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J. H. Neher and M. H. McGrath were two electrical engineers who wrote a paper in 1957 about how to calculate the capacity of current (ampacity) of cables. [1] The paper described two-dimensional highly symmetric simplified calculations which have formed the basis for many cable application guidelines and regulations.
The electrical resistance of a uniform conductor is given in terms of resistivity by: [40] = where ℓ is the length of the conductor in SI units of meters, a is the cross-sectional area (for a round wire a = πr 2 if r is radius) in units of meters squared, and ρ is the resistivity in units of ohm·meters.
Conductors installed so that air can freely move over them can be rated to carry more current than conductors run inside a conduit or buried underground. High ambient temperature may reduce the current rating of a conductor. Cables run in wet or oily locations may carry a lower temperature rating than in a dry installation. A lower rating will ...
In electromagnetism, current density is the amount of charge per unit time that flows through a unit area of a chosen cross section. [1] The current density vector is defined as a vector whose magnitude is the electric current per cross-sectional area at a given point in space, its direction being that of the motion of the positive charges at this point.
The resistivity can be expressed using the SI unit ohm metre (Ω⋅m) — i.e. ohms multiplied by square metres (for the cross-sectional area) then divided by metres (for the length). Both resistance and resistivity describe how difficult it is to make electrical current flow through a material, but unlike resistance, resistivity is an ...
The current British Standard for metallic materials including wire is BS 6722:1986, which is a solely metric standard, superseding 3737:1964, which used the SWG system. The IEC 60228 , used in most parts of the world, defines standard wire sizes based on their cross-sectional areas as expressed in mm 2 . [ 3 ]
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is the length from to a fixed point M on the conductor, ε 0 {\displaystyle \varepsilon _{0}} is the vacuum permittivity . Using this method, the self capacitance of a conducting sphere of radius R {\textstyle R} in free space (i.e. far away from any other charge distributions) is: [ 2 ] C = 4 π ε 0 R . {\displaystyle C=4\pi \varepsilon _{0}R.}