A) 5 terminal ET70115C.
Voltage selector set to 240.
B) BUT you say you have 120Volts from breaker >>> confusing, so I'm proceeding on basis that you meant to say 240V from breaker.
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C) Review timer:
5-terminal ET70115 timer has NO and NC terminals
NC is normally closed >> this terminal has power when timer is OFF
NO is normally open >> this terminals has power when timer turns ON
For water heater, you want NO terminal 4 so water heater turns ON when timer turns ON
D) Wire for 240Volt water heater
Two hot wires from breaker connect to terminals 1 and 2. This powers timer clock.
Jumper wire connects between terminal 2 to terminal 3. This sends power to the switch circuit.
Wires to water heater connect to terminals 1 and 4
E) 5-terminal timer turns off 1 hot wire to 240V water heater.
Turning off 1 hot wire will stop 240V circuit from working; Water heater thermostats also turn off one hot wire.
240V water heater has black and red wire.
Wire from terminal 4 should connect to Black wire on water heater.
Diagram shows two wires on terminal 1
Two wires on terminal 2
One jumper wire on terminal 3
One wire on terminal 4
Zero wires on terminal 5