There is a slightly warm wire coming from the home fuse box. We replaced all of the parts in the old hot water heater, including the wire to the fuse box. A new wire was run from the box with a new circuit breaker. Everything to code and the new wire is still a bit warm all of the way to the water heater about 15 feet away. Am thinking about buying a new hot water heater, but am concerned about what would make even the replacement wire a bit warm.

Thanks for your help

Is this a job you did yourself? Was it INSPECTED? If not the question should be directed to who did the job. While wires under load can feel warm to the touch, they can be checked as a simple loose connection can cause excessive or uneven heat. Or the appliance is drawing too much power. It should be checked and verified if it's a concern.

You didn't give any specs or descriptions or testing results so hard to say if it's just you noticing a new install, or something serious or defective in some way.

Did you replace the old element with the same wattage element? Some people think they can get hot water faster with a higher wattage element. Yes you will but can overload the existing wire. What is the breaker rating and what is the AWG of the wire?

I'm wondering if they have a Water Heater that requires a 30 Amp Circuit, but is on a pair of #12's. May never know since I saw no reply to your answer.