Advice for converting Sears Craftsman 220V compressor plug to washing machine plug

[Elmo]

I just want advice for the conversion I just did before I plug it all in.

Details:

1. I have a two-phase 220V Sears Craftsman compressor which uses a 220 volt plug with one blade sideways (otherwise it looks like a normal 120V grounded plug).

1. I have no sockets in the house which fit that 220 volt plug.

2. I have a three-pronged dryer socket in the garage with 220 volts (two hots and a neutral).

1. I just attached a new 3-pronged male dryer cord to a female 220 volt plug with the one blade sideways.

Before I plug in the compressor to the recepticle to the dryer cord to the recepticle, would you have any concerns about safety or other?

I assumed the neutral in the compressor plug was the center wire (looks like a ground pin) while I assumed the neutral on the 3-pronged dryer cord is the L-shaped center pin. Is that the correct assumption for the shared neutral?

[Evan]

Ummm yeah... Why would you want to plug a 20 amp 240 volt appliance which uses a NEMA 6-20 cord configuration into a 30 amp outlet made for a NEMA 10-30 cord configuration by means of using a cord adapter which you have created yourself...

There is a reason why the outlets for vastly different amperage ratings do not fit into each other... SAFETY...

Why not have a correct 20 amp 240 volt NEMA 6-20 outlet installed in your home so you can use your compressor safely on a circuit which is not capable of supplying more power than your appliance is rated for which could damage it or cause an accident of some kind...

~~ Evan

[mm]

Huh?

Instead of assuming such things, why not get a meter that measures resistance and figure out whicih wire goes to which prong in each cord? Hint: There is no neutral in a 220 volt appliance or its cord. Two wires will be hot relative to ground and the ground wire should not be connected to the other two.

Also get a meter that goes up to 250 VAC and measure the voltage at all three slots of the receptacle, relative to ground. Then you'll know which 2 slos are hot, and which two prongs of the cord will be hot, when connected to the appliance, and which need to be connected to each other electrically (in the appliance).

This is not to say I think the project is a good idea. No comment on that.

[Bill]

I've seen thousands of dollars of damage done from people replacing plugs and not realizing what they are doing....

Have the proper outlet, wiring, and circuit breaker installed!

[Jon Danniken]

Nonsense. I have a lamp here which states not to use more than a 100 watt bulb, and uses an 18 gauge cord.

It plugs into a 15A receptacle, fed by a 14 gauge wire.

All of this is perfectly to code.

Jon

[jamesgangnc]

Yes, the L shaped prong is the nuetral on a 3 wire dryer. The other two are the hots. Your compressor more likely needs 2 hots and a ground. While what you are doing is not exactly by the book it will run your compressor. The safety issue is that you will be using your nuetral line for a ground and using a 30 amp circuit where a 20 amp one is called for. What you really need to do is install a dedicated

220 outlet in the location of your compressor. Have you looked at the compressor to see if it can be run on 110? Many motors have alternate wiring that allows them to run on 220 or 110.

[Jon Danniken]

Yes. Find out if your compressor expects the third terminal to be a ground; the third wire on your dryer is likely a neutral. If such is the case, you will need to install a properly grounded outlet for your compressor.

As to sharing the receptacle between a dryer and a compressor, I have done that with a welder, and to be honest, it gets old really fast. Eventually, you will want to install a dedicated circuit, and the sooner the better.

Jon

[George]

I don't believe you will find a code reference that allows what the OP described since your example doesn't scale.

[RBM]

Your setup will work as you describe. The Nec prohibits using a 20 amp receptacle on a 30 amp circuit. Provided that the motor has built in overcurrent protection, I would install a 30 amp dryer cord set directly on the compressor

[RBM]

The third terminal on a three wire dryer outlet is a neutral/ground, and by code must original in the main service panel and not a sub panel

[Elmo]

I agree with Evan, the right way to power the compressor with the NEMA

6-20P plug (20amp rating), is to hire an electrician to add a dedicated 220V NEMA 6-20R receptacle with a 30 amp dedicated circuit breaker.

Or to have that electrician swap out the existing NEMA 6-20R with a NEMA

6-20R and to swap out the 30 amp breaker with one with only a 20 amp rating.

However, the cost of hiring the electrician to change the circuit is more than that of buying a new compressor with, say, standard 110 volt power.

I use the compressor maybe once every six months. Unknowns in the adapter setup are what I'm asking about here.

The unknowns to overcome by asking this post are: a) How do I test if the NEMA10-30R is on a sub panel or not? b) Did I select the right pin ground in the NEMA 6-20R receptacle as the L-shaped neutral in the NEMA10-30P plug?

[Elmo]

I did use the Fluke DMM to measure the connections; I was just double checking which was the ground/neutral because I didn't wire the house in the first place.

