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Derating is necessary because multiple conductors carrying full-load power generate heat that may exceed the normal insulation temperature rating. (NEC 310.16) The NEC also specifies adjustments of the ampacity for wires in circular raceways exposed to sunlight on rooftops, due to the heating effects of solar radiation.
For example, the United States National Electrical Code, Table 310.15(B)(16), specifies that up to three 8 AWG copper wires having a common insulating material (THWN) in a raceway, cable, or direct burial has an ampacity of 50 A when the ambient air is 30 °C, the conductor surface temperature allowed to be 75 °C. A single insulated conductor ...
The table below assumes DC, or AC frequencies equal to or less than 60 Hz, and does not take skin effect into account. "Turns of wire per unit length" is the reciprocal of the conductor diameter; it is therefore an upper limit for wire wound in the form of a helix (see solenoid ), based on uninsulated wire.
Edison-base lamp sockets (called screw shell devices in the NEC) are required to have the neutral conductor attached to the outer screw shell [NEC 200.10(C)]. [5] In actual practice, the neutral terminal is silver colored, the line and load terminals are brass or (rarely) painted black), and the grounding screw is usually colored green.
The equation in section 310-15(C) of the National Electrical Code, called the Neher–McGrath equation (NM), may be used to estimate the effective ampacity of a cable: [3] = (+) (+), In the equation, T c {\textstyle T_{c}} is normally the limiting conductor temperature derived from the insulation or tensile strength limitations.
Certified and labelled for use in areas with specific hazardous conditions: for indoor and outdoor use in locations classified as Class II, Groups E, F, or G as defined in NFPA standards such as the NEC. 10: MSHA. Meets the requirements of the Mine Safety and Health Administration, 30 CFR Part 18 (1978). 11: General-purpose.
In Canada and the United States, the Canadian Electrical Code (CEC) and the National Electrical Code (NEC), respectively, use the circular mil to define wire sizes larger than 0000 AWG. In many NEC publications and uses, large wires may be expressed in thousands of circular mils, which is abbreviated in two different ways: kcmil [1] or MCM. [2]
The ampacity of a conductor, that is, the amount of current it can carry, is related to its electrical resistance: a lower-resistance conductor can carry a larger value of current. The resistance, in turn, is determined by the material the conductor is made from (as described above) and the conductor's size.