On Wed, 26 Jun 1996, Jon N. LeChevet wrote:
> Ray:
>
> Nick is connecting to a dryer plug (probably 40 amps) and that plug MUST
> have a neutral. The reason is that control and auxilliary circuits often
> use 110 volts, even on a 220 volt machine. For example; the dryer light
> runs off 110 volts, the motor relay (if it has one) runs off 110 volts, etc.
> Simply running two phases and a safety ground guarantees that the safety
> ground carries current and this is a no no.
I never thought of that, Jon, obviously you know a hell of a lot more
about it than I do (a little knowledge is a dangerous thing). But now, if
others will bear with me, I'd like to indulge my curiosity. I am 99% sure
my dryer has a three prong plug. So does my electric stove. Certainly,
each has 110 v circuits for things like the lights, so the three prongs
must be 110, 110, and neutral. Neither appliance has a separate ground.
Apparently, then, modern 110 volt appliances are grounded automatically
when you plug them in, but modern 220 volt appliances are not generally
grounded unless you run a separate wire to a ground. Why not simply have
a 4 prong plug? You'd think that very few homeowners are going to know
how to attach a separate ground. In fact, my stove was installed by the
appliance store, and they never ran a ground.
Jon continued:
> Your advise to Nick will get his compressor to run. The use of wrongly
> coded wire and neutral/ground interchanges is common and a safety hazard to
> people who may have to work on the compressor at a later date. The
> compressor calls for two phases and a safety ground - the plug has two
> phases and a neutral.
> a. Buy some proper 3 conductor cable and run the black and red to
> each side of the compressor. Run the black and red to each hot side of the
> dryer plug. Do not connect the white or green/copper to either the plug or
> the compressor. Run a separate and permanent safety ground from the
> compressor motor frame to a ground point.
That obviously is a better way to wire it. I do think, though, that it
would be going to extremes to discard the wire he has already bought. If
he gets the proper wire, he is only going to use two of the conductors.
That is going pretty far to avoid having a hot white wire inside the
junction box. I'd hope that anybody who opens the junction box on the
motor is going to be sensible enough to unplug the unit first. I would
definitely do it as you say, but I would use the white and black
conductors in the cable he has, wrap a few turns of black tape around the
end of the white wire in the motor's junction box to indicate it is hot,
and run the separate ground. Anybody who assumes the white wires in a
junction box cannot be hot hasn't changed many light fixtures.
The stores are full of tools made to run on 110 or 220, like my Delta
Contractor's saw and my dust collector. I would have to look, but I would
wager my saw uses a line cord that is color coded for 110. The
instructions for changing the saw to 220v operation don't advocate buying
a new line cord. I'm not sure what the instructions for converting my
dust collector to 220 said; they were incomprehensible and bore no
relation to the wires in the junction box. I hope I did it right.
Thanks for the comments; I learned some things that were well worth
learning.
Ray
Ray Gibbons Dept. of Molecular Physiology & Biophysics
Univ. of Vermont College of Medicine, Burlington, VT
gibbons@northpole.med.uvm.edu (802) 656-8910
|