I have a 240 volt air compressor. I have had it wired with a three prong dryer cord wich consists of a white, black and green wire. There is only a place on the compressor for these 3 wires. The problem I have is the outlet in my new garage calls for a four wire cord that has white, black, red and green wires. How to I wire this?

When replacing a 4 prong cord with a 3 prong cord you put black and red wires to the outside posts of your terminal block. The white wire from your three prong dryer cord goes to the center of your terminal block. The yellow and green wire from the dryer go together and attach to the cabinet ground.

Four-Wire: Green

The green wire is the ground wire, and it is the difference between a three-wire and a four-wire cord. (In the three-wire system, the ground wire and the neutral wire are the same.) The green wire connects to the dryer frame. Look at the cord-connection terminals on the back of the dryer. Very close by, you should see a screw sticking out of the metal panel. It will be labeled "external ground connector" or something similar. Connect the green wire to that screw.

Four-Wire: Black

Most appliances in your house run on 120 volts of electricity, some on 240 volts. The dryer needs both--240 volts to run the drum and heating element, and 120 volts to run the timer, thermostat and other components. For this reason, the dryer cord has two live or "hot" wires--one black, one red--rather than a single hot wire. The dryer draws 120 volts from each hot wire to get the 240 volts it needs for the heavy work, then pairs one hot wire with the neutral wire for the 120 volts to run the smaller functions. Look again at the connection terminals on the back of the dryer. There should be three screws in a horizontal row. Connect the black hot wire to the terminal on the left.

Four-Wire: Red

Connect the other hot wire, the red wire, to the terminal on the right. (In truth, you could connect either hot wire to either hot terminal, but it's standard procedure to put the black on the left and the red on the right, so do so.)

Four-Wire: White

The white is the neutral wire, and it connects to the center terminal. There may be a metal strap attached to this terminal. That's a ground strap. When a three-wire cord is attached, it serves the same function as the ground screw mentioned above.

Three-Wire: Gray

If you have a three-wire cord, then all three wires will be gray. They're arranged side-by-side with the hot wires on the outside and the neutral wire in the middle. Attach the hot wires to the left and right terminals and the neutral wire to the center terminal. Make sure the ground strap is attached to the center terminal.

is the white neutral wire connected to the neutral terminal on the dryers power terminal block in the back: it should be (RED to L1 then the BLACK to L2) they can be on either terminal as 2 power wires are not polarity sensitive, then as i said WHITE to the N or NEUTRAL terminal and the GREEN safety ground to any metallic point on the dryer,then check for a nominal 240 volts a.c. on the RED & BLACK terminals

Unplug the dryer then remove the terminal block cover and disconnect the 3-prong cord from the dryer. Note that the cord has red , black, and white wires but sometimes the red and black wires are both replaced by same color wire, usually gray and can switch places. There's also a ground strap wire also connected to the center terminal together with the white (neutral) wire.

Get yourself a 4-prong cord and note that it has red, black, white, and green/yellow wires but sometimes, like the 3-pronged one, the red and black wires are both replaced by same color wire, usually gray and can also switch places. All you have to do now is connect the 3 wires in the same manner they are connected as 3-prong cord to the terminal block. The ground strap wire now just need to be connected to the remaining 4th wire, the green/yellow wire instead of the center terminal together with the white (neutral) wire. Reinstall the terminal block cover and it's done.

It can be observed that converting a 3-prong cord to 4-prong cord is merely splitting the neutral line and the ground thereby increasing the number of prongs from 3 to 4 whereas, on the other hand, converting a 4-prong cord to 3-prong cord is merely joining the neutral line (white) and the ground (green/yellow) thereby reducing the number of prongs from 4 to 3.. The links below might enlighten you further on this matter.

