S.C. Code Ann. § 56-35-30: Auxiliary power units.

(A) For the purposes of this chapter, operating an auxiliary power unit, generator set, or another mobile idling reduction technology as a means to heat, air condition, or provide electrical power, as an alternative to idling the main engine, does not constitute idling an engine.

(B) For a vehicle equipped with an auxiliary power unit designed for idling reduction, the gross vehicle weight or axle weight used to determine the fine for a violation of commercial vehicle weight restrictions is the actual gross vehicle weight or axle weight reduced by five hundred fifty pounds.