220 power question

[john whittaker]

I plan on making an extension chord ~ 10’ long to power my saw off the dryer outlet. My 3HP 220 table saw motor requires 2 hots + ground. I have an older style dryer outlet that has 3-prongs. I’ll assume this is 2 hots + a neutral.
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I’ve searched previous posts and found that it is OK to use this outlet to power my 220 tools…and also have read that you should not use the 3-pronged dryer outlet to power tools. Since the dryer is wired directly to my main panel and it is connected to earth ground, I can’t see why this will not work or would be unsafe.
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If I use this 3-pronged outlet to power the saw, do I connect the neutral to ground? Or is it unsafe and should I stay away from this idea.<O:p</O:p
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Note that this is a very temporary power supply as the shop will be moving soon. No sense in putting in new sub panel (main panel full) at this stage. So if I use the dryer outlet will it be safe and where do I connect the neutral?

Any thoughts???

[Russ Massery]

I if the dyers 220 they are 2 hots and a ground. that's what 220 is......

[Rob Will]

No neutral is required for any true 220v device.

Some exceptions to watch out for would be accessories like a 110v machine lamp or a 110v coil in a magnetic starter. Anything on 110v will utilize a load carrying neutral back to ground.

Many new electric ranges have a 4 wire setup: (220v across the hot wires plus separate neutral and ground conductors).

Your cord needs to be 10ga or larger.

Rob

[john whittaker]

Let me clarify what I plan to do. Would appreciate some feedback so I don’t blow up the house or worse yet….burn out the table saw.

The power cord that came on my table saw is about 10’ long.

Basically what I plan to do is make the cord coming off the table saw about 20’ long by adding about 10’ of 10 gage 600V max rated wire and putting a 3-pronged dryer plug (10-30P) on the end so I can plug it into the dryer outlet. After the move I can cut it down to size.

The red and black wires (hots) coming from the motor will be connected to the hots on the dryer outlet. The green wire on the motor will be wired to the “L’ lug on the dryer plug.

Does this make sense? Will it work?
Thanks for the help.

[Rob Will]

John,
You simply need to make a separate extension cord with the correct male/female ends to extend what you have. No need to go into the motor connections. On my farm, we run 15hp 3phase augers and a 50amp 240v welder this way. Just get your wires correct when you make up the plugs.

You might want to jot down the NEMA number or make a pencil tracing of what plugs you have before you go to the electrical supply store. They will most likely have several male / female plug styles that are very similar to what you have.

Rob

[Barry O'Mahony]

Technically, this would be a code violation. Dryer outlets are not "22oV" outlet at all, since they require both 120V and 240V to operate. The older 3-wire outlets, now a code violation to install, are 3-pole, 3-wire, ungrounded 120/240V outlets.

Your saw needs a 2-pole, 3-wire grounded 240V-only outlet, That's two hots and a ground, as opposed to what you have on the old-style dryer outlet which is 2 hots and a neutral.

However, you've noticed that the neutral goes back to the main panel, and gets hooked up to the same place where the ground wire would go. So why would it be unsafe to use the existing plug? Well, there a some circumstances, especially in older homes, where than might not be the case. Maybe the circuit isn't a dedicated one, and there is another appliance somewhere, using the neutral as a neutral. Or, the circuit orignates in a subpanel, where the grounds and neutrals are not tied together. Note that is is very common in some parts of the country, e.g., California, to wire up the "main panel" as a subpanel, whereby the Service Disconnect is outside next to the meter base, possibly located next to a few big-amp breakers such as a hot tub. In all cases like this, the neutral lead can ned up not being at ground, and you can get a shock from touching the saw and a grounded surface at the same time.

Now you say this is a temporary situation. You say the circuit terminates at the main panel, and the main panel contains the Service Disconnect. And this will only be a temp thing, and you promise to install a proper plug on the saw when you move it to its new shop, and to install a proper circuit for it. You certainly won't sell the saw at some later time to unsuspecting person with the ungrounded plug on it. If all those things are true, then the only issue would be that, technically, it's a Code violation: the wire is white instead of green, and the wrong plug/receptacle is being used.

