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  • Extension Cord For Power Tools-Questions.

    I have been reading and looking at power tools. I have looked up the proper guage for extension cords. I have a 12 guage cord which is good for up to 16 amps at 100 feet. Looking in some manuals it does not recommend a cord over 50 feet. But everything I look up says 12 guage is good for up to 16 I think and up to 100 feet.

    So why would a tool manual so dont, but all over the net it says it is ok.

    The thing I was looking at is a Makita Circular Saw. It is 15 amps and only recommends using a 12 guage cord up to 50 feet.

    So can someone shed some light on this?

  • #2
    Re: Extension Cord For Power Tools-Questions.

    The amperage leaks out as it moves down the length extension cord. By the time it travels that 100' there isn't much amperage left. ............... OK, OK, I don't have a clue.

    On the other hand, whenever I need an extension cord for any of my tools I always use a 12 gauge cord. I actually have several of my stationary tools hooked up 100% of the time to some 10' long 12 gauge cords. I've never had any problems with any of those tools not operating correctly.
    Teach your kids about taxes..........eat 30 percent of their ice cream.

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    • #3
      Re: Extension Cord For Power Tools-Questions.

      I have a circular saw that I have been using at the end of a 100' 12 gauge cord ever since it was new. Got it in 1971. It hasn't slowed down near as much as i have!

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      • #4
        Re: Extension Cord For Power Tools-Questions.

        Just did a quick check of the instruction table that is provided with my Ryobi portable table saw which is 15 amps. I believe the table is pretty much an industry standard.

        For a 15 amp draw, 12 guage wire is recommended up to 50 feet (which you already know.

        Looking at 100 feet, 12 guage wire is recommended only up to 7 amps and at 12 amps the recommendation is for 10 guage wire. There is nothing recommended about 12 amps for that length.

        Part of the problem of course is that a 15-amp motor only draws 15 amps when it is free wheeling and under no load. Power tools, especiall table saws don't operate that way and you will most likely need in excess of 15 amps on start-up and then more than 15 amps as you load the motor with you cutting.

        Since the recommendation for 12 gauge is only up to 7 amps at that 100' distance, you seriously put a 15 amp motor in jeapardy of starving and over heating. There is a safety factor in what the cord and wire can withstand of course... but like a water hose, a particular diameter can only handle so much pressure/volume. In the case of an electrical wire, there is resistance and the longer you make the cord, the more resistance there will be. The diameter or guage means more capacity per length, but regardless of the so-called rating given my the manufacturer of the cord, it is the manufacturer of the tool and motor that drives it, which makes the best recommendation for design efficiency.

        Simply put, a 15-amp motor tied to a 100-foot of 12 guage cord would be starving for power, thus bogging down, overheating, and a good chance of burning out the motor. That is why they provide these little referance tables.

        I hope this helps,

        CWS

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        • #5
          Re: Extension Cord For Power Tools-Questions.

          This article seems to cover things fairly well http://findarticles.com/p/articles/m...2/ai_n7586142/

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          • #6
            Re: Extension Cord For Power Tools-Questions.

            Forget the cord maker's ratings.

            The issue is that a cord - ANY cord - is essentially a resistor in the line. You only have 120 volts available out of the wall. "Some" of that is used pushing the current past the line cord resistor, and what is left is used up to push the current through the motor windings. A thin wire has more resistance than a fat one, pretty much inversely proportional to cross sectional area. And a long wire has more resistance than a short one, pretty much directly proportional to length.

            Current times voltage equals power. An AC induction motor wants to maintain its speed... which by design is related to the line frequency. Without getting into a full-blown discussion (search on AC induction motor for this) the bottom line is that if too much voltage is lost in the CORD, then there is less voltage available for the motor. The motor tries to maintain speed under load and thus will draw more CURRENT to make up for the lack of voltage... it needs the power it takes to maintain speed under the load it's working against.

