Cam, the iota spec sheet says max current is 11 amps. The specs also say it has an operating efficiency of 80% which would mean that the actual draw when the output is 45 amps @ 12 volts would be about 6 amps @ 120 volts. Here's a voltage drop calculator:

Voltage Drop Calculator .You want a voltage drop of 3% or less.

Note that we are talking 2 different types of loads, one inductive one resistive. With inductive loads the current increases as the voltage decreases. With resistive loads the current decreases as the voltage decreases. A battery charger is an inductive load and if the amperage is too high it will at best shorten it's life expectancy and at worst fry it. The water heater is a resistive load lower voltage and thus amperage will decrease it's efficiency.