A friend just got an air compressor, 2HP 230V motor, but the wire has no plug. Is there an easy way with a VOM to determine which are the hots and the ground. The wire from the motor goes to a box, and then a new wire (black, white, and green conductors comes out of the box. He wants to hook it up to a 6P-20 plug.
TIA
Bob
Replies
240 is just two phases of 120 hooked phase to phase instead of phase to neutral.
Normal wiring at a 240 volt wall outlet is described in my sketch below. The outlet should look about like a 120 volt except the flat prong slots are rotated 90 degrees, horizontal instead of vertical.
Color coding on motors can vary, especially on those motors importred from China, or even some from Taiwan.
Green is always ground. The black should be a hot line and white would be the neutral on a 120 volt circuit but if your motor is truely a 240 volt motor it is possibly the other hot.
Black-------------Phase 1 ---+--------------
+--120 volts |
White-------------Neutral ---+ +--240 volts
+--120 volts |
Red---------------Ohase 2 ---+--------------
Green-------------Ground
If you have ANY doubts about your understanding of the above, don't take a chance. Have the wiring done by a professional. It won't cost all that much, especially compared to a burned out motor, a wiring fire, or an electrocuted you.
Good luck!!!
I'm assuming this post is from the US, so there's some mix-up since those aren't the colors or the numbers of wires you would attach to a 220 plug, which would have two hot wires colored red and black, and a neutral in white, with a bare grounding wire. Some motors can be wired to run either 110 or 220, but your friend's appears to be wired for 110, with the black wire being the hot (meaning it connects to the wider blade on a 110 plug), the white being the neutral, and the green being ground. If i've confused you enough to make you get some pro help or buy a good book with illustrations in it, my work here is done! <G>
PS: What's a 6P-20 plug?
Splintie: Some motors can be wired to run either 110 or 220, but your friend's appears to be wired for 110, with the black wire being the hot (meaning it connects to the wider blade on a 110 plug), the white being the neutral, and the green being ground.
I'm sorry to disagree, Splintie, but you've reversed the hot and neutral blades in your statement above. The WIDE blade is the NEUTRAL (WHITE wire) and the NARROW one is the hot line (BLACK wire). The U shaped prong is ground (GREEN wire).
Gosh, that's embarrassing, and moreso for the fact that i knew i was posting late and to be careful, i actually had a wiring book laid on my desk opened to a pic, and forgot to think before typing or noticing that this was their "don't" picture. It's on page 139 of B and D's Complete Guide to Home Wiring. I picked that guide for it's clear pictures, but i wonder about some of the info since they suggest every outlet in a kitchen be a GFCI--more than one on the same circuit. I've already proved i only play an electrician, but this is dumb, isn't it???
Please let me know when i've stood in the corner long enough... :[
Splintie-
i wonder about some of the info since they suggest every outlet in a kitchen be a GFCI--more than one on the same circuit.
My code book requires two separate circuits for kitchen appliances and all outlets to be GFCI protected. The theory is, I believe, that anything could fall into water in the kitchen sink. Use either GFCI circuit breakers, or a GFCI outlet in the lead box in the kitchen, and then every outlet downstream from that one will be GFCI protected. There's no need for more than one GFCI device on the same circuit. GFCI circuit breakers are no longer expensive, so that's a reasonable place to put the protection.
Having posted my solution, I have another suggestion. Casual forums such as this one are no place to solicit or offer electrical advice. That stuff is potentially dangerous. Get advice from a qualified electrician or your building inspector, not from anonymous (me, for instance) sources on the web. Set your V Chip accordingly.
No i really meant what i said that time! <G>
Every outlet a GFCI ( @ $7), pigtailed "for safety's sake" into a 12/2 circuit, page 233. They also show the 12/3 method, which i thought was a bad idea...? I like the idea a master electrician i know who puts only two outlets on each circuit--it's still possible to overload it, but it gives a lot more headroom if you want to play musical chairs with your micro, toaster, and dry your hair while scrambling eggs in an electric skillet.
Therefore, i concur that one should not rely on one source of information, but i think the poster ended up with a lot more info from posting than from not.
I've been lucky so far, i guess. Only burnt down two houses, couple trees, and cremated a pan of garbanzo beans before i started getting the gist of this wiring stuff...
