What do wire gauge and AWG, BWG, SWG mean?

What is wire gauge?

Wire gauge is a universal standard used to specify the thickness of wires by describing their cross-sectional dimensions. There are four main types of wire gauges: metric wire gauge, British wire gauge (SWG), Birmingham Wire Gauge (BWG) and American Wire Gauge (AWG). China has opted to utilize metric wire gauges, while the United States tends to rely on the American Wire Gauge (AWG) measurement system. This article will present each wire gauge in detail.

Types of wire gauge

1. AWG

The American Wire Gauge (AWG), also known as the Brown & Sharpe wire gauge, is a standardized system for categorizing wire diameters. This gauge system has been used in the United States since 1857. Thick wires have better physical strength and lower resistance, but thicker wires require more copper to make the cable, making it heavier, more difficult to install, and more expensive. The challenge for cable design is to use wires with the smallest possible diameter to reduce cost and installation complexity while ensuring that the wires can handle the necessary voltage and frequency limits.

The use of wire numbers predates the official definition of measurements in 1735. It's worth noting that wire billets were originally made by forging as rolling and extrusion techniques had not yet been developed 300 years ago. Additionally, the measuring tools at that time were quite primitive. It is believed that these numbers were originally based on a series of drawn wires, with No. 0 representing the original rod and each subsequent number corresponding to the number of times the wire had passed through the draw plate. For instance, No. 10 would have been drawn through the plate ten times. In other words, every pull would add an extra number, and the wire blank would become a higher number accordingly. As the numbers increase, the wire becomes thinner.

2. BWG

Birmingham Gauge, which is sometimes referred to as the Stubs Iron Wire Gauge or Birmingham Wire Gauge, is not to be confused with the Stubs Steel Wire Gauge, although they are similar in nature. Whilst the Birmingham gauge is often abbreviated to G, it is important to note that this is different from the French gauge.

The gauge system ranges from the lowest gauge number of 5/0 or 00000, which corresponds to the largest size of 0.500 inches (12.700 mm), to the highest gauge number of 36, corresponding to the smallest size of 0.004 inches (0.102 mm). The size steps between gauges vary, ranging from 0.001 inches (0.025 mm) between high gauge numbers to 0.046 inches (1.2 mm) between the two lowest gauge numbers. Although there is no particular mathematical pattern, generally the steps decrease as the gauge number increases. When referring to wire or fine tubing, the gauge number specifies the outside diameter of the product, while for larger mechanical tubing, the gauge number specifies the wall thickness regardless of the overall size of the tube.

3. SWG

The British Standard Wire Gauge (commonly abbreviated to SWG or Standard Wire Gauge) was formerly utilized to indicate wire size according to BS 3737:1964 (which has since been withdrawn). It was also referred to as the Imperial Wire Gauge or British Standard Gauge. Though SWG sizes have lost much of their popularity, they are still employed in assessing the thickness of guitar strings and certain electrical wiring. Today, the more typical size measurement for wires employed in electrical installation cables is the cross-sectional area in square millimeters. The current British Standard for metallic materials like wire and sheet is BS 6722:1986, which solely adopts the metric system of measurement.

4. Metric wire gauge

None of the above systems of measurement is part of the metric system. The IEC 60228, which is widely adopted globally, provides a set of standard wire sizes determined by their cross-sectional areas measured in mm². In engineering contexts, it is often more practical to describe a wire in terms of its cross-section area than its diameter. This is due to the fact that cross-sectional area is directly correlated with the wire's strength and weight, and inversely correlated with its electrical resistance. Additionally, the cross-sectional area is indicative of the maximum safe current capacity for a metallic wire.

Among other things, IEC 60228 defines a set of standard wire cross-sectional areas: 0.5 mm²; 0.75 mm²; 1 mm²; 1.5 mm²; 2.5 mm²; 4 mm²; 6 mm²; 10 mm²; 16 mm²; 25 mm²; 35 mm²; 50 mm²; 70 mm²; 95 mm²; 120 mm²; 150 mm²; 185 mm²; 240 mm²; 300 mm²; 400 mm²; 500 mm²; 630 mm²; 800 mm²; 1000 mm²; 1200 mm²; 1400 mm²; 1600 mm²; 1800 mm²; 2000 mm²; 2500 mm².

Wire gauge specification comparison table: