Here's a copy / paste of HR 727 which defines what a "Low-Speed Electric Bicycle" is. If anyone has any further documentation, especially for North Carolina, I'd love to see it.
Federal Electric Bicycle Law HR 727
SECTION 1. CONSUMER PRODUCT SAFETY ACT. The Consumer product Safety Act (15 U.S.C. 2051 et seq) is amended by added at the end of the following:
LOW-SPEED ELECTRIC BICYCLES
SEC. 38. (a) Notwithstanding any other provision of law, low-speed electric bicycles are consumer products within the meaning of section 3(a)(1) and shall be subject to the Commission regulations published at section 1500.18(a)(12) and part 1512 of title 16, Code of Federal Regulations.
(b) For the purpose of this section, the term `low-speed electric bicycle' means a two- or three-wheeled vehicle with fully operable pedals and an electric motor of less than 750 watts (1 h.p.), whose maximum speed on a paved level surface, when powered solely by such a motor while ridden by an operator who weighs 170 pounds, is less than 20 mph.
(c) To further protect the safety of consumers who ride low-speed electric bicycles, the Commission may promulgate new or amended requirements applicable to such vehicles as necessary and appropriate.
(d) This section shall supersede any State law or requirement with respect to low-speed electric bicycles to the extent that such State law or requirement is more stringent than the Federal law or requirements referred to in subsection (a).