I was asked to diagnose a problem with a water heater; I discovered 1 of 2 elwments not working, I measured the amp draw of the working element at 18. 36 amps, the elements were the same wattage from the factory, the upper reset switch had a the lower element shut down, if I were to replace the reset ; both elements would draw 37.76 amps. The wire is 10 gauge romex with ground ; the breaker is a 30 amp federal pacific breaker, what should I do?

HW heaters are designed to only operate a single element at a time. I have 2X5000 and it is on a 10/2 with a 30 double. When the upper t'stat goes off, the bottom will come on until full temp and then they are both off. .

both elements would draw 37.76 amps

How did you arrive at that number?

What are the symptoms of the water heater?