            So the problem with long extension cords is that they are too big of a resistor and result in too much lost voltage (often called "voltage drop"). This causes the motor to pull too much CURRENT.

            Now the MOTOR isn't a really a resistor, it is coils, but the coils have resistance too. And we all know that a motor gets warm, right? Well the heat loss is proportional to the SQUARE of the current times the resistance.

            Let's say you have a tool that normally pulls 8 amps. But you put it on a too-long, too-thin extension cord and the voltage drops. The motor still works fine, the tool is working... but it's now pulling 10 amps. The HEAT in that motor just went up by a factor of (10/8)^2 = 1.56!

            HEAT kills electric motors. It melts the insulation on the windings, destroys the epoxies or urethanes that holds the windings in place, and can even boil the grease out of the bearings.

            And THAT is why you want to stay within the manufacturer's extension cord recommendation. It is not false conservatism, and even though your tool may work fine on that 100foot long 16 guage cord, you are seriously reducing its lifespan because it is constantly running a lot hotter than it was designed to.

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            • #7
              Re: Extension Cord For Power Tools-Questions.

              Originally posted by CWSmith View Post
              Part of the problem of course is that a 15-amp motor only draws 15 amps when it is free wheeling and under no load. Power tools, especiall table saws don't operate that way and you will most likely need in excess of 15 amps on start-up and then more than 15 amps as you load the motor with you cutting.CWS
              Not true, CWS.

              A 15 amp motor does NOT pull 15A when it is free-wheeling under no load. It pulls 15 amps when it is developing it's rated power. When it is running no load, it is pulling well less than 15A, although operating at a less favorable power factor (phase angle between voltage and current).

              If you overload your tool then an AC induction motor CAN and WILL pull excess current. Some AC motors can actually develop 3-4 times their rated power as a peak.... this is called the "breakdown torque". But if you make a habit of doing this to your tools you will end up buying a lot of replacement motors.

              BTW, this is a big part of the problem with so-called "peak power" ratings.. such as are commonly seen on tools, shop vacuums, etc. The peak power is not a LIE... it can reach those levels.... but it doesn't under normal operation and in the case of a some products like a shop vacuum, it simply CAN'T... the impeller can't possibly load the thing that severely. NEver pay attention to power claims on AC induction motors especially.... you need to look at the motors rated amps.

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              • #8
                Re: Extension Cord For Power Tools-Questions.

                Just to add to the confusion, realize that the plug on the wall where the extension cord is plugged into may already have a voltage drop. If you have 70' of 14 gauge wire running from the power panel to the plug on a 15 amp circuit, you already have some voltage loss. The total resistance and voltage loss has to be figured going all the way back to the breaker panel.

                Best to use an oversized extension cord to minimize any further resistance. I use no smaller than #12 for any power tools.

                Go
                Practicing at practical wood working

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                • #9
                  Re: Extension Cord For Power Tools-Questions.

                  Here you go

                  12 gauge between 50-100 feet up to 15 amps

                  http://www.askthebuilder.com/B174_Ex...ze_Chart.shtml

                  There is also a chart attached for up to 15 amps you can use 12 gauge
                  Attached Files

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                  • #10
                    Re: Extension Cord For Power Tools-Questions.

                    How hard do you run your saw? Under many (if not most) conditions, you would not likely have any problems. If your blade is sharp, you are making intermitent cuts, and you aren't feeding the saw too fast, you aren't likely to see anywhere near 15 amps (except for a a few ms on start up). As discussed earlier, what you want to avoid is starving your motor of voltage, which will cause it to over heat.

                    Use the 50 ft 12 gauge whenever you can. If you need to go out to 100 ft often or will be pushing your tools hard, get a 10 gauge. For an occasional need of a few basic cuts, a 100 ft 12 should work fine. If the blade binds on you, turn it off immediately. Let the motor cool before trying again, as a stalled motor pulls a lot of current.

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