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15 Amp breaker w/ 15 Amp motor??

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  • 15 Amp breaker w/ 15 Amp motor??

    Question for the electrical gurus.
    I need to purchase a chop saw in order to cut some 1-1/4" heat treated 4140shafts in half.
    However, all of the circuts in my house (including garage) have 15 Amp breakers.
    Most of the Chop Saw motors I've seen are 5HP, 15 Amp motors.
    So.......
    Will the motor repeatedly trip my 15 Amp breaker everytime I turn it on, or attempt to cut under load?
    ....Stupid house.....

  • #2
    A 15 amp 120vac circuit is rated for a continuous load of 12.5 amps. For a 15 amp load you need a 20 amp branch circuit. Incidentally, 15 amps at 117 vac is only 2.35 hp. Are you sure it isn't a 220 vac motor? That would be 5hp.

    Of course it might be vacuum cleaner horsepower...
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    • #3
      Originally posted by KiddZimaHater
      <snip>.......
      Will the motor repeatedly trip my 15 Amp breaker everytime I turn it on, or attempt to cut under load?
      ....Stupid house.....
      Short answer, it is a 50:50 gamble. All breakers have a tolerance range, just like mechanical parts. You may have one that is a bit higher in actual trip value and be OK or one that is a bit lower and it will trip over and over.

      If your box is close to the garage I would add a new 20 Amp circuit and be done with it.
      Paul A.

      Make it fit.
      You can't win and there is a penalty for trying!

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      • #4
        You will need 12 ga wire for a 20 amp circuit.
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        • #5
          The motor will draw ONLY according to the load, after the base unloaded draw.

          So if you force the saw, or cut stuff requiring max power, you may, or may not, open the breaker repeatedly.

          If you lay off, you may not have a problem.

          If the breakers are strongly reactive to the "instantaneous" load, as some types are (especially the ones that have two levers in a standard width package), the breaker may open on startup. That can happen with a 20A too.
          1601

          Keep eye on ball.
          Hashim Khan

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          • #6
            I'm gonna have to take a look at one in the store.
            Hopefully the motor can be rewired for 220v.
            I'd sure hate to go thru the hassle of adding a new 20Amp 120v circut.

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            • #7
              Originally posted by KiddZimaHater
              I'm gonna have to take a look at one in the store.
              Hopefully the motor can be rewired for 220v.
              I'd sure hate to go thru the hassle of adding a new 20Amp 120v circut.
              I'm not sure what that would accomplish unless you have a circuit in the garage with NO OTHER LOADS on it. ie no lights, preferably only one outlet and no other loads or outlets in the house, assuming you only have one circuit in the garage, how do you plan to power 110v tools / lights???

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              • #8
                Originally posted by KiddZimaHater
                Will the motor repeatedly trip my 15 Amp breaker everytime I turn it on, or attempt to cut under load?
                ....Stupid house.....
                It will work fine on that circuit, UNLESS you stall the darn thing. I looked at several and you're right they all say 15 amp 110 volt.

                I looked at mine and it says it is 15 amp also. It is not a 2.35 HP (Evan) motor.

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                • #9
                  Fred,
                  I have a 240v outlet that I plug my mill and bandsaw into, and it's on it's own circut.
                  But the 120v outlets in the garage (2) are on the same circut.. along with the overhead lights AND the overhead kitchen light fixture.
                  ...Stupid home builders....

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                  • #10
                    you could use a distro box ( a 60 or 100 amp panel) plugged into your 220 outlet and then use a 20A breaker and 12 ga wire. I did that so I could get more power in my garage using my welder outlet.

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                    • #11
                      If the shop is wired two phase, as in two hots of 110 each, then you can run a 220 load from that with little problem. You may have other loads on each of the 110 legs, but as long as you aren't adding more current draw than each leg can safely give, it should be fine. A 15 amp load on 110 translates to a 7.5 amp load at 220. If each or either 110 leg is already running say 6 100 watt lights, that's a draw of 5 amps. If you add the 7.5 amps to that, you're at 12.5 amps, which is right at the rating for continuous current through a 15A breaker. The starting current of a motor load might be too much for it if there's already 5 amps flowing somewhere else, but it normally would work.

                      Note that this may not be accepted practice for wiring, although it does work. If ever you are drawing from two hots for one load (220) then each leg would have to be wired to half of a dual breaker. Both hots would go out then if you trip it. That could mean that all your lights go out, unless you have at least one other 110 circuit on its own breaker. I think that is a no-no- it's not allowed by code to have all the lights go out when one breaker goes. It's not an electrical hazard, but here you are stumbling around in the dark- maybe even falling onto the saw just when someone else flicks the breaker back on-
                      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                      • #12
                        I have a Milwaukee 6178-20 abrasive cutter that is 120v 15a and alleged to be 5.4 hp. I hate to bust your bubble but I have 20a breakers in my shop and I can easily trip the breaker if I bear down on the handle.

                        If it can put out 5.4 hp power it will take more than the 15a it's rated at and I don't know how long it would last if you worked it that hard a lot. If you use it with some respect you can get by on a 20a circuit but I don't think a 15a circuit will work well.

                        If you run a new circuit for it run #10 with a 30a breaker.
                        It's only ink and paper

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                        • #13
                          In case it's not obvious, if you run a 30 amp breaker, you also need a 30 amp plug, and you make it easier to burn up the motor.

                          If the saw is rated at 15amp, and you blow a 20 amp breaker in use, you are basically overloading the motor and it's good that the breaker trips. I can do the same on a table saw by pushing too hard and/or cutting with a dull blade.

                          As for the 5.4hp - that's just a wacky BS "peak" rating - probably right at the point the rotor stalls and the magic smoke starts.

                          OP - Run the saw on the 15amp breaker. If it trips, back off the feed pressure a bit. The breaker is there to protect the wiring; maybe it will protect your motor also.
                          Last edited by lakeside53; 04-27-2012, 02:36 AM.

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                          • #14
                            The BS 5hp rating is taken from the amount of instantaneous torque the motor can produce when it is suddenly stalled hard to a stop. It is complete bull since it is measuring the stored angular momentum in the rotating mass, not the power the motor can produce.

                            The real horsepower before losses is determined by the amp draw from the nominal line voltage. That is 117 vac for a split phase circuit. Amps times volts divided by 745.5 watts per hp. Then subtract anywhere from 10% to 40% depending on the quality of the motor. The most that can be drawn continuously from a 15 amp circuit is 1462 watts by code. Less losses and that gets down to maybe 1170 with a decent motor with 80% efficiency. That works out to around 1.5 hp in the real world as long as you have good line voltage and small voltage drop.
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                            • #15
                              Originally posted by Evan
                              The BS 5hp rating is taken from the amount of instantaneous torque the motor can produce when it is suddenly stalled hard to a stop. It is complete bull since it is measuring the stored angular momentum in the rotating mass, not the power the motor can produce.

                              The real horsepower before losses is determined by the amp draw from the nominal line voltage. That is 117 vac for a split phase circuit. Amps times volts divided by 745.5 watts per hp. Then subtract anywhere from 10% to 40% depending on the quality of the motor. The most that can be drawn continuously from a 15 amp circuit is 1462 watts by code. Less losses and that gets down to maybe 1170 with a decent motor with 80% efficiency. That works out to around 1.5 hp in the real world as long as you have good line voltage and small voltage drop.
                              Evan - that's the REAL story. I don't know how these mfg's can get away with listing stuff like that. My dad would always laugh at the 2-5 HP motors of today. His old 5 HP motors you almost needed a fork lift to pick up.

                              Walt

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