What NPN transistor should I use?
You shouldn't use an NPN transistor for this project. There are parts out there that will work by an N channel, logic level MOSFET is a better choice. It's \$R_{DS}\$ in saturation will be very small, and it won't waste power as heat. Take a look at a part like this one. \$R_{DS}&lt;10m\Omega, P=I^2R = (5A)(10m\Omega) = 50mW\$ That is pretty good.

You'll implement it like this:







simulate this circuit &ndash; Schematic created using CircuitLab