This is a typical and very incorrect assumption. The wire must be rated at the full 60A. You see, there is something called the 80% rule. On a continuous load circuit the load cannot exceed 80% of the wire
rating. So then on a 55A wire the maximum is 44A, which is not acceptable. Here is a chart with options you can pick from:
Ampacity is the maximum current that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. Cerrowire's ampacity chart helps calculate the load requirement for a circuit.
www.cerrowire.com
BTW, I am pretty sure that anything other than the NM-B options must be in a conduit.