GS 20-124 (f) Every semitrailer, or trailer, or separate vehicle, attached by a drawbar or coupling to a towing vehicle, and having a gross weight of two tons, and all house trailers of 1,000 pounds gross weight or more, shall be equipped with brakes controlled or operated by the driver of the towing vehicle, which shall conform to the specifications set forth in subsection (e) of this section and shall be of a type approved by the Commissioner.
4000lbs by law.
But of course the size of truck vs trailer multiplied by the distance towed and how many bus loads of nuns you will pass should be accounted for.
Duane