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.
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.
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
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?
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.
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
The receptacle you show looks to me a NEMA 220v 20 amp receptacle,
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)
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
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