Nordyne 40 gal elec water heater recently started tripping the reset located on the upper coil on the actual unit (not the breaker in the fuse panel)

Your electrical testing probably not accurate, or upper thermostat is bad, or reset is tripped?
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#reset

http://waterheatertimer.org/How-it-works.html

Gene

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

Test elements.
http://waterheatertimer.org/How-to-test-water-heater-element.html
If elements test out, then replace both thermostats.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

More resources:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html
Otherwise, if you have some hot water, then power to unit is good and probably at least 80% rated voltage, breaker is good, and upper element is at least functional and probably working perfectly, reset button is not tripping so tank is not overheating.

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

Thermostat could be bad causing the breaker to trip due to it being shorted. It's either your breaker or the thermostat, more than likely the upper thermostat is bad.

it is possible it needs a reset. 1. turn off the breaker to make sure there is no power to the unit 2. open the top cover and there will be a red button like this http://www.tonyhoover.com/wp-content/uploads/2010/03/water-heater-upper-thermostat.jpg push it. It may have a plastic cover over or around it, leave this in place it is insulation for the wiring.
close it up carefully and then turn on the power and see what happens.

First a question. Did you fill the water heater until water ran freely at the hot taps.
If not you may have had an air pocket and blown an element. Water heaters will
always be shipped with the ECO in the reset position. The reset is a RED button
located under the upper cover for the thermostat. Shut off breaker to water heater
and remove this cover, and the fiber glass, then push the reset. Replace the fiberglas
and the cover securely and turn on power. You should actually hear the heater in most
cases. Remember that it takes about 45 minutes for a full tank of hot water from startup.

You posted under Rheem 40 gallon electric water heater, which is two-element 240Volt heater.
If this is not your heater, add a comment.

If you suddenly have no hot water, that narrows the suspect list.

Step1:
Check for tripped circuit breaker.
Open top cover on water heater and push in reset button on upper thermostat.
If reset button is tripped out, and will not push back in, then replace upper thermostat.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html
Put ear against tank and listen for bubbly-fizzing sound that says heater is ON.
If reset is not tripped, then move to step 2

Step 2: use tester or multi-meter and test across top two screws on upper thermostat for 240Volt.
If heater is not getting 240V, then test circuit breaker.
Move water heater wires from 240V breaker to another same-amp 240V breaker and see if that restores power.
You may need to replace breaker:
http://waterheatertimer.org/How-to-replace-circuit-breaker.html
If problem persists, then you might have a line problem between breaker and water heater.
Open cover on top of heater, and check connections on top of heater.

Step 3:
If heater is getting 240V, then upper thermostat or upper element is bad.
I suggest 30 minute troubleshooting which could save you a service call, and will be valuable information in the future:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, fixya expert speaks with you over phone while you work on timer.

If I understand correctly, you have electric water heater.
Heater suddenly stopped heating.

Four things cause sudden loss of water heater.
1) Reset button located under the top cover is tripped > turn off power, remove cover, press reset button > if reset keeps tripping, go to the troubleshooting link shown below.
2) Electric power to heater is interrupted > check circuit breaker > test for voltage > look for loose wire on breaker
3) Upper element burned out > test and replace element
4) Upper thermostat bad > replace upper thermostat

Reset button is best suspect.
Here is a link for complete troubleshoot and homeowner repair of electric water heater:
http://waterheatertimer.org/How-to-troubleshoot-electric-water-heater.html

did you turn the power on BEFORE you filled it with water???

this is a pretty common mistake. you will need to check the upper element to make sure its working.
most water heaters have an upper and lower themostat. the upper thermostat is the one we need to check.

use your volt meter...

turn the power off, test with your volt meter to make sure! remove one wire from the element, not the thermostat, and check resistance on the element. if no resistance, you burned the element, and will need to replace it.
replace with same wattage, usually 4500watts. it will be written on the element.
also, I like to push the reset button on the thermostat to make sure it wasn't tripped.

Problem could be a shorted element. Disconnect the wires from the breaker and try to reset it, remember to flip the switch in the fully off position before moving it to the on position. If the breaker doesn't trip then the problem is in the heater itself. If you have access to an Ohmmeter, with the power turned off check the continuity of the elements across the screw terminals and from the terminals to ground. There should not be a reading to ground and a high resistance across the terminal screws

need to know which breaker- house ele. panel breaker, which should be a min. of 50 amps,or the element breaker located inside either the upper or lower cover. If it is the element breaker/reset switch, this can be changed with great ease. Turn the power off and the ele. panel, check with a voltage meter to make certain power is off at the unit, loosen or remove the wires- making sure as to location of wiring, remove the switch and replace, reconnect wires, turn breaker back on,and you should be ready to go, This is assuming you have a 50 amp breaker in the electricial panel.