Wiring and plug for a 3 hp cabinet saw

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I have to wire for a 3hp cabinet saw. The motor is a Baldor and the tag on the motor housing says it is 12.5 amps at 230V. (It also says it will operate on a minimum of 203v at 13.7amps. It has an over-heat shutoff.)
Does this mean I can use a 20amp double pole breaker? What difference would it make if I used a 30amp? I think I'll use #10/3 wire (romex-type).
I need to wire a plug-lead from the saw's mag switch. I reckon a 20amp/230v male plug with recepticle to match would be correct. Again: what difference would it make if I used a 30amp setup?
Oh yeah, something else: I have two (unused) 15amp breakers. If I gang them with a tie bar, does this make the setup function as a double pole 30amp breaker?
I will appreciate comments and recommendations.
Best, David
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No problems with anything until you came to the two 15 amp breakers, you need a double 30 amp breaker for this setup. Myself, I would go with the 20 amp breaker 12 gauge wire, and an appropriate 20 amp plug, socket, and cord, but if you want to up it all to 30 amp breaker, 10 gauge wire and 30 amp socket, plug and cord I would not have a problem with that either. Assuming the length of run of the circuit is not excessive. The breaker is there to protect the wire, not necessarily what is connected to the circuit. Greg
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Everyone says that, but why shouldn't it also protect what is connected?
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Because, it is designed to protect the wiring. If you want to protect what is connected you should buy the tool with it's won built in protection.
If the breaker was designed to protect the tool, you would only be able to plug "1" tool into a circuit at a time. Many tools only pull 7 amps. Are you going to put in 7 amp breakers and what if you want to plug in 2, 7 amp tools into the same circuit? What if you want to run a dust collector and a TS on the same circuit? You can't have a breaker that will protect both and not trip when both are on.
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I know what it designed to protect, but why the heck not put a 15a breaker on the circuit instead of a 20 or 30? Doesn't cost anything so you pick up protection for free. I am not saying it required, or even necessary, but that doesn't mean it isn't preferable.
If he wanted to run a DC and a TS on the same circuit he would wouldn't have asked about wiring a 12a TS. And if he did want to run both, the 20a circuit wouldn't be adequate either!
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In the case of power tools, the rated current is usually less than the startup current, so you don't want the circuit to be *too* close to the rated current, or you risk occasionally tripping the breaker when you start the tool.
Hence, read the manual and do what they recommend.
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wrote:

Because that would limit him to using 14ga wire. It will take forever for the saw to spin up. (You're always talking about the importance of avoiding voltage drop -- think it through.)

No, but the slow startup speed for the saw *does* mean that.

Nonsense. 20A would be perfectly adequate for his TS and any DC up to about 2hp.
--
Regards,
Doug Miller (alphageek at milmac dot com)
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Doug Miller wrote:

No. A _smaller_ breaker only limits the circuit load to less than what it could otherwise be used for.
I see no sane reason to limit the breaker size to less than what the circuit wiring is rated for, either. One never knows what may come down the road as a desired addition, so having to go back and replace the breaker to take advantage of the circuit's full capacity is simply an unnecessary pita and cost (although not huge, another breaker for no reason other than undersizing it originally just makes _no_ sense).
--
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Of course you're right -- I hadn't had my coffee yet when I posted that. :-(
A 15A breaker would subject him to nuisance trips, though.
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Doug Miller (alphageek at milmac dot com)
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What is the point??? The circuit breaker is not a good protection for an appliance, it is not finely tuned enough for that purpose and it has no idea if the tool is getting too hot or not since the wiring near it is not getting too hot. A tool can get damaging hot and not draw too many amps to trip the breaker.
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No, I didn't.