[Russ Filtz]

If it's a dedicated line, it doesn't matter if the third wire is neutral OR ground! Neutrals are bonded to the ground bar in the main panel! (I would check this though as sometimes the ground is wired to something handy like a metal water pipe, but still the neutrals would need to be bonded to a ground rod somewhere) If you want to be technically correct, you could flip main breaker and swap the dryer neutral over to the ground bar, but it should basically be OK as is, if dedicated. The grounding wire is nothing special, just an extra neutral basically in case something goes wrong (at least on a 120V circuit!).

Barry, even if the panel is a "sub-panel" wouldn't the neutral just carry back down a neutral feed to the "Main" panel and be bonded to ground there? Neutral has to be bonded to ground somewhere for 120V circuits I would think. If you pull the main breaker and take off the panel shroud to get at the wires, it should be easy to see if the neutral is bonded to ground. The should be a piece of metal running between the two busses. If you want to make a sub-panel, you break off the metal tab connecting the neutral bus to ground.

[john whittaker]

Barry, as I understand it....you are correct that I am using a 3-pole, 3-wire, ungrounded 220 outlet, and I need a 2-pole 3-wire grounded outlet. If I confirm that the dryer circuit is wired directly to the main panel...and the main panel is connected to earth ground, am I correct in thinking the neutral in the existing dryer outlet is also grounded?....And it can be used to power the saw?

I understand that the new code requires separating ground & neutral but I promise and swear on my routers life that this is very temporary and will not sell the saw to anyone with the old 3-pronged plug.;) Heck I just bought the saw and would like to power it up and see how she flies.

[Russ Filtz]

A true neutral HAS to go to ground at some point. Otherwise there's no place for the electrons to go! A 220 works in a push/pull arrangement with two "hot" leads. With the 180 out of phase, each hot acts as a "neutral" for the other hot. With that, there's no need for a neutral to get rid of electrons into the ground.

In a 220 ONLY appliance, the third wire is a GROUND. Some appliances use both 120 and 220, so the third wire would be a neutral, which completes a 120 circuit off one of the 220 legs (actually 240 made out of two 120 circuits). Most new appliance that use both 120/240 would probably have a fourth wire, which would be a pure ground for both the 120 and 240 circuits (if wired this way the ground would need to be heavier gauge than the neutral since it would be possible to see current from both the 120 and 240 circuits at the same time). Not sure what the code says on that, it may allow the same gauge as neutral, but better safe I say for the few cents in wire cost.

Of course I'm not an electrician, but I did wire a full sub-panel in my garage with 120 and 240 circuits. Of course that's easy since I could follow code and wire my 240 plugs with a third wire GROUND! If you did any funky wiring to the house at all, you can always tape the existing wire to show the proper color at both ends (in case you forget later). Sound like you are just messing with the equipment cord though.

[Barry O'Mahony]

Barry, even if the panel is a "sub-panel" wouldn't the neutral just carry back down a neutral feed to the "Main" panel and be bonded to ground there? if problem is, if there is a subpanel, there may be another circuit, at 120V, on that subpanel drawing a siginificant amount of current. if that is the case, the neutral conductor from the subpanel back to the Main, since it isn't a superconductor but just a piece of copper with a finite resistance, will have a non-zero voltage drop across it. In this case, the neutral bar in the subpanel won't be "at ground", but will be at a voltage a little bit above ground.

In this case, the saw's frame is tied to the not-quite-grounded neutral in the subpanel. You touch the frame while you are standing on the concrete floor; maybe your shoes are a little wet. That return current that would normally flow on the neutral from the subpanel to the Main? Some of it flows through you. Maybe you don't feel a shock. maybe it's just a slight tingle. Maybe it's not there all the time, but occurs, say, while you're using the saw, when the termostat on a space heater on the other circuit flips on. The "tingle" startles you while the blade is spinning ... use your imagination.

I know it is a little difficult for laymen to visualize, but the intent of the NEC is to avoid these "sneak paths", as they tend to cuase wierd and nasty things like water pipes they shock you when you touch them, HVAC ducts that spark, etc. The "neutral" and the Equipment Grounding Conductors must be laid out carefully. Not following the Code, and not understanding the electric theory behind it, can really end up biting you.

[Russ Filtz]

I guess I see how the 120 current in that case could make a jump to the easiest path to ground. By having the neutral bonded in the main panel, the easiest path to ground would be through the ground bus to earth, and not backfed through the ground wire (or other neutral in this case) to your machine.

But, if it's truely a dedicated circuit, there wouldn't be any difference with using a neutral or ground wire, other than the infinitesimal extra resistance going from the neutral bus to the ground bus.