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Thread: Electrical Question

  1. #1
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    Electrical Question

    Anybody around here an electrician?
    I'm not completely dumb when it come to electrical stuff, but I don't really know a lot either. I thought I would ask my question here before I talked to my local electrician to see if what I want is even possible.
    Here's my situation,
    The power in the shop is controlled by a subpanel that is fed off of a 30a, 240V breaker on the main house panel. All of the circuits in the shop are currently 15A, 120v. The table saw I want to run needs either 20a, 120v or 10a 240V. What I want is to install a 15a, 240v, circuit for the saw and whatever else might need to plugged in there from time to time. I don't know a lot about power requirements so what I am wondering is would the 30a, 240v, coming into the shop be enough to handle the 15a, 240v circuit, and another one of 15a, 120v circuits drawing full current at the same time? I hope that makes sense.
    "If they don't know you personally, don't take it personal."

  2. #2
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    A table saw rarely uses full power .... only when you hear it groan while cutting. So in reality it will only use a fraction of the power available. You will be able to get lots of other things with intermittent load on that circuit.

    About the only woodworking machine that draws a lot of power for a lot of the time is a cyclone. Even when you aren't sawing or planing it is drawing the 22 or so amps for a 5 hp motor, etc. My 5 hp saw only draws 22 amps for very brief periods. My 5 hp bandsaw draws 60+++ amps for the 10 seconds that it takes to get the wheel spinning, then almost none while cutting. (A 30 amp circuit can support 100+ amps for a few seconds without harm.).

    Does that help?

    Simple Electrical engineering....
    Rule 1 if any wire or connector gets warm, it is inadequate (corrected to wiring, since motors naturally get very hot)
    Rule 2 If the lights go dim on that circuit, it is inadequate.

    Other than that, don't worry.
    Last edited by Charlie Plesums; 05-04-2015 at 02:06 PM. Reason: corrected from "anything" getting warm to wiring getting warm
    Charlie Plesums, Austin Texas
    (Retired early to become a custom furnituremaker)
    Lots of my free advice at www.solowoodworker.com

  3. #3
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    Charlie,

    That was a great reply...simple but says it all.

    Enjoy,
    JimB
    First of all you have to be smarter than the machine.
    VOTING MEMBER

  4. #4
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    Although I've always done all of my own electrical work, I was reluctant to add my comments to this thread but I will now.

    There are several things to consider in your situation.

    1. You say there's a 240v circuit to your shop. That means there should be two 120v circuits feeding the outlets you have. A 240v circuit comprises two 120v feeds,

    2. Adding a 240v circuit for a tablesaw will use power from the full feed to your shop. The only other thing that should be on at the same time is lights, unless you have a dust collector.

    3. If you have a dust collector, or are planning to add one, something to consider is the load on each side of the 240v supply. One way to balance that load would be to run the table saw on one side of the feed and the dust collector on the other side. That means both would be wired for 120v. When my shop was a 2-car garage, that's the way I set it up initially.

    4. Make a drawing of the way your shop is wired now and consider the options for keeping the load balanced as much as possible.
    Bill Arnold
    Citizen of Texas residing in Georgia.
    NRA Life Member and Member of Mensa
    My Weather Underground station

  5. #5
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    Quote Originally Posted by Bill Arnold View Post
    ...
    2. Adding a 240v circuit for a tablesaw will use power from the full feed to your shop. The only other thing that should be on at the same time is lights, unless you have a dust collector.

    3. If you have a dust collector, or are planning to add one, something to consider is the load on each side of the 240v supply. One way to balance that load would be to run the table saw on one side of the feed and the dust collector on the other side. That means both would be wired for 120v. When my shop was a 2-car garage, that's the way I set it up initially.
    ...
    Bill, i disagree slightly - not a big deal (you get partial credit) but as a one-time engineering professor I would call it the wrong answer.

    I would run both the saw and the future dust collector on 240 volts. Why? At 240 volts, the current (amps) is half as much as the same motor draws at 120 volts. The loss in the motor, becoming heat (not good) is proportional to the square of the current, thus when run at 120 volts the motors generate roughly 4 times as much internal heat as they do at 240 volts. Lots of people argue (wrongly) that the motor is more powerful run on 240 volts; the answer is that it will last longer and be more efficient (since it generates less heat).

    Balancing the two sides of a 30 amp 240 volt feed is not a big deal, but your argument about not running multiple machines at a time is good. Lighting yes, dust collector yes, and any one machine, but not multiple machines. Balancing the 120 volt load (outlets) on the two sides of the 240 volt circuit just allows you to get full use of the two 30 amp 120 volt feeds - the 240 volt load will inherently be perfectly balanced between the two sides.

    In fact, I would recommend lighting on a separate circuit if at all possible, so that you can shut off the 240 volts and work on anything, or if you pop the 240 volt breaker, you are not plunged into darkness.
    Charlie Plesums, Austin Texas
    (Retired early to become a custom furnituremaker)
    Lots of my free advice at www.solowoodworker.com

  6. #6
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    Charlie, you need to re-think what you said about internal heat of the motor. Because of the way a motor is wired, the internal components use the same POWER regardless of whether it's set up for 120v or 240v. The power used and the efficiency of the motor are what determine the heat buildup.
    Bill Arnold
    Citizen of Texas residing in Georgia.
    NRA Life Member and Member of Mensa
    My Weather Underground station

  7. #7
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    Why am I even bothering to read this thread?
    "Folks is funny critters."

    Think for yourselves and let others enjoy the privilege to do so, too. ~Voltaire

  8. #8
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    Quote Originally Posted by Charlie Plesums View Post
    In fact, I would recommend lighting on a separate circuit if at all possible, so that you can shut off the 240 volts and work on anything, or if you pop the 240 volt breaker, you are not plunged into darkness.
    Do you mean a separate circuit from the house for the lights, or from the sub-panel in the shop? The lights are already on their own circuit from the sub-panel. I was thinking of a single plug on the 240v circuit from the sub-panel, that's it. I would just plug whatever was needed at a time into it.
    Last edited by Michael Tulak; 05-04-2015 at 07:24 PM.
    "If they don't know you personally, don't take it personal."

  9. #9
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    Quote Originally Posted by Bill Arnold View Post
    Charlie, you need to re-think what you said about internal heat of the motor. Because of the way a motor is wired, the internal components use the same POWER regardless of whether it's set up for 120v or 240v. The power used and the efficiency of the motor are what determine the heat buildup.
    Interesting, but I have already decided that I am going to run the table saw on 240v. Now I just have to figure out how to do that.
    "If they don't know you personally, don't take it personal."

  10. #10
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    Quote Originally Posted by Michael Tulak View Post
    Interesting, but I have already decided that I am going to run the table saw on 240v. Now I just have to figure out how to do that.
    The advantage of running any piece of equipment on 240v is voltage drop over distance. If one has a very short length of electrical cable from the distribution box to a table saw, it makes no difference whether it is set up for 120v or 240v. As one increases the distance, the resistance of the electrical wire causes an increasing voltage drop. The voltage drop is determined by the current (amps) being drawn. The lower current in each leg of a 240v circuit causes less of a voltage drop; i.e., the motor runs more efficiently.
    Bill Arnold
    Citizen of Texas residing in Georgia.
    NRA Life Member and Member of Mensa
    My Weather Underground station

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