Edited 10/5/2002 10:44:32 PM ET by SPLINTIE
They also show the 12/3 method,
Please note that this is a comment, not advice.
By the 12/3 method, I assume you mean achieving two 120v circuits with only one cable run. Each circuit is protected by its own 120v. breaker. But, the breakers must be tied together by a common handle so that if one trips, the other is tripped as well. Saves some wiring, but the two circuits aren't totally independent.
Well, somehow I got the advice I needed, which is to check each lead against the chassis of the motor, and hopefully between the two hots will be about 240V/10A or lets see, would that be about 24 ohms resistance (or lets say in the 10-40 ohm range since we're comparing AC to DC and its been awhile since I was in school, as opposed to teaching it). This is a pretty old american motor which someone rewired using a regular 12/3 (black,white,green) extension cord.
As to 12-3 romex (generically X/3) for two 120 circuits needing a dual breaker, yes and no. You can put them on a dual breaker with a single joined trip lever, but you can also put them on two separate breakers as long as both breakers are on different legs so that the current in the neutral is the difference and not the sum of the currents on each hot. The problem with X/3 wiring is that everything in a service panel becomes a single wire just inside the clamp, and sometimes if things are getting moved around, two hots from an X/3 could end up on the same leg. If I were to ever wire with X/3, I would mark the black wire with some red tape to indicate it was connected to a red somewhere else in the panel. Where I think this is especially dangerous is with the new half size breakers, where someone who isn't thinking could put both hots from an X/3 onto each half of the breaker, thus putting them on both on the same leg.
After the new service panel went in, I did a master listing of all breakers. Each breaker is listed by position, and all devices it controls are layed out in their order on the wire run. I'm most of the way through labeling the outlet covers on the inside as well. Garage sale stickers are great for this. I also labeled each J-box with the circuits that are connected within or pass through. In addition, the electricians put in 4 separate runs of X/3 wire, so each breaker with a hot from them also has a special notation. I live in a 1910 house that is now on its third generation of wiring, and you take nothing for granted. Especially since it started out as single family, spent a long time as an up-down duplex, and now is being turned back into single family. One of our first discoveries was that the downstairs apartment paid for the power to the upstairs refrigerator, which was on the same circuit as the downstairs ceiling lights. THAT must have been annoying. If you do live in an older house, I believe it pays to document what is what. You might be surprised as i have been on numerous occasions here, but not so much anymore.
OK, Splintie, you can come out of the corner now, but you must promise to be good.
On the serious side, I must agree with Don about soliciting and giving advice here on electrical matters. That was why I put a trailer on my other post that if what I said didn't connect, call an electrician.
The problem with giving advice is that I have no idea of the experience or aptitude or even common sense of the person to whom I am addressing the advice. For a parallel example, check out the thread about the guy repressurizing aerosol cans with an air compressor. Yipes!!!
In the future, I will be very cautious about the advice I give, referring people to books and professionals rather than being too free with advice that might misunderstood or misapplied. I will however freely advise on factual matters and point out potential danger zones.
I have no idea of the experience or aptitude or even common sense of the person to whom I am addressing the advice.
Yet another good reason to use a pseudonym.
I'm not so alarmed as everyone else about pressurizing the aerosol cans, since i have a compressor i fire up almost daily. Besides, you can learn a lot from bad role models: the guy down the lane got cut apart when he was down in the hole with his well water pressure tank and it blew--i decided to spring for a new guage before installing mine!
The green will show continuity to the motor frame, and the other two not, but the black and white will have continuity to each other. On a NEMA 6-20P plug (20A, 250V, 2 AC hp), looking at the business end with the ground (U-shaped) on top, the lower right blade is horizontal, the lower left is vertical. Both straight blades are hot. There is no neutral, nor will it fit into a 5-15R (15A, 250V, 2 AC hp), but a 15A plug (6-15P) will fit into the 20A (6-20R) receptacle. Tape both ends of the white wire with black or red to indicate hot. The motor should have integral overload protection, or there should be a motor starter with overload heaters. All switches must be 2-pole (you've got 2 hots, after all; even though breaking either one will break the circuit, you must disconnect all non-grounded conductors together).
Be seeing you...
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