I currently have a 220 volt compressor outlet that I want to convert to a 4 prong outlet to connect my generator. I have the new 4 prong outlet, a new 4 prong generator cord, and a circuit breaker panel interlock kit. Question is, the current 220 compressor outlet had 3 wires, but the new plug has 4 prongs. How or can I use safely the 3 wires with the new 4 wires outlets that for my generator cables and outlet? Thanks!

Sounds like the old cable was connected to two phase conductors and ground...
You cannot use the existing three conductor cable,
You need a (4) conductor cable for connection to the supply Neutral...

Thanks for your response.

Actually the 220 line for the compressor was run, but never hooked up. It just has the box, but no outlet was ever installed.

There are 3 wires inside the box, hooked up to a 30 Amp double pole breaker in the panel. All I would have to do is install an outlet.

My question is, since the run is currently a 3 wire (2 hot and 1 ground), how can I either convert or use the current 3 wires for a generator 4 wire plug and cable outlet? What do I do with the neutral on the plug if there isn't a 4th wire?

Thanks again!

As you have indicated the generator was designed for a (4) wire connection...
To install this properly you will need to install a fourth conductor...

You cannot do this without the neutral...

Thanks again!

How do I install a 4th? Can I run an 8 gauge wire through the conduit? And is so, where on the circuit breaker box do I connect the 4th? I would assume a neutral bar?

What if there is weren't a neutral bar, how else could this be done?

If the overcurrent device for this generator circuit is 30 ampere, #10 awg would be fine.

... the fourth wire is most likely a neutral, so yes on the neutral bar...

If there is no neutral you do not (will not) have any 120 volt outlets..

Thanks much! Very helpful.