depending on the particular switch you have, it'll have a resistance on it that cuts the voltage down when you have it on less than full speed - heat is how the excess juice is 'thrown away' by that resistor
shouldn't be a problem if everything is in good condition with original gauge wires, and the only thing you might look at is if all the wire connections are good and tight, as loose connections also generate heat, and make surre you have a fuse on the circuit
Bill
