The National Electric Code, also knows as NFPA-70, is the standard that building officials use to define what is legal to use in electric services of all kinds. NFPA is the National Fire Protection Association. The Electric Code began almost as soon as Edison wired some of his new lights in New York, and the first electricity caused fire happened. Soon thereafter the National Fire Protection Association began writing safety codes aimed at preventing fires and promoting safe wiring techniques. Today, more than a century later, the Electric Code is republished every 3 years. As new inventions and new applications for electricity are ever expanding the code book expands and is revised to define the new applications. The 2005 edition has been updated to include recent developments in available materials and equipment. The code has 772 large pages detailing every electric installation from common house wiring to installation requirements for 35,000 volt transformers. The tables detailing how many conductors you can put in a conduit go on for 50 pages in the appendix.
The problem most people have with the code comes from its complexity and scope. If you are a handyman, or even an electrician, it takes years of study to learn all its requirements, and even then they change it on you. It can be helpful though. One year I was installing wire for a welder in my garage. I found a paragraph that allowed me to derate the current load of the conductors based on the duty cycle percentage of the welder. I saved quite a bit of money on heavy wire that wasn't needed, and had a safe installation. The code is so long and complex that several other books have been written to help professional and amateur electricians understand the code, translating it's stilted language and detailed requirements into plain English for common applications. The "Illustrated Guide to the National Electric Code," by Charles R. Miller, is one of the better ones. There are also numerous how-to handbooks on electric wiring based on the code. For example, "Wiring Simplified," by Richter and others. is an easy do-it-yourself guide. However, even an illustrated guide book is not the code. When the electric inspector comes out to review your work you need to know that you wired it correctly. You can bet he's read the code and will be approving or disapproving your work based on its requirements.
Over the years I've had to repair and straighten out many half-baked and outright dangerous electric installations. Many of them were done by well meaning men who just didn't know how electric wiring was supposed to be done. I recommend that everyone doing wiring from homeowner to seasoned electrician, to electric engineer ought to own a current copy of the National Electric Code. The 2005 edition will be current until 2008.