People have asked us—quite often, actually, “What is AWG or American Wire Gauge?” Besides the name, it helps to know the number gets lower as the wire size goes up! Before you run that electrical conduit, know what size wire to put inside it.
What is AWG: The Basics
It’s easy to be confused by wire gauge, especially since, as the number gets lower, the size of the actual wire goes up! AWG stands for American Wire Gauge and is a specification of sorts that gives specific wire dimensions for, among other things, electrical wires. The sizes (gauges) range from 0000 (“four aught” the largest) to 40 (the smallest). The term “wire gauge” refers both to how wire is made and the electrical resistance of the wire. You can figure that a wire doubles in size every 6 steps in gauge. In our desire to explain wire in terms of electrical use, however, we’ll deal mostly with a wire’s electrical resistance – more on that later.
Wire Gauge and Electrical Current
Since we’re talking about electrical wire, for all practical purposes wire gauge determines the amount of electric current it can safely carry, as well as its electrical resistance. Wire gauge can also, as a matter of simple calculation, tell you the weight of the wire per unit of length. This is because all current electrical wire is made primarily of copper.
What About Aluminum?
Aluminum wiring almost never rears its head in residential construction except for larger gauge stranded aluminum wire (larger than #8 AWG). However, homes built or remodeled during the ’60s and ’70s used it quite a bit as copper prices skyrocketed. Aluminum serves as a good conductor but has a lower ampacity (the point at which the current causes the cable to begin to melt down). It also expands and contracts more, and it corrodes. As you can imagine, those who have aluminum wiring in their homes might want to take a look at updating their system or at least having it inspected regularly.
The Consumer Products Safety Commission Weighs In
Aluminum wire was used for branch circuits starting in the early 1960s and used through the mid-1970s, largely because of the copper shortage during those years. Experience has proven, however, that solid aluminum wire is not nearly as reliable as copper wiring. Older aluminum wire is soft and has a greater tendency for thermal expansion than copper wire. The aluminum wire expands and the dimension changes as the current heats it. When the current stops and the wire cools, it contracts and leaves gaps between the conductor and the terminal. This process is referred to as “cold flow.” It is typically the cause of arcing and overheating.
Aluminum wire also oxidizes much more readily than copper wire. Aluminum oxide, which acts as an insulator, can also result in poor connections and cause arcing and overheating. It is always a good idea to have anti-oxidant paste on aluminum wire terminations.
American Wire Gauge, Current, and Cable Diameter
OK, back to copper wiring and wire gauge measurements. Here is how the gauge rating reflects cable size and determines the amount of current that can flow through the wire:
As you can see, the larger wire (lower gauge number) can support more current, and thus a higher breaker size.
AWG (American Wire Gauge) vs. SWG (Standard Wire Gauge)
The wire gauge system we use here is known as American Wire Gauge (AWG) or “Browne & Sharpe”. It was developed in the U.S. for the electrical industry and is designed for use with non-ferrous metals (non-magnetic metals with no iron). The number of the gauge corresponds to the electrical resistance of the wire. Thicker wire can have more electrons running along it, hence it has lower resistance and a lower gauge number. Conversely, thin wire carries fewer electrons, and it has a higher resistance and a larger gauge number.
SWG is a British-developed system, but before you write it off, realize that it’s also used in America—just for ferrous metals and non-electrical applications. Remember how we defined gauge in the beginning? Well, SWG gauge numbers (since they don’t apply to electrical cable) specifically correspond to how many times the metal needs to be drawn through a die plate to reduce it to the desired diameter. As the machine draws the wire through, it gets progressively thinner. This results in a higher SWG number.
The British made wire long before us… but we figured out first how to pass electricity across it on a regular basis and turned it into an industry.
Determining the Right Size for Electrical Circuits
Determining the correct gauge of wire for your application involves checking the amount of amps (current) you expect to pull on the wire. A standard residential home will have circuits with a breaker that is usually rated to 20 amps (15 for older homes and certain specific uses or 14 gauge wiring). If you draw more power than the circuit or cable was designed for, the circuit breaker is designed to “trip” or shut down power so that you don’t reach the ampacity (melting) point of the wire.
A rule of thumb is the 20% rule. Only use 80% of the wire capacity. This is a good way to compensate for variables like wire length, expansion, surges, etc. If you’re wiring up a house or adding a new circuit, dropping in 2/12 cable and a 20A breaker is most likely the way to go. Now, if you are adding an electric dryer, electric oven, or some other high current device then you may need to use a compatible wire that meets the specifications for that product.
Hope this helps. Got questions? Post them in the comments below!