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Thread: Magnetic Motor Switch to protect Table Saw from surge

  1. #1

    Magnetic Motor Switch to protect Table Saw from surge

    I only have a 230v/30amp outlet in the garage for my table saw which is rate for 230v/8.4 amps. I can't change the outlet but it seems that a magnetic motor switch that is rated to 230v and 12amps would protect myself and the saw from a power surge. Is there anything wrong with this line of reasoning? I will have a professional install the switch.

  2. #2
    Join Date
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    Yes, there is something wrong with your reasoning. The only thing a magnetic contactor will protect you from is a restart when the power comes back on after an outage. It won't even provide brownout protection because the contactor will continue to hold long after the voltage has dropped to a level that is harmful to the motor.

    That being said, there are motor starters with sophisticated circuitry that can cut out on over/under voltage situations, but they get quite expensive.

    Let's be clear about what you mean when you say power surge. Are you talking voltage spikes or an extended over voltage situation? For voltage spikes from a lightning strike, or from noisy electronic devices connected to the system, you can surge protectors that mount to the panel or to the machine that will take care of that.

  3. #3
    Join Date
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    If you are concerned about voltage surges, such as lightning strikes to the power system, this switch is not going to provide protection. The advantage of a magnetic motor switch is if the power goes out, the switch is shut off, so when the power returns the motor won't restart. This is a good safety idea, but does not help nor protect motors from high voltage spikes. This can be, and should be mitigated using a whole house surge suppressor, like the ones you put on your computer and electronic equipment. These are pretty cheap insurance to protect in case of lightning and other large power surges. Mine cost about $230 installed.

  4. #4
    Joseph, if I'm understanding correctly, I think you're concerned about the mismatch between the table saw's current draw (8.4A) and the receptacle's 30A rating? If so, that's not a concern. Presumably, you regularly plug household devices (lamps, vacuums, toasters) which draw only a few amps into receptacles rated for 15 or 20A.

  5. #5
    Ditto Frank & Eric.

    A surge suppressor will give you protection from power surges feeding from the grid to the tool. But I'm guessing this is not quite what you're worried about...??

    If you are worried about the saw being overloaded and pulling too much amperage, I would not worry. The saw is still protected by the 30A. breaker (CB) and its not terribly uncommon to see CBs rated for 3-4X the load they are supplying. (Assuming of course that the CB is fed by proper wire size.) I doubt you'll have the saw at some middle-ground overload condition for very long - say 150%. It is too much for the motor, but no sweat for the CB. Either you'll finish the cut, or the motor will lock and trip the CB. (We all routinely plug our 0.3A phone chargers into receptacles with 15A CBs.)

    If for some reason you need to run the saw at very near 100% load for extended periods, then you may very well need a new starter - - just not for the starter. Rather you'd need to make sure either the new (or old) starter is compatible with a suitably sized overload relay. This relay trips based on its specific current handling settings, matched to the motor AND the wire size between receptacle and motor.

    (What Dan said too - - he just types faster than I do).
    Last edited by Malcolm McLeod; 02-25-2019 at 4:20 PM. Reason: Dan's faster - aaaAAARRRGGhhhhh

  6. #6
    Join Date
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    In my experience with electronics, nothing protects against lightning except unplugging everything.

  7. #7
    Join Date
    Apr 2017
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    Michigan
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    Instead of having to crawl to the plug on my TS I installed a DPDT switch. The protects from lightning coming to attack my VFD thru the hot and neutral legs. Fool me once...

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