British Standard Wire Gauge (often abbreviated to Standard Wire Gauge or SWG) is a unit for denoting
wire size given by
BS 3737:1964 (now withdrawn). It is also known as the Imperial Wire Gauge or British Standard Gauge. Use of SWG sizes has fallen greatly in popularity, but they are still used as a measure of thickness in guitar strings and some electrical wire. Cross sectional area in square millimetres is now the more usual size measurement for wires used in electrical installation cables. The current British Standard for metallic materials such as wire and sheet is BS 6722:1986, which is a solely metric standard.
History
SWG was fixed by
Order of Council
An Order of Council is a form of legislation in the United Kingdom. It is made by the Lords of the Privy Council (in practice, ministers of the Crown).
Orders of Council differ from Orders in Council in that, while Orders in Council are orders ...
August 23, 1883. It was constructed by improving the
Birmingham Wire Gauge. It was made a legal standard on 1 March, 1884, by the British
Board of Trade
The Board of Trade is a British government body concerned with commerce and industry, currently within the Department for International Trade. Its full title is The Lords of the Committee of the Privy Council appointed for the consideration of ...
. SWG is not to be confused with
American wire gauge, which has a similar but not interchangeable numbering scheme.
Standard
A table of the gauge numbers and wire diameters is shown below.
The basis of the system is the
''thou'' (or ''mil'' in US English), or . Sizes are specified as wire diameters, stated in thou and tenths of a thou (mils and tenths). The wire diameter diminishes with increasing size number. No. 7/0, the largest size, is (500 thou or ) dia., No. 1 is (), and the smallest, No. 50, is ( or about ).
The system as a whole approximates an
exponential curve
Exponential growth is a process that increases quantity over time. It occurs when the instantaneous rate of change (that is, the derivative) of a quantity with respect to time is proportional to the quantity itself. Described as a function, a q ...
, plotting diameter against gauge-number (each size is a approximately a constant multiple of the previous size). The weight per unit length diminishes by an average of approximately 20% at each step. Because the weight per unit length is related to the cross sectional area, and therefore to the square of the diameter, the diameter diminishes by approximately 10.6%:
:
However, the system is
piecewise linear, only approximating the exponential curve loosely. Thus, it runs in constant steps of () through the range No. 49 - No. 39 and of () through No. 39 - No. 30.
See also
*
Wire gauge comparison chart
*
IEC 60228, the metric wire-size standard used in most parts of the world.
*
Circular mil, Electrical industry standard for wires larger than 4/0.
*
American Wire Gauge (AWG), used primarily in the US and Canada
*
Stubs Iron Wire Gauge
*
Jewelry wire gauge
*
Body jewelry sizes
*
Electrical wiring
*
Number 8 wire, a term used in the New Zealand vernacular
References
{{reflist
Wire gauges
3737