Please trim and quote more carefully. Those are dpb's words, not mine.
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Doug Miller (alphageek at milmac dot com)
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Doug Miller wrote:

On top of which, I have no idea what was the point trying to be made... :)
(And, btw, I knew you knew the previous point, only figured it should be clarified/corrected for the record...)
--


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LOL
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Ok, you asked why shouldn't it also protect what is connected. I explained why it shouldn't, not that it could not be done. It shouldn't be done because the breaker is used as a last resort to protect you home wiring. It shouldn't be done because of the reasons I mentioned. It can be done, but it shouldn't.
Additionally, The protection needs to be located at or very near the motor. The farther away it is the slower it will react to an overload especially if the tool that is overheating is doing so slowly. Breakers tend to not work very quickly. I regularly run a 1100CFM dust collector, a 15 amp router, fan, and radio on a 15 amp circuit. No problems as long as the DC and Router are started and brought up to speed 1 at a time. Breakers are designed to take overloads and not trip immediately and are usually rated at less than what the wiring in the house will handle. That may not be fast enough to save a tool. Thermal protection built into a tool tends to work at the threshold.
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Consider a 20 amp circuit. Beyond 20 amps, the wire may melt and cause fire. Below 20 amps, all is safe.
Consider what you plug into a 20 amp circuit. It could have *any* current rating, from a tiny lamp cord to a power tool. The breaker can't protect what's plugged in unless everything you plug in is capable of carrying 20 amps safely.
So, devices plugged into a circuit which desire overcurrent protection must provide a *suitable* protection themselves.
Note that in the UK every device has a suitable fuse in the plug, which protects the device.
To recap, a circuit breaker...
* CAN protect the hardwired circuit
* MAY protect some plugged-in devices
* CANNOT protect ALL plugged-in devices
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"Toller" wrote:

By definition, a circuit protective device, protect the insulation on the conductors of the distribution system.
Overload devices provide the protection for the device.
About the only thing they have in common is protection, but they of necessity, approach it from different directions.
Lew
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wrote:

Because that's not what it's there for. Do you want a 1/2 amp breaker to protect the circuit that you have a 60W table lamp plugged into?
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Doug Miller (alphageek at milmac dot com)
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Toller wrote:

Four reasons: 1. The circuit breaker has no way of knowing what is plugged into it, might be a 5 watt night light, might be toaster oven, might be an arcwelder. Generally there are multiple sockets tapped off each branch, with who knows what plugged into them. You don't want the circuit breaker to pop until the current is enough to overheat the wires. 2. Electric motors draw enormous amounts of current getting started. To avoid popping circuit breakers, you want to furnish the motor plenty of current to start with. The starting load only lasts a few seconds while the motor is coming up to speed. Your 12 amp motor might draw 30-40 amps at startup, and then drop down to a couple of amps after to getting going. If you put it on a 12 amp breaker, it will pop that breaker on every startup. 3. The house circuit breaker is supposed to prevent house fires. Without protection, a short circuit will heat the wire, running thru the wooden studs of your wall, up to red hot. Then the studs catch fire and your house burns down. Circuit breaker size is set by the branch wire size. Rule of thumb: 14 gauge wire = 15 amp breaker, 12 gauge wire = 20 amp breaker, 10 gauge wire = 30 amp breaker. Doesn't matter what you plug into the branch circuit, choose a breaker that's right for the wire. 4. Stationary saw motors always have a thermal protection switch built into them that shuts the motor down if it gets too hot. They don't need any more protection. Speaking of which, should the thermal overload pop, you want to be aware that it might spontaniously reset, starting the saw up again. You want to keep your hands clear of the blade until you unplug the tool.
Was it me, I'd run your 220 volt saw on a 12 gauge branch circuit protected with a pair of 20 amp breakers, one in each hot lead. Use two conductor plus a ground wire cable. I'd make sure the iron frame of the saw was connected to the green ground wire. Back at the terminal box make sure the green ground wire goes to ground.
David Starr
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Except that it's a Code violation to put the 20A receptacle he intends to use on a 30A circuit...
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Doug Miller (alphageek at milmac dot com)
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wrote:

That is why I said, "but if you want to up it all to 30 amp breaker, 10 gauge wire and 30 amp socket, plug and cord I would not have a problem with that either." My intent was everything was to be 30 amp. I suppose it could be translated otherwise. Greg
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