Using 3-wire 220V dryer outlet for tablesaw

[Tom Henderson2]

My garage/shop is a little light on electrical service -- only one circuit for general garage use. I hope to run some new circuits in the near future but in the meantime I'd like to make the best use of what is there.

The good news is that the washer and dryer are located in the garage too. So there is a 110V circuit for the washer and a 220V outlet for the dryer.

I'd like to run my tablesaw from that 220V outlet. The saw motor is 110/220 convertable so that is not an issue. I realize the motor performance won't be much different, but it will free up the other outlets for other applications.

But I'm wondering about the outlet in the wall... it is a "3-wire non-grounding" outlet, which I THINK is a NEMA 10-30R. I've attached a photo. The house was built in 1984 if that makes any difference.

The "non-grounding" aspect of this bothers me.... is there a safety aspect to using this for a tablesaw application? The dryer runs fine so I presume it will work, but I'm wondering if it is smart to use it for a saw.

Second, aside from the grounding issue are there any reasons that using that 220V outlet would not be adviseable? I seem to recall reading something once that suggested the 3-wire dryer outlets wouldn't work for welders, etc but don't recall the details.

Thanks in advance.

Tom H.
Ventura, CA

[Chris Padilla]

Tom,

That is indeed a NEMA 10-30R: 30 Amp, 3 pole, 3 wire receptacle. It is mainly used for the older-style electric dryers. We've always had a gas-dryer so it only needed a 120 V outlet to run.

I used such an outlet for my TS for a while until I redid my garage and had no issues. What it has, as you outline, is a red (hot), black (hot), and a white (nuetral) but no green or bare wire ground. In other words, it is dual voltage: 240 V to run the dryer heater and 120 V to run the dryer light, motor, and maybe some other things.

Assuming (BIG ASSUMPTION) this is wired to your main house panel (and not through a sub-panel), the neutrals and grounds are bonded together. If it goes through a sub-panel, then the neutrals and grounds are separated and you might have a cause for concern although mine was wired like this and as I told you, I used it for a long time and had no issues but that doesn't mean anything, really. It is just my personal experience.

I assume you've looked at the wiring just to be sure? Maybe there is a ground in there you can use and you'd just need to get a new 4-wire outlet?

[John Morrison60]

Tom

I ran into this condition installing a new oven.
My outlet was 3 wire (house was built in 1988) and new oven called for 4 wire connection.
However, since the oven did not call for a neutral lead (to provide 110v lead), it was OK to
use the existing white wire (neutral) to provide ground.

As long as your saw does not need a 110 feed along with the 220, you should be able to
use the two hot leads as is, and use the neutral/ground provided in the plug to connect to your equipment ground green wire.


I am not an electrician, no guarantees made to the info.
This is what I understood was OK by code when I put the new oven in.
To pull a new 4 conductor, 10 guage wire, is likely both expensive an difficult.

Good Luck.
John

[Tom Veatch]

The picture shows a NEMA 10-30R receptacle. Normally this would be wired so that the L shaped slot is connected to the Neutral (white) conductor and the two slanted slots to the two hot (red/black) conductors. The circuit can then provide both 220 and 110 volts to the dryer.

Is this the ONLY outlet on the circuit? If it is, and the neutral (white) wire in the circuit is connected to the grounding strip in the breaker box as it normally is, then there is no functional difference between this receptacle and the 2 pole grounding receptacle, NEMA 6-30R. The neutral (white) conductor effectively becomes the grounding conductor IF THERE ARE NO OTHER OUTLETS ON THE CIRCUIT.


It's playing fast and loose with the intent of the electrical code, but, if it were me doing it and I:

1. was certain that there were no other outlets on the circuit
2. was certain that the neutral wire from that outlet connected directly to the grounding bus
3. used the existing outlet for the saw by unplugging the dryer before plugging in the saw so that there's no possibility of the dryer running while the saw is plugged in

then I'd use the existing outlet to power the saw. Now, I'm just telling you what I'd do. I'm not advising you to do the same.

