Hi! I need to rewire the electrical connection. I'm wondering if a 20amps electric break is enough. Does anyone knows how many amps does it takes set on 125 F?

one of the elements has burnt out and needs replaced . call a local plumber to save you the headache of diy

follow this test and fix it. God bless you
If there is no hot water, first push the reset button on the high-temperature cutoff; test the thermostat and replace if faulty (see below). Test the heating elements and replace if faulty (see below).
If there is not enough hot water, adjust the thermostat settings. As needed, reset, test and replace thermostats and heating elements if faulty.
If the water is too hot, adjust the thermostat setting
An electric water heater is a 240-volt appliance that can deliver a fatal shock. Always turn off power to the heater at the electric service panel before beginning work, and verify that power has been shut off. Make sure others in the house know not to restore electricity prematurely. If you have any doubt as to whether electricity has been turned off, seek professional assistance.

Which delay timer do you have? 9 pin, 11 pin, etc?
What voltage are you using?
What is amp rating on timer?

Copy following links for general and specific wiring diagrams:
http://waterheatertimer.org/How-to-wire-pin-timers.html
http://waterheatertimer.org/images/10-pin-wiring2-1000.jpg
http://waterheatertimer.org/images/ON-delay-timer-diagram-900.jpg

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

You need three #8 wires for AE125. Two #8 wires for AE115.

1) Copy following link for wiring information:http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.html

2) The AE125 has 3 elements, and requires 3 wires.Each wire is sized #8 copper, and is connected to 40 amp breaker, up to 100 feet. Over 100 feet, use 50 amp breaker and #6 wire. Oversizing breaker and wire is smarted and more efficient than undersizing.

3) Often a separate subpanel is installed to provide 3 extra spaces.

http://waterheatertimer.org/images/Main-subpanel-Tankless-600.jpg


4) Makes sure main breaker panel can accept additional 120 amps of draw without overampping panel ... and starving other appliances of full power.... that will damage appliances, overheat motors, and damage main panel and overheat main breaker. You want to oversize, not undersize electric service when installing tankless electric, which is why I do not recommend tankless electric installation, including issues of costing more to operate, and expensive repairs, and general money risk-nuisance of owning tankless vrs tank-type heater.
http://waterheatertimer.org/Troubleshoot-household-electricity.html#stress


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

check all water pipes going in and out.tanks are sealed except for these connections.

OK well at 120 Volts at say 25 Amps (well below blowing potential) that equals 6000 WATTS 6KW, on a near short circuit. going through it...You would melt TOO.
I always use Ceramic ones myself, look they get HOT real HOT, BUT shouldn''t melt.
But in saying that your Electric shower shouldn't draw that much anyway, in reality. it may draw 10A - 15A, at most? more like 5A - 10A,
So i suspect you have a fault to earth somewhere you need to call an electrician. You may just have discovered a very bad fault. Some sort of short or leakage to earth or something?
A shower shouldn't draw anywhere near enough current to cause this type of occurrence. Please disconnect it by tripping the breaker and call a professional. get that melted one changed anyway.

its 3 phase not 4 Your wired wrong one of those should be a nuetral or ground

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

go to rinnai web site they have all the descriptions of what is there and how it is together, i hope this helps you out in your venture