The National Electrical Code was developed by, and is still maintained by the National Fire Protection Association (NFPA). A number of years ago the code was updated for photovoltaic and wind power systems based on research done at Sandia National Labs in New Mexico.

A good start on understanding the NEC code is this document called "Photovoltaic Power Systems and the National Code". It was written by John Wiles, one of the main researchers at Sandia, and is an expanded version of the section of the code that deals with photovoltaic systems. The basic idea of the NEC code is to avoid fires, and other associated problems caused by insufficient wiring.

The NEC code document itself costs about $65, and is available from the National Fire Protection Association. Although the actual code as published needs to the one you should use for all design decisions, here is a copy of a rough draft of the code that can give you an idea what the NEC actually looks like. Warning, this is a big document.