Four-prong to Three-prong Electric Dryer Conversion
Three-prong to Four-prong Electric Dryer Conversion

It's either you convert the 3-prong outlet to 4-prong or the 4-prong cord to 3-prong. But the latter is easier and more feasible. Unplug the dryer then disconnect the 4-prong cord from the dryer. Note that it has red, black, white, and green/yellow wires but sometimes the red and black wires are both replaced by same color wire, usually gray. The red and black wires are lines L1 and L2 reversibly, meaning they can switch places, while the white and green/yellow wires are the neutral line and ground, respectively.

Get yourself a 3-prong cord and note that it has red , black, and white wires but sometimes, like the 4-pronged one, the red and black wires are both replaced by same color wire, usually gray and can also switch places. All you have to do now is connect these 3 wires in the same manner they are connected as 4-prong cord to the terminal block. The wire strapped to the dryer cabinet and connected to the green/yellow wire of the 4-prong cord wire is now left hanging. The last step is to connect this hanging strap wire to the same terminal on the terminal block (center) where the white wire (neutral) is connected and it's done.

It can be observed that converting a 4-prong cord to 3-prong cord is merely joining the neutral line (white) and the ground (green/yellow) thereby reducing the number of prongs from 4 to 3. On the other hand, converting a 3-prong cord to 4-prong cord is merely splitting the neutral line and the ground thereby increasing the number of prongs from 3 to 4. The link below might enlighten you further on this matter.

Electric Dryer Prong Conversion

You have to replace the dryer's 3-prong cord to 4-prong. Get yourself a 4-prong cord then on the terminal block at the rear of the dryer, connect the wires of the 4-prong cord to where the same wire colors (red, black, and white) of the 3-prong cord connects to. The only modification needs to be done is on the green/yellow stripe wire that connects from the white wire terminal to the dryer's cabinet. Remove this wire (green/yellow stripe) from the white wire terminal then connect it to the same wire color (green/yellow stripe) of the 4-prong cord and you are done. The new wire configuration would then be red, black, and white on the terminal block and green/yellow stripe wire on the cabinet.

Ok here is how to hook up looking at dryer. Left brass screw is hot put red or black wire on. Center screw silver is neutral put white wire on. Right brass is also hot put the red or black wire on which one you have left. The green you ground to the dryer body or you can put with the white wire.

The reason why you have to hot wires is that it is a 240 volt dryer. Each hot wire caries 120 volts. The hot wires are the red and black. The white is a neutral witch is also a ground

I hope this helps. If you have any questions please ask before you rate.

Good Luck And Thank You for the post

You can replace the 4 prong with a three prog cord found at Lowes etc.

First, disconnect the black wire, one of the "hot" connections, from the left side post by removing the terminal screw. This is one-half of the 240-volt connection.
Next, disconnect the red wire, another of the "hot" connections, from the right side post by removing the terminal screw. This is the other half of the 240-volt connection.
Then, disconnect the white wire from the center connection. It is used for the neutral connection.
Now, disconnect the green wire from the machine case by unscrewing the green screw.
The next step is critical to return your electrical connection to factory condition before installng the 3-prong cord! Locate the white machine wire that is now connected to the neutral (center) screw terminal. Disconnect it from there and install it under the green ground screw.Connect the outer two wires (hot wires) to the outer two screws, obviously, one on each screw.
Now connect the center wire (neutral) to the center post. You'll notice the white machine wire is now the case ground connection, unlike the 4-prong cord connection in which the green wire from the cord makes the ground connection.

The old three prong, the vertical prong is neutral (white wire), the two slanted are 220 volts. The new plug, the extra prong is ground (bare wire). Shut off the outlet at the breaker panel, and remove the outlet to examine the wiring to it. It should be as above--the 220 volt wires are red and black or black and black. The ground wire is bare or green insulated. Inside the dryer case, tie the green and white wires to the dryer case, and tie the two outside wires where they went with the old cord. Check with an electrician if you are unsure about this.

The following link explains how:

http://www.fixya.com/support/r793520-3_prong_4_prong_power_cord_conversion

If you have questions, please let me know. I hope this helps you.