I had a 220 volt outlet installed in the garage of my new home to accommodate a 6.5hp air compressor, which requires 220 volts / 20 amps. My compressor has a three wire plug, but the new home has a four wire outlet, so I bought a four wire appliance cable. The pressure switch on the compressor has only hot/neutral and ground.. . How do I connect a three-wire piece of equipment to a four wire outlet?