I had a 220 volt outlet installed in the garage of my new home to accommodate a 6.5hp air compressor, which requires 220 volts / 20 amps. My compressor has a three wire plug, but the new home has a four wire outlet, so I bought a four wire appliance cable. The pressure switch on the compressor has only hot/neutral and ground.. . How do I connect a three-wire piece of equipment to a four wire outlet?

I am working backwards, the worst part of this is at the end of my advice.

Assuming your in USA, the supply voltage is 240 volts.

The compressor may be rated 220 volts, OK to connect to 240 volts.

What exactly does the nameplate on the AC state? It probably states 230 volts.

If the comp is 220-240 volts, it only needs two hots and an equipment ground, not the neutral you mention.

So, the four wire cord should have black, red, white, and green.

Connect green to green screw at the pressure switch.

Connect black and red to the input of the pressure switch.

Cap the white neutral wire. Not needed.

This is all assuming the 4 wire receptacle is wired properly.

You state the outlet was installed specifically for the air compressor. Someone did not check the specs of the AC, and the wrong outlet was installed.

Oh and here is the best part, your going to freak out.

A 20 amp 240 volt circuit is not sufficient for a 6.5 HP 220/230/240 volt motor.

So, not only is the wrong receptacle is installed, the circuit size is much too small to allow the motor to operate properly.

Sounds like back to the drawing board for you.

Either get a much smaller compressor, or have a new larger circuit and receptacle installed to match the unit you have.

I cannot tell you the max motor size until you confirm the size wire and circuit breaker and supply voltage you actually have now.

First off, confirm the actual amperage rating of the motor. The 6.5hp is likely marketing HP, as in "developed" HP. That 6.5 is likely a fantasy.

Also, saying it "requires 220 volts / 20 amps". What does that mean? Does it require a 20A circuit (which I suspect), or does it draw 20A (which I doubt)?

TK is certainly correct, whoever put in a 4-wire receptacle for a compressor was not very familiar with wiring motors.

I have a feeling you have one of those big name compressors that is rated in impossible HP and your 20A circuit will be fine.
I have a 60gal IR vertical that is plugged into the existing 20a/240v receptacle that my older 3hp 30gal one was plugged into.

Ahhh, now I get it.

Quote:

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Originally Posted by stanfortyman

First off, confirm the actual amperage rating of the motor. The 6.5hp is likely marketing HP, as in "developed" HP. That 6.5 is likely a fantasy.

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A 6.5 HP motor is not a standard NEMA size motor, as it is not listed in NEC Table 430.148 Full load currents Single Phase AC motors.

But what do I know, if someone says they got a 6.5 HP unit, I take it at face value.

Amazing how "marketing" is so close to "scamming".

Hey, how I look at it if a customer asks for circuit to handle a 6.5 motor, that's what they get, possibly a grossly oversized circuit.

More money in my pocket. I did my due diligence.

Great advice Stan, Deb needs to be absolutely sure the unit is really what is is advertised as.

Buyer Beware.

Excuse me?

I just want to impress myself with how smart I am?

Why exactly did you come here where it is advertised "Live Answers From Real Experts"?

Did you want wrong information from a stupid person?

Based on the information given BY YOU I gave the best answer I could.

The make and model number would have been handy in your first question.

Also letting us know that the manual states that a 20 amp circuit is required would have been good too.

Why does the manual I find on line state a few differences in the info you provide?

http://www.devap.com/repair_pdfs/AC/...S/l6560v-1.pdf

1. 15 amp min circuit not 20 amp
2. 240 volts not 220 volt
3. 6.5 PEAK HP is NOT a 6.5 HP motor

Hey, next time I will impress you with how stupid I can be.

For FREE by the way, no matter how good or bad the advice is.

Of course, you will never know which is which.

Maybe after the fire dept leaves you will realize you may just have gotten bad advice, FOR FREE.

You're right, of course. What you get for free is usually worth what you pay for it. You obviously don't know enough about stationary air compressors to know that the HP rating refers to the pump and not the electric motor that drives it.

I will cntact a qualified electrician to resolve my issue.

