Different regions of the world have different systems and conventions for measuring wire (conductor) size. The European system uses square millimeters of cross-sectional area, while the marine industry in North America uses the American Wire Gauge (AWG). The North American automotive industry has something different!
Origins of the AWG Gauge System
The AWG system has its roots in the manufacturing of jewelry in ancient Rome. Fine gold wires were made by pulling gold rods through dies with a small tapered hole, with the inlet to the taper large enough to accept the rod. This process is known as drawing wire. It reduces the diameter of a rod or wire by a certain amount, stretching the rod or wire in the process. To reduce a large rod to a small wire without breaking the wire, the wire must be pulled through a series of ever smaller dies. When a wire is given a gauge number (e.g., 18 gauge) it refers to the fact that it has been pulled through this number of successive dies to reach this size. Over the years, there were multiple iterations of this system, with many wire-making companies having their own set of dies and proprietary gauge numbering system. The British finally standardized this.
Evolution of the AWG Gauge System
The AWG gauge system in North America, derived from the British, was standardized in the 1850s, based on the dies used by a particular U.S. company. The bigger the AWG number, the more dies the conductor has been pulled through, and the smaller the conductor. We end up with a counter-intuitive result: as the wires get smaller the numbers go up!
The gauge system was introduced before the advent of machinery. There was a limit to the size of the largest rod that a person could pull through a die to start the size reduction process. This rod was given the number ‘0’. The first die a rod is pulled through is number ‘1’, and so on. With modern machinery it is possible to begin the process with much larger rods and correspondingly larger dies, but there are no numbers left to give to these dies! As a result, we stack zeros. The AWG system goes up to four zeros (resulting in ‘0000’, or 4/0, or ‘four aught’ conductors), and the British system up to seven zeros. A ‘two/aught’ (00, or 2/0) conductor is larger than a ‘one/aught’ conductor (0, or 1/0), whereas a two-gauge conductor (AWG 2) is smaller than a one-gauge conductor (AWG 1).
The Metric System
The AWG system is used extensively in North America, especially in the marine industry. In Europe and many other countries in the world, conductor sizes are measured in terms of their cross-sectional area – i.e., the physical size of the copper conductor. This is typically expressed in terms of square millimeters (mm²) but sometimes instead of area the conductor’s diameter is used, expressed in millimeters. Either way, the bigger the number the larger the conductor, and the number itself accurately expresses the actual physical dimensions of the copper conductor. It makes a lot more sense than using gauge sizes! In all cases, the AWG number or the millimeter number refers to the dimensions of the copper in the conductor and does not include any insulation.
Unfortunately, there is no direct correlation between AWG and metric conductor sizes, and no formula that converts one to the other. The only way to compare them is to create a table with the AWG sizes, and what each one translates into when converted to mm². It will be found that this almost never results in whole numbers in terms of mm², and almost never exactly matches metric wire sizes. This can make it a little tricky when converting conductor ampacities and overcurrent protection device ratings from one system to the other.
Conclusion
Despite its counter-intuitive nature the AWG gauge system has a rich history that has evolved over time. It is not going away any time soon! Understanding this system is crucial to proper conductor sizing in the North American marine industry.
To learn more about selecting the right conductors for your boat, check out our wire sizer and our Boat Electrics 101 course!
