Yes, they are required by the national electric code.
It does not
It is a electrical code rule taken from the National Electrical Code book. It has to do with multi wire branch circuits.
It is what the electrical code book in the United States is called. In questions and answers it is usually shortened to the letters NEC.
The most current edition of the National Electrical Code. At this time it is "NEC 2008."
National Electrical Code
role of national electric code in the design of electrical installation
2008 National Electrical Code - Table of Contents - Chapter 6 Special Equipment - Article 668 Electrolytic Cells
NFPA 70
Yes, they are required by the national electric code.
It does not
It is a electrical code rule taken from the National Electrical Code book. It has to do with multi wire branch circuits.
In the USA, the National Electrical Code is NFPA 70. There are several national fire codes, including National Fire Code (also called Uniform Fire Code); NFPA 1 (National Fire Protection Association) and the International Fire Code (International Code Council). Each of these references hundreds of other codes and standards, including Life Safety Code, Building Code, Electrical Code, etc.
It is what the electrical code book in the United States is called. In questions and answers it is usually shortened to the letters NEC.
National Fire Protection Association (NFPA).
National Electrical Code (NEC)
The most current edition of the National Electrical Code. At this time it is "NEC 2008."