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Yeah, you could do that. Shut off the main breaker. Remove the 240 breaker pair and disconnect the wires to it. Replace with a 120 breaker of the same amperage. Attach hot wire (probably pick the black one, to keep color meaning clear and uniform in the box). As bill suggests, wrap some white tape around the red wire near the ends (in both breaker box and garage outlet) to mark it as the neutral wire.
Ground and red-now-marked-white-with-tape both get connected firmly to the ground strip in the box. Replace 240 receptacle in garage with 120 and connect wires there. Turn on main breaker and test.
I’m not an electrician. But that’s what they’d do.
PS- if it’s in the garage, maybe use a GFI outlet.
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I would pay the $300 on this one. I’ve been looking into messing with 220V outlets for a bitcoin mining project, and as I understand those outlets, you’re supposed to have two 120V hot lines, one neutral line, and (ideally but not necessarily if the house is older) a green ground line. If this installation is missing the neutral line, or if the house ground has been substituted for neutral, something’s amiss.
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There is no detailed map of your home's electrical system in existence, they simply do not keep details of every circuit, box or receptacle. If anything it would only be the utility company that had any records at all and that would only be the meter drop and meter location. As long as the work is done properly you will not have any issue and there will be nothing to worry about and no insurance issue ever.
You are talking about 20 minutes of work and it is well within the scope of DIY as long as you strive to do it right.
Change the breaker, if you have 12 gauge wire then you want a 20 amp single pole breaker of the same brand and type required by the panel.
Re-code both ends of the red wire to white using white electrical tape and attach the white wire to the neutral buss. If you need to add a piece of white wire to reach the buss, it is perfectly safe and legal to use a wire nut inside the electrical panel.
Change the 240v receptacle to the correct 120v receptacle. Black wire to the brass screw, white to the silver screw and green or bare to the ground screw.
Turning the main breaker off will usually kill most everything in the panel downstream of the main breaker. In some panels their could be some pass-thru lugs that remain energized but these are seldom found in residential panels. If in doubt TEST with a multi-meter/volt meter. Never rely on a non contact tester to prevent electrocution.