The ground in the two-phase 220v compressor NEMA 6-20P plug should be the center pin and the ground/neutral in the NEMA10-30R dryer receptacle should be the L-shaped center prong.

The two questions I would like to ask here are: a) How do I know if the NEMA10-30R is on a sub panel (it's inches from the main panel on the other side of the wall outside the garage)? b) Are my tests correct that the ground on the 20-amp NEMA 6-20P plug is "equivalent" to the ground/neutral on the 30-amp NEMA10-30R receptacle?

[Steve Barker]

I'm suspecting that out of all these replies, NONE are correct. What i suspect is that you have a 20a plug on a 120V compressor and you just don't have any 20a 120v outlets in your house. Better look a little closer at everything before you feed that unit 240v.

[Elmo]

You know, I bought it that damn Sears Craftsman compressor on a whim in a sale in the 80's. It was (on sale), about 350 or 400 bucks.

Then, when I brought the huge thing home, I realized I didn't have an outlet for it, so I just let it sit for a year or so. Then I moved, and didn't have an outlet for it, so I let it sit for a few years. Finally I moved to a place that had the outlet, and I used it every six months or so to blow up the kids toys. I moved again, and guess what, no outlet.

Funny thing is that I've seen Sears compressors on sale over and over and over and over again for, guess what, just about 350 or 400 bucks. They never changed prices in decades. Amazing.

In hind sight, I never should have bought it as I barely used it due to the cord and the lack of needs.

However, I still have it. And I actually want to blow up a kids toy and then figured I'd plug the NEMA 6-20P into an adaptor consisting of a NEMA

6-20R and a NEMA10-30P and then connect to the NEMA10-30R fused by a 30-amp circuit for my 20-amp compressor.

I was just asking advice, specifically: a) How do I know that I'm NOT on a sub panel? b) Did I pick the correct ground/neutral?

Is there anything else I didn't think of (before I plug it in)?

[Steve Barker]

does it look like this:

[Elmo]

Let's look at this from a "safety" standpoint, which is your point.

The "proper" way to do this, all will agree, is to spend more than the compressor is worth to install a 20-amp dedicated NEMA 6-20R receptacle.

However, from a safety standpoint, if anyone can show me how plugging the

20-amp NEMA 6-20P through an adapter to the 30-amp NEMA10-30P receptacle is inherently more dangerous, then I'll listen.

Of course, I do realize that the "equipment" won't be protected to 20 amps (it will be protected to 30 amps); but I'm not at all worried about the

30-year old compressor burning up.

The "house" wiring is still protected to 30 amps, which is what matters.

And "safety" is the same (as far as I can tell) since we're using exactly the same two hots and the same ground/neutral.

Or did I miss something critical in that analysis? Advice always welcome.

[Steve Barker]

Here's a better comparison, scroll down to the 20a section. It seems it could be a 120v OR a 240v depending on which side the sideways blade is on.

[RBM]

Very good point. Most of these small compressors run 120 or 240 volts, and do require a 20 amp plug @120 volt. It is entirely probable that the OP is looking at a 20 amp 120 volt plug and not a 20 amp 240 volt plug, which are very similar looking

[dpb]

jamesgangnc wrote: ..

It is ground serving (per applicable Code of the time) as ground/shared neutral.

It is _exactly_ by current book for 240V service w/ the 240V load there is no need for the neutral; therefore the ground is no longer serving as a shared neutral only as ground.

No, NO, _NO_! There is no safety issue; as above the 3rd conductor in a shared utility (dryer) outlet _IS_ the ground conductor; the NEC formerly allowed it to be shared function of also serving the neutral. It is wired to the ground bus, _NOT_ the neutral.

The second misconception here is that there's some proscription against a higher-rated circuit supply a lesser-rated load--that again is simply nonsense. The 30A circuit breaker/fuse is there to protect the circuit, _NOT_ the load; the load will have its own overload protection for that purpose.

While I'll agree it's certainly a nuisance factor to have a humongous

30A dryer plug on the end of the cord for a small compressor, it is not an issue whatsoever from a safety (or Code, for that matter) standpoint.

--

[Elmo]

I agree that's the by-the-book method. However, the cost of having an electrician replace the 30-amp NEMA 10-30R with a 20-amp NEMA 6-20R is more than the cost of a compressor that runs off of 110.

I didn't realize the 30-year old Sears Craftsman compressor COULD run on

110. How would I know? (I'll try to dig up a model number and google it after this reply.)

But wouldn't the neutral line ALREADY be used as a ground in the NEMA

10-30R dryer circuit were I to plug in a dryer to that circuit?

Isn't the compressor the same as the dryer from an electrical standpoint with respect to the ground/neutral wire?