Any idea what this line is at the top of an electric water heater?
The one that connects to both the hot and cold line
That would be a mixing valve, google it 
It mixes hot water with cold to prevent scalding.
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As has been stated, looks like this tempering valve…
Why wouldn’t you just turn the thermostat down at the heater itself?
Not the primary reason its installed in that location…
https://www.grainger.com/ec/pdf/Cash-Acme-Tank-Booster-Mixing-Valve-Brochure.pdf
It results in getting more capacity out of your water heater. Especially for those of us on load control programs.
Because if the water in the tank gets below (roughly) 22 degrees, you run the risk of forming the Legionella bacteria which causes Legionnaires’ disease. Keep the water in the tank at 140 degrees and the bacteria can’t form.
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Thanks guys
there was not one thing right with the electrical supply to that tank…
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Yeah, I know it was not secured properly
Just so you know it is 10 guage with a 30 amp breaker