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I am refurbishing an old Craftsman 10" table saw that has been collecting dust in the back of my shop for years.

I don't remember exactly how I acquired it, but to my recollection the one time I tried to start it sparks flew from the on/off switch. The switch controls an integrated 110vt receptacle which the motor plugs into. All is factory original.

I tested the motor by plugging the motor power cord directly into a wall receptacle and it runs fine. I would like to resume using the factory On/Off switch (because it is a safety switch- pull On, push Off) but wanted to check it first.

Opening the switch box I discover that the switch is double-pole, i.e. opens and closes both the hot and neutral wires that lead to the motor receptacle.

  • why did the manufacturer install a switch that opens/closes both the neutral and hot?
  • can I safely use the switch to only open/close the hot (the neutral side is dead/open)? I can wire the neutral to bypass the switch...
Machavity
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Jimmy Fix-it
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7 Answers7

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The technique is called double-switching. According to this article, on AC equipment the technique is used to avoid dangerous conditions when hot and neutral are reversed, as often happens with outlets that have been improperly wired. It is permitted by this exception in NEC 404.2(B):

404.2(B) Grounded Conductors. Switches or circuit breakers shall not disconnect the grounded conductor of a circuit. Exception: A switch or circuit breaker shall be permitted to disconnect a grounded circuit conductor where all circuit conductors are disconnected simultaneously, or where the device is arranged so that the grounded conductor cannot be disconnected until all the ungrounded conductors of the circuit have been disconnected.

jbeldock
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Probable reason: they made ONE saw assembly and used a motor that could be configured as 110 or 220. For 220 you would break both lines, for 110 you don't need to, but there is nothing saying you can't (so long as your switch breaks both lines together), so it's just easier to have everything the same.

JRaef
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I don't think anyone can answer why that it a double pole switch or how the saw was originally wired.

You can remove the neutral wires from the switch and connect them together. Make sure you carry the neutral integrity to the power source so you don't end up switching hot. Also, did you determine why you had sparks flying when you tried turning it on before? Check that on-off switch thoroughly.

JACK
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My understanding is that should a short happen down stream of the motor on either side, the safety switch cuts both sides because you are dealing with 110v AC here.

If there was a short to ground anywhere, the motor could still spin (at half speed) due the fact that AC alternates directions with every cycle.

Breaking both poles at the same time means that no electricity can flow in either direction.

Stephen Bailey
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Does the cord have a grounded plug (3 prong?) or 2 prong? If 2 prong is it polarized (one prong slightly wider than the other, which is the neutral prong). If the saw is old enough not to be grounded, nor have a polarized plug, it's really important to have a double disconnect if the saw in plugged in backwards, very easy and "normal" to do if not grounded or polarized. So I agree with the post above, that if the saw is plugged in backwards, with a single pole switch, you'd be cutting off the neutral, rather than the hot, which is NEVER a good idea.

Note that the Neutral is at ground potential, meaning there should be no voltage between the neutral and ground under normal operating conditions.

If I were rebuilding the saw, given it's age, I'd replace the cord with a 3 wire cord with a grounded plug (you can buy those with the plug pre-wired at Home Depot or Lowes, replace the switch with another double pole switch and attached the ground (green) wire to someplace metal on the frame for proper grounding. Be sure to pay attention to the amperage draw of the motor and that your wire and switch are of the proper size. You should be able to find that on the nameplate on the motor. Not knowing that right now, I'd suggest at least 14ga wire and 15-20 amp double pole switch. It wouldn't hurt to go to 12ga wire which is rated at 20 amps (might be hard to find pre-wired, might have to buy your own plug). Hope this helps.

George Anderson
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Table saws, can remove a lot of material very quickly. This makes them very effective but it also makes them very dangerous. That is the whole reason your manufacturer used a special switch in the first place.

A double pole switch considerably reduces the chance of the motor turning on unexpectedly due to a fault in either the switch or the wiring.

This is not the place to cheap out, trying to make a faulty original switch work by bypassing one side of it or fitting an inappropriate modern switch. Fit a modern switch designed for the purpose. Preferably one with "no volt release", so the saw doesn't start up unexpectedly after power is lost and restored.

Peter Green
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A table saw motor contains components like capacitors and will switch into generator mode when unpowered. That can leave enough voltage while the saw runs out to cause twitching or flailing, and the vicinity of a rotating table saw blade is a bad location for twitching or flailing. Without a neutral to base that voltage off, it's a lot harder to get that voltage to cause damage as you'd need to touch two rather than one conductor.

There is a reason table saws usually have double interruptors, and it's not a good place to try trading safety against a few dollars.