According to to ohm's law, if your resistance increases and voltage stays the same, then amperage will increase in direct proportion. So if your system has too much resistance, then your amperage is going to be too high and things will start getting hot.
High resistance could be in a loose connection, bad ground, corroded wire/connection, gauge/thickness of wire too small, broken wire strands, etc.
You also have to take into account that voltage is increasing so amperage increases as well.
If you at a look at my write-up, I used 10ga. wire to and from the BAP. When you start increasing voltage, you need heavier gauge wire to flow that extra amperage, especially if you are running it from the front of the car, like most people do (that's why I mounted mine in the back, as close to the fuel pump as possible). You may just be overpowering your wiring and causing it to get hot.
Also, check the wiring at the BAP unit itself. When I got mine (used) it had broken strands and loose wiring right out of the unit. I opened it up and resoldered some new, heavy gauge wire at the transformer.
If you have burned through 2 units, you need to start looking at your installation
