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2) A 120 volt duplex receptacle requires (1) one hot wire connected to the brass terminal of the receptacle, (1) one white neutral wire connected to the silver terminal of the receptacle and (1) one ground wire connected to the green screw on the receptacle.
3) Since you have metal conduit and assuming the metal conduit originates at your main electrical panel, you don't require a separate ground wire since the conduit acts as the equipment grounding conductor.
However, the receptacle will require a neutral wire and the neutral wire can only be white or gray insulated and no other colors allowed for the neutral. Yes, per the 2011 edition of the National Electrical Code, you will need a white or a gray neutral wire originating at the electrical panel neutral bus bar.
Whether you are wiring a duplex 120 volt receptacle or a plug-in cord that will be connected to a 120 volt receptacle, a white neutral wire is always required.
4) In addition to the wiring, the existing 240 volt double pole breaker will need to be de-installed at the panel and replaced with either 15 or 20 amp Single Pole breaker. The amperage of the breaker, whether 15 or 20 amp will be determined by the load.
5) Most likely your existing wiring for the water heater was using 10 AWG stranded copper wires. If re-using the 10 AWG hot', they will not properly fit under the brass screw terminal on a duplex receptacle since 10 AWG wire is thicker than the required 14 or 12 AWG needed. You will need to splice either a 14AWG or a 12AWG short jumper piece of copper wire in order to terminate on the receptacle brass screw.
6) A 15 amp breaker requires 14 AWG copper wire and a 20 amp breaker requires 12 AWG copper wire
7) It is recommended that no more than (8) eight receptacles be wired to a 15 amp breaker and no more than (10) ten receptacles be wired to a 20 amp breaker.
8) Refer to the link below for assistance in wiring a grounded duplex receptacle. This will be the 1st picture shown on the link.
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