|Forum topic by charlie_c||posted 08-21-2010 04:35 PM||2420 views||0 times favorited||1 reply|
08-21-2010 04:35 PM
I apologize if this is question has already been answered elsewhere in the forums, but I was unable to find anything in a search of the forums that gives me the specific answer I’m seeking.
I’m purchasing a new 3 HP cabinet saw that will draw 18 amps @ 220 volts. Until I can get my shop properly wired for 220, I would like to use an existing dryer outlet (30 amp, older 3 prong style). That socket is in use for the dryer, so I cannot simply change it out for the appropriate socket for 220 volt, 1 phase power tools and still remain married.
I am concerned about a recommendation I received to simply wire in a three prong dryer plug to my saw. As I understand it, the older 3 prong dryer outlets do not use a ground plug—the center “ground” is actually a “neutral” plug to provide 120 volt current to the controls for the dryer. It behaves like one of the flat prongs on a standard 120 volt plug in a circuit connecting it to one of the flat prongs on the dryer plug. (Connecting across both flat prongs on a dryer plug makes the 220 volt circuit). If that is indeed the case, then grounding my machine to the neutral plug simply makes the frame of my machine 120 volts “hot” with respect to a true ground. Simultaneously touch a machine wired in this manner and the frame of a 120 volt machine wired to a “true” ground and I get fried! Newer dryer outlets dating from some time in the 1990s use a 4 prong plug to provide a true ground, but my house was built in 1973.
I suppose that even in houses built in 1973 a ground wire was run to the metal box housing the dryer socket, so one option would be to update my dryer outlet and plug to the 4 prong configuration required by current electrical code. Then there should be no problem wiring a 4 prong plug to my saw—I just ignore the “neutral prong”. That said, I am not an electrician with experience wiring these circuits, so I would appreciate any feedback from someone who is.
FYI, a good reference for plug and socket configurations is: