Wiring and plug for a 3 hp cabinet saw

[Doug Miller]

Curious: what's the nameplate amperage rating on the saw? How far is the receptacle from the breaker box? And is the circuit wired with 12ga wire as it should be?

[Swingman]

... and, in the past the plug that comes standard on the 3HP Delta Unisaws is a NEMA 6-15P. It will work with a 6-15R, 15A receptacle, as well as a 20 amp, 6-20R receptacle.

[Leon]

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.

[David Starr]

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

[dpb]

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).

--

[Doug Miller]

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.

[Chris Friesen]

It doesn't "need" to be sized that big.

For dedicated motor loads it's *permitted* to use an oversized breaker should it be required. I have a 3HP motor on a 20A circuit, and I've yet to trip the breaker.

Chris

[Leon]

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.

[Doug Miller]

No, I didn't.

Please trim and quote more carefully. Those are dpb's words, not mine.

[David Todtman]

Thanks everyone. I plan to use 12ga wire, 20amp 2-pole breaker, and matching plug/receptacle.

Best, David

[LRod]

Not just the Uni--I have that same plug on my DJ20 and on my DC380--all 3HP motors.

Unless Doug corrects me, I believe the use of a 6-15 receptacle on a

20A 240V circuit is permitted by the same exception as the 2-15 on a 20A 120V circuit is. Those -15 receptacles aren't "rated" for 15A, they're "keyed" for 15A. In any event, I think all of that goes out the window if it's a single receptacle and the only one on the circuit (note a duplex receptacle is not a single receptacle).

[Doug Miller]

12-2, right? Not 12-3?

Excuse me if I'm telling you things you already know, but there is occasional confusion on this part. For premises wiring such as NM cable (Romex), the number after the dash indicates the number of current-carrying conductors, and does *not* include the ground. Thus, 12-2 Romex has three wires in it, and

12-3 has four.

xx-3 cable is used for electric dryers, cooktops, ovens, etc. because those appliances also have 120V circuits (timers, motors, etc.) which need a neutral.

Pure 240V loads, such as a table saw or electric baseboard heater, use only the two hot conductors and thus need only xx-2 cable. With ground, of course.

[dpb]

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...)

[Doug Miller]

Exactly right, LRod.

[David Todtman]

Doug, Your previous comment is right on target--I was subject to the confusion you referenced. You saved me the extra expense of the 3 conductor wire. I'm getting 12/2. Thanks.

[LRod]

In addition to that labeling confusing a lot of people, back in about

1960 when I worked in a hardware store, if you wanted a ground with your NMC you had to state "with ground." If you asked for 12-2, you got a two conductor wire, no ground. If you wanted a ground wire, you had to ask for 12-2 w/ground. That's no longer the case--now you get a ground with your NMC whether you want it or not (and why wouldn't you?).

In those days, two wires went to your electric water heater or your pool pump--both hot. I know--I grew up in a house in FL which we built in 1956. We had a pool installed in 1960 (which, watching the electrician, who was the dad of a classmate, was the beginning of my education on electricity). There was no ground required.

Obviously, that, too, is no longer the case.

[Phil-In-Mich.]

This question is OT to the original post:

Based on the OP's motor's description, what information on the tag for the Baldor motor is missing that would make one suspect the motor is really a 3 ph, 3hp motor?

For example, I find a 3hp motor at a flea market, and the seller claimed it was from a table saw, what clue would I look for to tell me it is 3 phase? (and thus should put it down, since I ain't going to get 3 phase in my house, no way, no how.)

Wouldn't a 3 phase also be listed as input voltage 203 to 230 VAC phase-to-phase?

Phil

[Leon]

LOL

[Leon]

Something else to consider when buying from a flea market, you need to be sure of the spin direction. Left and right tilt saw motors spin in opposite directions.

[Edwin Pawlowski]

+

The nameplate will say 3 PH

Voltage can vary from 208 to 480. Typical single phase won't go over 230.