Expert: Master Bob - 7/16/2005

Question
I have a 2hp electrical air compressor.  It can run on either 240 or 110 volts.  My father wired it for 220 for me.  He has passed and I need to change the power to 110 volts.  How do I do that? I have to change the plug.  But what else do I need to do to make sure it is wired properly.  This is a craftsman air compressor.

Answer
Hi EJ,

  Based on your description...and as you mentioned...the compressor cord's plug will need to be changed back to a 120 volt plug end to fit the receptacle....more then likely a standard NEMA 5-20P or possibly a NEMA 5-30P. The 240 volt plug end may have been either a NEMA 6-15P or 6-20P...providing 2 120 volt hots and a ground to the compressor.

For more on NEMA plug and receptacle Configurations, see:

http://www.leviton.com/sections/techsupp/nema.htm


 But - what needs to be noted here is that when you convert back to 120 volt from 240...the current (in amperage) will now double on the wiring..(although the motor still draws the same total amount of power)..so the cord wire size also MAY need to be changed. For example, the typical 2 HP motor draws 12 amps on each of the 2 hot wires at 240 volts...but on a 120 volt feed...it will now draw 24 amps on the hot and neutral wire...making the cord now need to be a #10 gauge instead of a #12 gauge....so that needs to be kept in mind. Also...the breaker and wire feeding the compressor wall box will need to be changed. If it had a double pole 20 amp breaker at 240 volt at the panel...it may now need to change out to a single pole 30 amp breaker....and #10 wire all the way.

 Also....you may need to change around internal connections at the compressors wiring box to adapt it back to 120 volts...and it should be marked on the backside of the compressor's wiring compartment cover...or on stickers on the wires themselves....check for this with the unit disconnected...and make a note on paper of all connections before any are changed.

 Hope this helps...if you need more details or have new questions...please follow-up. If you do follow-up, please include the model number for this Craftsmen compressor....I may be able to pull more specific electrical detail.

Best Wishes,

Bob Osgood
Master Electrician

RKO ELECTRIC

www.rkoelectric.com