Tablesaw 220 volt plug Question - Which configuration???

[Rob Russell]

OK - let's clear up some misinformation that's starting to pop up here.

Richard and Frank, the correct wiring for a straight 240v, single phase machine (in the US, at last - Europe is a different story) would be 2 hots + grounding. There would be no neutral.

The reason that you don't need a neutral is not because the neutral and grounding conductors are eventually bonded at the main panel. The reason is that a 240v, single phase motor only needs the 2 hots to run. No current flows back through the grounding conductor (unless the motor has an internal short, but that's a different story). You could hook up a 240v, single phase motor to just the 2 hot legs and it'd run fine. The grounding conductor is for safety. If you're running a 3-wire circuit for a 240v motor, it would be 2 hots + grounding (green or bare wire), not 2 hots (black + red) + neutral (white).

TJ III - the new NEC requirements for 4-wire, 240v circuits applies to new dryer and range circuits, not motor circuits.

Mike is correct about there being NEC rules about motors. In general though in our residential workshops, and especially for starting something like a tablesaw which is a lightly loaded start, treating the circuit as a normal branch circuit is fine. For a 3 HP saw, a 20 amp circuit should be fine. I would use twistlocks because they guarantee a more secure connection - the L6-60 series is what you'd want.

The one option here is if you want to run a router table in your saw's extension table, you'll need a neutral. One way to do that is just run a plain old extension cord for the router when you need it. Another is to run a 4-wire circuit for the saw (hot, hot , neutral, grounding), run a 4-wire cord from the wall plug to a junction box that you mount on your saw. On that box you'd have a 240v, 3-wire outlet to plug your saw into and a couple of 120v receptacles for your router, drill, etc. Obviously - the simplest route is a 3-wire receptacle for the saw and run a separate extension cord for the router if/when you need it.

Rob
(experience amateur/homeowner electrician)

[Tim Sproul]

You want a NEMA L6-20R for the receptacle if you want 20A, 220V service to the saw. It is a 3 prong, twist-lock receptacle. You'll obviously also need to go get a matching plug. There is also L6-30 commonly availabe which is rated for 30A. I can get these at HD or Lowe's around me.

Actually....I have a couple L6-20R receptacles. Want 'em? PM me. I use L6-30 in my shop....and can't remember why I have a couple L6-20R around.

[Glen Blanchard]

Thanks to all the advice here guys. I appreciate the help.

Tim - Thanks for the offer, but the electrician is coming out tomorrow afternoon. Thanks anyway.

I have got sufficient information to carry on an intelligent conversatin with the electrician now!!!

[Mike Wilkins]

You may have to wait until the machine arrives to see what, if any, type of
plug it has. Most machinery comes without a plug, as the manufacturer does
not know what type of outlet you have. You may have to get the electrician
to schedule a return trip.
I used industrial-type twist lock plugs from the big box. They are pricier than
the standard type of plug, but I have a tendency to go overboard. And you
have to have use outlets for these plugs to fit.
Good luck and watch those fingers.

[Jim Becker]

You may have to wait until the machine arrives to see what, if any, type of plug it has. .

Or...pick a style and change the plug on the saw if it happens to come with one that is different than the outlet! Call back costs for the electrician may very well be much more expensive than buying a new plug for the saw.

[Richard Wolf]

OK - let's clear up some misinformation that's starting to pop up here.

If you're running a 3-wire circuit for a 240v motor, it would be 2 hots + grounding (green or bare wire), not 2 hots (black + red) + neutral (white).

Rob
(experience amateur/homeowner electrician)

I don't know Rob, I have (7) 240v machines in my shop. Not one of them has a green or bare wire in them. They all have 2 blacks and a white or black, red and white.

Richard

[Rob Russell]

If those machines have no 120v loads, then each of those is miswired in 1 of 2 ways.

First, they could have been wired where that white conductor is connected to the grounding bus. In that case, the conductors are misidentified - the grounding conductor is supposed to be green or bare by code.

They could also have been wired with a neutral instead of grounding conductor. If your machines are wired to a subpanel, you have a potential alternate path for current from another load to return though the neutral bus, to your machines and through you. Not a large chance, but it exists. If the machines are wired to the main service panel, then the neutral and grounding busses are bonded and electrically there is no difference there. Either, way - they should have been wired as a grounding conductor, not a neutral.

The machines would run just fine either way because the 240v motors don't need either the neutral or grounding conductor to operate.

[Phil Ordway]

Frank, if you have 3 blades on your 220 plug, it is not grounded. 220 requires 2 hot leads and 1 common lead. If the motor also has a green fourth wire that is a ground, but it is not the norm on this type of equipment.

