Had a 220 outlet for an oven that we had altered to 110 which means one breaker switch is not used. I want to revert to 220 for an electric oven. Old outlet remains. What should I see when I open it up and how can I revert to old outlet?

Welcome to the Forum.

A 240 volt circuit requires a double pole breaker in the panel, wire properly sized and an suitable outlet.

New installations will use a 4 wire configuration, but, you may have a 3 wire arrangement if it was an older job. While I suspect you could simply reconnect everything, upgrading to 4 wire would be an option.

Be careful.

Bob.

If you used the oven circuit for a standard 15 or 20 amp receptacle without changing the breaker to a 15 or 20 amp you made a huge mistake.
If you did the right thing and replaced the breaker with a 15 or 20 amp breaker then you need to change it back to the size for the oven, assuming the wire size is correct.
1. What size and pole breakers to you have in the panel now?
2. How many wires in the circuit?
3. What is the size of the wires, and are they copper or aluminum?
4. How many amps does the oven draw?

If it is a 120/240 volt load it must be a 4 wire circuit as mentioned, but if it was a 3 wire circuit installed and permitted under the pre-1996 NEC then its grandfathered in as long as you don't extend the wiring.

This was originally done year 2000. Breaker currently has two 20 amp switches, one of which is currently not in use. Figured it is the portion of the wiring not in use. Haven't check the back of the receptacle unit, yet to see how many wires, their size or metal. I don't have the amp requirements with me but will confirm when I get the paperwork out. Thanks for your help, guys!

If it was installed in 2000 it is probably a 4 wire circuit, assuming it was done with a permit. You must use a 4 wire receptacle. If the 1996 or newer NEC was in effect in your area when the circuit was installed and it had a 3 wire circuit for a 120/240 volt load it was not up to code when installed so it must be replaced. Most ovens require 120 and 240 volts, so you need 2 hots, an insulated neutral, and a grounding wire. Is it a range unit or just an oven, if its just the oven some are direct wired. A 4 wire feed is needed for a 120/240 volt load regardless of if cord and plug connected or direct wired. As you know, the amperage draw must be considered, and compared to the wire size and breaker size.