I'd add a 10-30P plug to the saw's power cord with the ground wire (green) connected to the L shaped blade on the plug and the two hot (red/black) conductors connected to the two slanted blades on the plug. Under the circumstances specified above, the only difference between the 10-30 and a 6-30 wiring configuration from the receptacle to the breaker box would be the color on the insulation of the grounding conductor.

[Chris Padilla]

Is this the ONLY outlet on the circuit? If it is, and the neutral (white) wire in the circuit is connected to the grounding strip in the breaker box as it normally is, then there is no functional difference between this receptacle and the 2 pole grounding receptacle, NEMA 6-30R. The neutral (white) conductor effectively becomes the grounding conductor IF THERE ARE NO OTHER OUTLETS ON THE CIRCUIT.

Tom,

As I understand it, in a sub-panel, the whites (nuetrals) and green/bare (ground) are separated. They only get bonded together at the main panel. That is why I inquired about the dryer being wired from a sub-panel or the main panel. Also, dryers are usually wired on a dedicated circuit so there *shouldn't* be anything else on that circuit.

[Tom Henderson2]

Is this the ONLY outlet on the circuit? If it is, and the neutral (white) wire in the circuit is connected to the grounding strip in the breaker box as it normally is, then there is no functional difference between this receptacle and the 2 pole grounding receptacle, NEMA 6-30R. The neutral (white) conductor effectively becomes the grounding conductor IF THERE ARE NO OTHER OUTLETS ON THE CIRCUIT.

Hi Tom (and all that have replied)

This is the only outlet on that circuit....

BUT -- don't know if it matters -- but this is a condo. So what is in my garage is a sub panel of sorts. The service entrance is on the end of the building a couple doors down. Dunno if this changes the picture or not.

Thanks for the info.

-TH

[Chris Padilla]

Tom H.,

Just open the panel and peer inside. If the white and greens/bares are bonded together, you are at the main panel. If they are separated, you are at a sub-panel.

[Jim O'Dell]

On my Square D panel, it isn't real evident where the two bond. It is a bolt that goes in that bonds the two. So be careful looking at the panel to determine if it is a main or sub. Jim.

[Chris Padilla]

My main panel just has a couple very long "bars" with several screws in them that all the white/green/bare wires terminate to. The large ground wire from the pole ties into the top of it like in the pic...for the Main panel. Sub-panels will have a nuetral bar AND ground bar...not connected to each other.

[Tom Veatch]

... Dunno if this changes the picture or not...

It might. The neutral conductor carries current during normal operation. The grounding conductor does not. If the neutral on the dryer circuit bonds directly to ground, there is no more danger of the current in the neutral conductors of other circuits feeding back through the "neutral/ground" of the dryer circuit than there is of it feeding back through the grounding conductor of any of the other circuits.

However if the neutrals all bond together at the subpanel and are separate from the grounds, then there may or may not be a practical problem. The neutrals and grounds will eventually bond together at the service entrance, but there will be a significant wire run where they are separate.

Depending on the amount of resistance in the neutral conductor from the subpanel to the main panel, current from the subpanel to the main panel in the neutral conductor will develop a small voltage to ground at the neutral bus in the subpanel. Therefore, there will be a small voltage to ground in the neutral wire of the dryer circuit. If that is used as the grounding conductor for the table saw, then there will be a small voltage to ground on the frame and other "grounded" parts of the table saw.

Whether or not that voltage is significant is impossible to say from my vantage point. The amount of that voltage is dependent on the resistance of the neutral path from the subpanel to the main panel and on the instantaneous current flowing in that neutral path (Ohm's Law, V=IR). It may be enough that you experience a distinct tingle when you touch the frame of the tablesaw or it may be totally negligible. It's simply impossible to say. That is why the NEC prohibits grounding conductors from carrying current in non-fault conditions and why I included the condition that the neutral and grounding conductors be bonded at your breaker panel before using that circuit to run your tablesaw.

I hope I've explained that clearly enough to get my point across. The bottom line is that if your dryer circuit goes to a subpanel where the grounding conductors are not bonded to the neutral conductors, then there is a potential for an unexpected voltage to ground on the presumably grounded portions of the tablesaw.

Safe? Unsafe? Probably safe, but "probably" is not the same as "certainly".