I need the diagram

your manual is located here
http://manuals.adventurerv.net/Atwood-Water-Heater-Service.pdf
http://manuals.adventurerv.net/Atwood-Water-Heater-Service.pdf

Copy following links for wiring electric water heater:
http://waterheatertimer.org/How-to-wire-water-heater-thermostats.html
http://waterheatertimer.org/images/WH-w-combined2-1000.jpg

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Is this electric water heater?
Does it have 1 or 2 elements?
http://waterheatertimer.org/How-to-wire-water-heater-thermostats.html
http://waterheatertimer.org/images/WH-w-combined2-1000.jpg
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Open following links for typical wiring illustrations:
http://waterheatertimer.org/images/Water-heater-wiring-w-numbe.jpg
http://waterheatertimer.org/How-to-wire-water-heater-thermostats.html
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

Links below show specific wiring diagrams and explain 240V with two hot wires vrs 120V with hot and neutral wires.

Then if you have more questions, add a comment and I will fill in blanks:
http://waterheatertimer.org/How-to-wire-water-heater-thermostats.html
First diagram will be for your water heater.

http://waterheatertimer.org/B220C.html

Add a comment for more free help.
Also take advantage of fixya expert assistance live.
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F76 is a communication error. It usually occurs if you connect or disconnect a quick connect cable or remote control when the tankless heater is connected to electricity. (If you have a quick connect system, make sure you only have 1 remote and 1 quick connect cable.) To get rid of the code, try disconnecting electricity for 20 seconds, reconnecting electricity, and turning the remote on. If the code returns, you may need to replace your quick connect cable, remote cable, or even remote control

try this diagram ..


Please vote !!

this should help, althought they don't list first alert, they have several different wiring diagrams available.

http://waterheatertimer.org/Compare-13-electric-water-heater-timers.html

According to the user manual below, it's a 50 amp heater, therefore, you need 6 AWG electrical cable and a 50 amp double pole (220 volt) breaker in the main service panel.

You need two hot wires and a ground wire. The cable would be specified as 6/2 w/ ground. The 2 hot wires in the cable would be red & black or black & black -- these attach to L1 & L2 respectively. ( L = Line )

More info, let me know.

Charlie

http://ecx.images-amazon.com/images/I/A1b%2BxE2vITS.pdf