Richard

220 Requires two hot leads and a ground, NOT a neutral.

[Bill Lewis]

If your machines are wired to a subpanel, you have a potential alternate path for current from another load to return though the neutral bus, to your machines and through you.

Just to clarify this a bit further, the reason for this is that the neutral and ground bars are kept separate in a subpanel.

[Richard Wolf]

220 Requires two hot leads and a ground, NOT a neutral.

So Phil, why do some 220, single phase applications have 4 wires. Do they have 2 grounds?

Richard

[Jim Becker]

So Phil, why do some 220, single phase applications have 4 wires. Do they have 2 grounds?

Because some appliances (dryers, ranges) require both 240v and 120v to operate. The timer/clock/lights typically are 120v and the "real business" of these appliances requires 240v. In order to support both voltages on the same line/breaker, you also need to have the neutral for the 120v. It's NOT the same as a ground. Confusing, of course, because they are bonded in the main panel. Nevertheless, they are different and code requires them to be separate from a wiring standpoint. The ground (green or bare) wire serves both the 240v and 120v "sides". So you have four wires, two hots (one of which is also used for the 120v "side"), one neutral and one ground.

But our 240v tools generally do not require dual voltage. Therefore, they only require the two hot legs and a ground. The two hot legs effectively balance each other as they are 180º out of phase with each other as the current alternates. The neutral performs that balancing in a 120v circuit. (Rob, please correct me if I'm inaccurate in any way on this)

[Rob Russell]

Because some appliances (dryers, ranges) require both 240v and 120v to operate. The timer/clock/lights typically are 120v and the "real business" of these appliances requires 240v. In order to support both voltages on the same line/breaker, you also need to have the neutral for the 120v. It's NOT the same as a ground. Confusing, of course, because they are bonded in the main panel. Nevertheless, they are different and code requires them to be separate from a wiring standpoint. The ground (green or bare) wire serves both the 240v and 120v "sides". So you have four wires, two hots (one of which is also used for the 120v "side"), one neutral and one ground.

But our 240v tools generally do not require dual voltage. Therefore, they only require the two hot legs and a ground. The two hot legs effectively balance each other as they are 180º out of phase with each other as the current alternates. The neutral performs that balancing in a 120v circuit. (Rob, please correct me if I'm inaccurate in any way on this)

Jim, that's pretty much correct.

The reason no neutral is needed for 240v machines is exactly because the 2 legs are 180 degrees out of phase with each other. If you put a 120v/60Hz AC line on an ocilloscope, you'd see a nice sine wave with 60 cycles per second. If you did the same thing with the 2 hot legs of a 240v circuit, you see 2 sine waves - each the perfect mirror of the other. The current that would normally flow back through a neutral conductor is exactly cancelled by the subtractive effect of the waveforms.

On a 120v circuit, there is no cancelling waveform. The neutral doesn't "cancel" the hot leg, it creates the connection to ground so that there is a voltage difference (called "potential") so that current can flow. That's why the neutral is a current-carrying conductor. On a 240v circuit - that potential exists between the 2 hot legs.

[Steve Cox]

Rob, I'm not an electrician but I spent a few years as an Electronics Technician and I have to question some of the info in your last post. All our residential 220V is single phase. What you are describing is two phase power. Moreover, if the two phases cancel each other out, I think our motors would just sit there and hum. I'm not sure I'm correct but I'd like a little more info on the phase cancellation.

[Rob Russell]

Steve,

What I described is the typical center-tapped, 120/240 single phase fed to most residences in the US. It's not "2-phase". There is apparently a true 2-phase, I believe that some areas around Philadelphia have it. How it's different from the single phase we're all used to - beats me. I've only worked with 120/240 single-phase and 120/240 Delta 3-phase.

On a 240v motor, the phase cancellation between the legs is such that the current drawn on 1 leg at any instant in time is the reverse of the current on the other leg. That's why the NEC allows multi-wire circuits, where multiple hot legs share a neutral. The current returning via the neutral is the difference between the current drawn on the individual legs. 15 amps in leg A and 10 amps in leg B = 5 amps back on the neutral. A 240v motor is just an example of where the currents between the 2 legs will be perfectly balanced and therefore results in no need for a neutral.

Rob

[Steve Aiken]

I suppose this is fodder for a different forum, but since we're on the topic....

I've never really understood 240V single phase, especially in the multi-wire circuit. The current in the 2 legs don't really 'cancel each other out' does it? The current from each leg still has to get back to the panel then to the transformer, right? In a multi-wire circuit with different current in each of the 2 legs, the neutral carries the difference because not all the current can make it's way back through the hot legs.

Am I out-to-lunch on this?

Steve