Electrical Wiring in the Home/electric air compressor motor


QUESTION: Bought a new compressor and wanting to wire up in shop. It says it is 220 volt motor. I am wiring a cord with a plug in direct to the pressure switch. When I took the cover off of the switch I see there are only a black and white wire coming from the motor to the switch. The cord coming from the wall and 220 source is 3 wires, black , white and green. I can plug the white to white , black to black and there is a grounding screw for the green on the pressure switch. Should there be a green wire coming from the motor to the pressure switch?

ANSWER: Chuck, absolutely,  you want an uninterrupted EARTH GROUND   NOT NEUTRAL  FROM THE MAIN THROUGH PRESSURE SWITCH.  You may also want to consider installing a fused disconnect,  as breakers are designed to protect wire, not devices.

Breakers are very slow in reacting to a motor fault where a dual element fuse[s] will give you a lot more protection for the motor, and a disconnect is not that expensive,  even a decent used switch would work.  But if you tied up a significant amount of money for this compressor, I would not rely on the motor internal thermals or the breaker.

But yes good question,  the EARTH ground runs from the motor frame to the main and through all devices on the way.

Other wise your ground protection stops at the pressure switch. Protection is not really the correct wording,  but you want that path to the main,  for the same reasons they changed the rules on three wire connections for appliances,   to four,  one is an EARTH ground,  not a neutral to divide up the 220,  an earth ground to move a ground fault to the breaker and hopefully trip it off.

Most do not understand the idea behind the ground wire,  but it is a low resistance path designed to get the over current signal to the breaker as fast as possible, should a fault occur.

Good question!

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QUESTION: So what you are telling me it is not necessary but would be a good source of protection. Hooking up the way it came from factory, just the 2 wires from motor to switch, would be OK and would run OK with no motor damage?

Exactly,  it will run fine  you need the ground extended   the fuseable disconnect is up to you, the truth is motors have almost zero protection with a breaker as the only over amperage trip,   

A fuse will catch a surge or other issue,  where a breaker might   it just depends on what you have in the compressor and bounce that off the cost of a fused disconnect,   which if you look on surplusrecord.com  might be one there for next to nothing,  but no it is NOT necessary to operate the motor or create any havic,  or do anything bad,  it is just better protection for your motor,  and make sure your earth is extended from motor frame to main    and you got it,  

any issues  let me know,  
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Electrical Wiring in the Home

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Will Babbitt


Electrical issues of all types, wiring, control, appliances, components, specialty in Electric MOTOR or APPARATUS trouble shooting, electric motors, electrical problems, single phase, three phase, DC, capacitors, elevator MG-Sets, modifications, reverse engineering, VFD Drives, single to three phase convertors. Repair of most any electrical/mechanical/electronic apparatus, OEM, AC, DC, Industrial Applications, Three phase, single phase failure mode, determining the root cause of the equipment failure BEFORE failure repeats, Antique appliances, electric motors, fans, ceiling fans, base mount fans, poor equipment performance, Modifications, habitual failures, vibration, redesign, obsolete issues, collectible items restored, rewinding. Owner of EMR Repair Inc www.emrrepair.com More information on electric motors, under Engineering/Motors Ask about any electrical or mechanical problems in the home, office, or even at your work. B2B or business to business CONSULTING I can do but it would be a much more complex issue, that would require a significant amount of time. repair@mearservice.com from there we can discuss and begin work on about anything. Industrial electric motors, controls, troubleshooting, vibration, alignment, any type of industrial or commercial electric/electrcical equipment For the home owner, renter, apartment if you have odd things going on, describe to me, photos are normally very helpful, appliances, heating and air conditioning issues, light switches, installation, DIY PROJECTS FOR THOSE NOT EDUCATED IN ELECTRICAL, we can do some things, but for safety and sanity reasons, I may refer you to a local electrician, and help you find one, help you with pricing and even speak directly with anyone you hire if you have issues,


Over thirty years with a major repair and sales company, VP of Operations, and former owner of my own specialty repair shops, MEAR Services local to Kansas City Missouri EMR Repair Inc., located in Kansas City Missouri, 2014 sold both businesses to JCI Industries, Lee's Summit Missouri wbabbitt@jciind.com Evaluation and repair of electrical/mechanical apparatus. Electrical and mechanical repairs, trouble shooting including, vibration and balance issues. About.com provides this service, and the huge costs to provide help on about any subject. They provide the servers, the people to vet the experts, costs that would be the same as operating a good sized business, They should be appreciated for what they do, the huge costs and the great help they provide for free, Electric motor questions, can be answered here but there is a dedicated category that has other experts to help and add to a solution. Other common questions, are noises in the walls, breakers tripping, devices not starting or shutting down for no apparent reason, smoke alarms buzzing, thermostat change outs, or sizing wire or needing information for a new project, but anything can be resolved, anything, with enough effort and patience, DIY, with electrical is possible, but in most cases the use of a simple ohm meter or volt meter is needed, they can be purchased at any hardware store for a few bucks. If you have smoke, or smell smoke, don't be writing me, call 911 the Fire Department has no problem looking for smoke smells, better then spraying water on a blazing fire. DO NOT GET CAUGHT INVESTIGATING AND TRAPPED IN A BASEMENT LOOKING FOR A BLOWER FAILING< As to the tip jar, it is up to you, it is appreciated, every expert spends out of pocket that I know, we spend on IP time, computer wear, printing documents, books, and of course our time, but if we prevent a $400 service call, we did well,

Former IBEW, EASA, IEEE, UL, ..............


10 plus years, various Technical Schools, followed by Industrial College Credit and non Credit Courses, electronics courses, experience in most any electrical apparatus, electric motors and generators, AC OR DC, fractional to above 5000HP, other electrical apparatus, slip rings, sleeve [plain] bearings, lubrication, identification-no data plate, control components, service and sales. I have continued night schools for decades, the list is extensive. I have been in the business of repairing most anything with wires or mechanical parts for decades, I have some helpful hints and directions you won't have to dig up. Or hopefully not have to pay a huge service call invoice to find.

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