HF DC? It's suppose to be rated at 20 amps so I assume that's really the only tool where I should use a 20 amp breaker then right?
                    A 20 Amp breaker would simply be to protect the dwelling from fire due to overcurrent in the wiring, caused by a fault (likely catastrophic short) in one of the loads or perhaps the wiring itself.

                    It (15 Amp breaker used with single tools) should be adequate and provide maximum protection. You would want to use a larger breaker if you were running several loads on a single circuit, whose total was more than 15A but less than 20A. But the possibility exists, unless the individual loads were fused or protected by internal circuit breakers, any device on the line could be damaged by overcurrent.


                    I believe you are incorrect about the HF 2HP DC. It is rated to draw 14 or 15 Amps and I have confirmed mine draws 14.7 amps (67 Amps on startup) when running with no hoses attached - pretty much the maximum airflow condition and hence maximum current condition. I run mine on a 15A circuit without tripping any breakers.

                    from the HF webpage:
                    Powerful 2 HP dust collector creates a dust free working environment. Develops over ten times the suction of most shop vacuums. Works with a 4'' hose to pick up large chips from jointers, saws, shapers and planers. Hose sold separately.
                    • Locking casters
                    • Lockable on/off toggle switch
                    • Motor: 2 HP, 110V, 14 amps, single phase
                    • Bag capacity: 70 gal.
                    • Air flow: 1600 CFM
                    • Single stage
                    • Filtration: 30 micron
                    • Hose inlet: 4''
                    • Overall dimensions: 75-1/2''H x 33''L x 22''W
                    • Shipping weight: 145 lbs.
                    • CSA-US certified

                    ITEM 45378-7VGA
                    Last edited by LCHIEN; 12-31-2006, 03:14 AM.
                    Loring in Katy, TX USA
                    If your only tool is a hammer, you tend to treat all problems as if they were nails.
                    BT3 FAQ - http://www.sawdustzone.org/forum/dis...sked-questions

                    Comment


                    • #11
                      Very good information, Loring! I guess I will have to re-analyze my shop circuits and adjust accordingly.

                      Comment


                      • #12
                        Originally posted by LCHIEN View Post

                        I believe you are incorrect about the HF 2HP DC. It is rated to draw 14 or 15 Amps and I have confirmed mine draws 14.7 amps (67 Amps on startup) when running with no hoses attached - pretty much the maximum airflow condition and hence maximum current condition. I run mine on a 15A circuit without tripping any breakers.
                        [*]Motor: 2 HP, 110V, 14 amps, single phase
                        However, I think NEC calls for most circuits to NOT carry more than
                        80% of its maximum load. In this case, I would think a 20A circuit would be
                        better suited.

                        http://ecmweb.com/mag/electric_sizing_circuit_breaker/

                        Paul

                        Comment


                        • #13
                          Originally posted by LCHIEN View Post
                          A 20 Amp breaker would simply be to protect the dwelling from fire due to overcurrent in the wiring, caused by a fault (likely catastrophic short) in one of the loads or perhaps the wiring itself.

                          It (15 Amp breaker used with single tools) should be adequate and provide maximum protection. You would want to use a larger breaker if you were running several loads on a single circuit, whose total was more than 15A but less than 20A. But the possibility exists, unless the individual loads were fused or protected by internal circuit breakers, any device on the line could be damaged by overcurrent.


                          I believe you are incorrect about the HF 2HP DC. It is rated to draw 14 or 15 Amps and I have confirmed mine draws 14.7 amps (67 Amps on startup) when running with no hoses attached - pretty much the maximum airflow condition and hence maximum current condition. I run mine on a 15A circuit without tripping any breakers.

                          from the HF webpage:
                          Powerful 2 HP dust collector creates a dust free working environment. Develops over ten times the suction of most shop vacuums. Works with a 4'' hose to pick up large chips from jointers, saws, shapers and planers. Hose sold separately.
                          • Locking casters
                          • Lockable on/off toggle switch
                          • Motor: 2 HP, 110V, 14 amps, single phase
                          • Bag capacity: 70 gal.
                          • Air flow: 1600 CFM
                          • Single stage
                          • Filtration: 30 micron
                          • Hose inlet: 4''
                          • Overall dimensions: 75-1/2''H x 33''L x 22''W
                          • Shipping weight: 145 lbs.
                          • CSA-US certified

                          ITEM 45378-7VGA
                          I wasn't sure either so just went out and copied what my actual DC says and I quote;

                          Central Machenery Model 45378, DC- 2 HP 70 Gal.,
                          Motor 2 HP, 20 amps Peak,
                          Single phase, 60 Hz
                          110 volts,
                          RPMs 3450,
                          Power cord 5 1/2" 16 AWG x 3C gauge.

                          I wonder why ours differs so much. I bought mine about 2 years ago maybe that's it? Also the 16 AWG power cord puzzles me. 16 AWG? Sure seems light to me.
                          May you die and go to heaven before the Devil knows you're dead. My Best, Mac

                          Comment


                          • #14
                            Originally posted by maxparot View Post
                            A bit of additiional info for optimizing your dedicated circuits.

                            Although Loring's info on breakers is quite correct there is a couple of things that can help support a high draw device on a circuit:

                            Increase the gauge of wire you use to run your circuit. Even though a 15 amp circuit calls for 14 awg cables using 12 or 10 awg lowers circuit resistance. Likewise running conduit and using stranded thhn wire instead of solid wire will lower resistance. Using the combination of a larger gauge and stranded wire will insure you never choke your tool.
                            I am confused on this b/c the code does not change the gauge w.wire style. A contractor I know says that there is no advantage in resistance either way. Could you explain? I know some people who think that solid wire is better.
                            Andrew

                            Comment


                            • #15
                              Originally posted by Andrew Benedetto View Post
                              I am confused on this b/c the code does not change the gauge w.wire style. A contractor I know says that there is no advantage in resistance either way. Could you explain? I know some people who think that solid wire is better.
                              It has more to do with run length and wire size then wire size alone. I will not use anything smaller then #12 when running lines. You run #14 and decide you want 20A at a later date and you are a screwed puppy.

                              Stranded vs solid is personal preference in most cases. Stranded wire pulls through conduit easier but is a PITA otherwise. You land up with pigtails to outlets and lightswitches.

                              Comment

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