Need Help Wiring Old Compressor

[Thomas & Paige]

I have an old Craftsman Twin Cylinder air compressor that has no plug/cord wired in at the moment. I need to learn the correct wiring procedure in order to connect this 230V motor to my existing 50amp 250V plug in my garage. The house was built in the 60's and all the wiring is original. Therefore, I have provided pictures of each component in question, sorry you have to cut and paste.

First here are pictures of the existing 50amp 250V Plug in my garage.

Second are pictures of the compressor and its' motor face plate, wiring, and wiring diagram.

Third is a picture of the wiring block on the compressor.

My intentions are to purchase a NEMA 10-50P and 10ft of 12-3 power Cord and simply wire the Red and Black wire to the two terminals on the wiring block. Leaving the white wire disconnected. Is this the proper thing to do or am I going to cause some sort of electrical hazard? The red and black are my two hot wires right? By the way where is a good place to look for the 10-50P, I have tried my local Home Depot and Lowes but they don't carry them. Any help would be greatly appreciated.

Thomas

[RBM]

Your compressor draws 16 amps, so you don't need to connect it to a 50 amp outlet or circuit. Change the fuses or breaker to 20 amp and change the outlet to 20 amp 240 volt and get a matching cord and plug, (3 wire) Run the cord to the pressure switch, connect the two power wires to the switch(line side) and the ground wire to a ground screw. It looks like you've already got the cable going from the pressure switch to the motor

[buffalobill]

we don't know if this machine works. does it have a ground fault? with a digital multimeter check for faults to ground from the bare metal to the connection terminals. then the device should be grounded and tested before you go thru all this wiring. i don't see a green ground wire in your wiring. you'll need one. test ac the red and black should be hot and hot, and need to read 220 across them to run the device. remove and replace the rusty plug and buy a proper sized outlet, cord, wire, and breaker. don't accidentally ground either of the #2 and #4 motor hot terminals it says on one of your photos.

[DanG]

The receptacle you show looks to me a NEMA 220v 20 amp receptacle, reference:

The red and black wires shown are the LINE hot leads. Each one should be wired to a separate breaker in your panel and those two breakers should be in a single block or have their toggles wired together. I would expect these breakers to be labeled 20. If they are truly 50 amps, the breakers could be changed to provide better protection to the motor. The white wire shown should go to ground in your panel.

In your wiring diagram, it shows lugs 2 and 4 to be LINE. If you can maintain the same color code as the receptacle, you would hook red and black to 2 and 4, it does not matter which is which. 220 does not need neutral. There may be a lug for ground and/or it would be typical to crimp on a wire fork or ring to install under a case screw on the motor. This ground can be passed on to the compressor frame, the pressure regulator, etc. These lugs are already tied to the pressure regulator with the white and black leads in the black cord.

I can't read the writing on your pressure regulator block. I would bet that the existing white and black say LOAD. There should be two more wire slots that say LINE. These would correspond to the incoming LINE wires of your new cord that will hook to the red and black at the existing receptacle. I would bring the existing receptacle white/ground to the regulator and terminate in the case clip shown.

(top posted for your convenience) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing . . . . DanG (remove the sevens) [e-mail]

[T-Man16]

Thanks RBM, I plan on going with a 20amp 240V disconnect along with matching outlet and plug. The wiring had me confued because I did not know that a 240V circuit does not need a neutral and rather each hot wire trades off as being the neutral. I kept banging my head about how the electrons will flow back to the panel, but now I know. Now the white/neutral wire can be used as a safety ground so long as I label it as such (green tape) at the outlet and panel. So the red and black will go to the line side of the pressure switch and the white will get attached to the housing of the pressure switch because it is close in proximity and common to the chasis of the motor. Do you think originally the plug would have used the white/neutral wire as a safety ground? A consumer would not have known to label the outlet/panel with green tape back in the early

70's when this compressor was bought...don't you think?

[T-Man16]

Buffalobill, The compressor has sat for a long time but It did work that last time my father had it pluged in...4-5 years ago. I checked for continuity between the bare terminals on the pressure switch to the motor chasis and had none. Therefore, we have no faults to ground. The green safety ground wire is not in the original outlet from the early 1960's they simply did not practice this at the time. Only the outlets in areas where moisture is a concern do I find the green safety ground wire in my home. Therefore, this is the original wiring in the home and I had no intentions of changing it. However, as stated above I can utilize the white/neutral wire in the original outlet as a safety ground if I attach it to the metal chasis of the pressure switch and then recode this wire as green ground. I do read 240v across the red and black wire in the outlet. Thanks for the info

[T-Man16]

DanG,

The outlet is labled 250V 50amp and resembles most closely to the

10-50P on the website you provided. I don't have much experience switching out breakers so I am going to leave them in place and use a 20amp 240V disconnect at the outlet.

Thank you for pointing out the fact that 240V does not need a neutral, this is what confused me the most. I kept banging my head wondering how electrons will flow back if both the obvious terminals on the pressure switch were occupied by hot leads. It wasn't until you mentioned this and I looked it up in a time life sereis book on basic home wiring that I realized the hot wires trade off acting as neutrals. There is no obviuous ground lug but I do know the chasis of the motor is common to the clip attached to the metal box housing the pressure switch since there is continuity when tested with my DVM. So do you think originally the cord from Craftsman simply grounded the white/neutral wire in this manner? A consumer would be unaware at the time that it was acting as a safety ground..right?

The pressure switch block is labled motor on one side and line on the other. The leads from the black cord (white /black) are attached correctly as these have never been messed with. I will attach the red and black to the line side of the pressure switch.Then, I will attach the white/neural to the clip in the picture as this is what holds the screw that attaches the cover to the pressure switch.

Thank you for the info Thomas

[RBM]

If you have a cord and plug arrangement, you don't need a separate disconnect and conversely, if you install a disconnect switch, you don't need a cord and plug