Search results
Results From The WOW.Com Content Network
The most common residential and small commercial service in Canada and the U.S., single split-phase, 240 V, features a neutral and two hot legs, 240 V to each other, and 120 V each to the neutral. The most common three-phase system will have three hot legs, 208 V to each other and 120 V each to the neutral.
The neutral conductor of a 3-phase, 4-wire system and the middle conductor of a 2- phase, 3-wire system must have at least 2 separate and distinct earth connections with a minimum of 2 different earth electrodes to have a satisfactory earth resistance; The earth electrodes must be interconnected to reduce earth resistance
Where two voltages are given below separated by "/", the first is the root-mean-square voltage between a phase and the neutral connector, whereas the second is the corresponding root-mean-square voltage between two phases (exception: the category shown below called "One Phase", where 240 V is the root-mean-square voltage between the two legs of a split phase).
However, it is a common misconception that "hot" conductor colour-coding is required by the Code. In the United States, colour-coding of three-phase system conductors follows a de facto standard, wherein black, red, and blue are used for three-phase 120/208-volt systems, and brown, orange or violet, and yellow are used in 277/480-volt systems.
Two-phase power can be derived from a three-phase source using two transformers in a Scott connection: One transformer primary is connected across two phases of the supply. The second transformer is connected to a center-tap of the first transformer, and is wound for 86.6% of the phase-to-phase voltage on the three-phase system.
This allows one to choose single-phase AC power at either 110–120 volts between phase and neutral or 220–240 volts between phase and phase. Since these two modes do not need three phases there is also a dark yellow-orange four-pin connector available designed for a single-phase 110–120 or 220–240 volt load.
The National Electrical Code (NEC), or NFPA 70, is a regionally adoptable standard for the safe installation of electrical wiring and equipment in the United States. It is part of the National Fire Code series published by the National Fire Protection Association (NFPA), a private trade association. [1]
The process for obtaining a formal interpretation is outlined in the front of the code book. The NESC is currently published on a 5-year cycle. Urgent safety matters that require a change in between code editions are handled through a Tentative Interim Amendment (TIA) process. Original work on the NESC began in 1913.