I am going to wire an outlet for a compressor in my shop.  Should I connect to 120V or 240V?  I’ve had a few questions regarding the topic of what is best, 120 or 240 connection when you have a choice on a motor, like on a compressor or a pressure washer.  One particular question from one of my website visitors involved a situation where a new larger compressor was purchased to replace the old one that had a single voltage motor rated at 120V/7A, and the new one was more than double the size.  The old unit was connected to a dedicated 15A/120V circuit, and he was quoted a healthy sum to upgrade this circuit to larger wire, and a larger breaker.  The simple solution to this would be to change the existing circuit from a 120V to a 240V supply as the new motor was dual voltage.

Here is a link to an article that deals with this particular situation:  How to Convert an Outlet or Receptacle from 120v to 240v.

The current draw on the new motor was 16A on 120V.  This would require supply conductors of #12AWG, up from the existing #14AWG, and the breaker from 15A up to a 20A (could be allowed to be up to 40A for a dedicated motor circuit).  By switching the motor to 240V, you can keep the same wire size – simply make the required changes to the connections inside the panel, and install a 2-pole breaker.  You would also need to change the receptacle and compressor plug (if cord connected) to a 240V configuration. This is a much better solution than running new wire, especially if the panel isn’t close to where the compressor is, and if the walls are finished.

If this is a new installation, it is always best to run a supply of 240V instead of 120V if it is a dual voltage motor. With 240V, you will have a little more torque, and it will come up to speed a bit faster, but the biggest advantage is less line loss and voltage drop, thus saving some operating costs.  You may also realize some cost savings on the size of the supply wire or cable that you need to run as well.

A motor running at 240V will consume half of the amps that it will draw while running on 120V. We pay for electricity by the kilowatt hour (KW/hr.) so if we consider a motor that draws 16A on120V, this is 1920 watts. (Volts x amps = watts).  The same motor on 240V draws 8 amps, for the same total of 1920 watts (neglecting power factor, a little more technical than we are going to get here).  However, the difference is in line drop (volts lost) and line loss (power lost) in the conductors.

This motor running at 120V will have twice the amount of line drop (amps x resistance of the wire = volts) because it is drawing twice the current, and it will experience four times the line loss (power in watts), because amps(2) x resistance of the wire = watts.  This is wasted power that is lost in the form of heat.  The longer the wire, the more significant these losses become.

The other consideration and potential cost savings here is that you can use #14AWG wire to run the motor on 240V, but you will have to buy at least 12 AWG to run it on 120V. This would be a cost savings, however the breaker may cost you more as you need a 2-pole breaker for the 240V motor, but only a single pole for the 120V set up.

Remember that you will have to know and understand how to make the correct splices in the motor junction box so that you don’t put 240V on a motor wired for 120V (likely damaging the motor), or 120V on a motor wired for 240V (may not start, or be very sluggish).

In conclusion, it is always best to use 240V if the option is available to you.