Am I missing something? I read TK's rant about "impressing myself with how smart I am" and don't see why it was posted. Who questioned him or his intelligence? I sure do not see any post that does that. What gives? Was there a post removed? I'm totally confused as to what transpired.

Missouri, check out the "disagree" comment above that post. I agree with TK that it was uncalled for.

Sorry Stan... I don't have any posts from DEB except the original one and the one right after TK's. Must be some glitch at my end... or as I like to call it... that F***ing AOL.
But I appreciate the explanation

No, no. Not a post. One of those "rate this answer" comments underneath TK's post.

Quote:

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Originally Posted by debloooz

You're right, of course. What you get for free is usually worth what you pay for it. You obviously don't know enough about stationary air compressors to know that the HP rating refers to the pump and not the electric motor that drives it.

I will cntact a qualified electrician to resolve my issue.

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Deb, your probably right about my knowledge about "stationary air compressors", so to test your theory, I always accept a challenge to learn more, and I went out to learn how these air compressors really get rated.

That is what makes an electrician better, and try not to make the same mistake twice. I really strive to give good, accurate, and safe advice. I truly hate people getting hurt and fires burning down buildings.

I also apologize for my flippant comment about making money installing circuits that were too large for an unsuspecting customers. I really don't do that in my everyday business. I truly try to be consultative and do no more and no less than needed.

If I had been given the job connecting your unit, based on your information, if I did not do due diligence, this may have occurred.

I do know that stationary means just that, it is intended to remain in one place, as opposed to the opposite of portable, so lets move onto air compressors of your style.

I did not know that the HP rating was for the pump, never heard that before. HP in electric appliances was always for the motor, so I automatically went there, that's all electricians usually know, at least this one.

And keep in mind, I am not trying to impress myself with how smart I am. I did that years ago. That was not the context in my trying to answer your question.

I came across some interesting information that you may find helpful in making a decision in purchasing an air compressor.

Seems DeVilbiss Air Power Company was one of several plaintiffs listed in several Class Action Suits, along with some other popular AC makers accused of misleading of consumers regarding the "Peak HP ratings of small air compressors.:

Campbell Hausfeld
Ingersoll-Rand Company, Ltd.
Coleman PowerMate, Inc.

I found this at:

Campbell Hausfeld Devilbiss Air Power Ingersoll Rand Coleman Powermate Inc Lawsuit Attorney | Lawyer Air Compressor Manufacturers Settle Underpowered Motor Action for $40.6 Million

There is quite a list of brand names involved, along with various settlements to correct advertised ratings.

This information was corroborated and confirmed at several other websites:

Excerpt from:

Evaluating True Horsepower and CFM Ratings of Air Compressors
Air compressors for the home or small shop have been advertised and sold with ridiculously inflated horsepower ratings.

And

Excerpt from:

Air Compressor Buyer's Guide
Horsepower rating really matter?
The answer is no.
When an air compressor is built, it requires a motor or engine to power the process of producing air. The horsepower is not the only component of the equation. It has to be combined with the compressor pump that it runs to produce the air needed. The bigger the horsepower does not necessarily mean more air. Remember to keep your required cfm in mind. Horsepower does not make you work more efficiently, but horsepower working in unison with a quality pump that produces enough cfm for your needs.

BEWARE: Make sure when shopping that you look for the running horsepower and not the peak horsepower you get at startup. Some manufacturers have falsely advertised or implied the peak horsepower to be the normal run horsepower. At start-up, there is a surge of power needed to start the motor and can be 1.5-3 times the amount of running power needed. This means the horsepower need for the motor peaks as well, but then drops back down to normal. Many of these manufacturers are involved in a class-action lawsuit for misrepresentation. They have used man’s machismo of “the more horsepower, the better” to their advantage to make sales. These aren’t cars we’re talking about!

Just to name a couple.

So, I hope I helped to change your opinion of the intent of my advice.

If anything, you certainly helped me, and even better, any other readers of this post, in understanding air compressor ratings.

Oh, but wait a minute, you already bought one of these.

Change Buyer Beware to Owner Beware.