Wire gauge is easy to confuse, especially because as the number gets smaller, the actual wire gauge gets bigger! AWG stands for American Wire Gauge and is a specification that gives a specific wire size for wire etc. Sizes (specs) range from 0000 ("the largest quarter") to 40 (the smallest). Wire gauge is a term that refers to the way the wire is made – i.e. the wire is drawn through a die. In theory, a higher gauge indicates the number of times the wire is pulled through the die to reduce its diameter. You can calculate that the size of the wire doubles in size every 6 steps.
People often ask us, actually, "What is AWG or American Wire Gauge?" Besides the name, it's also helpful to know that the numbers get smaller as the wire size increases! Before you run that conduit, know what size wire to put in it.
What is an AWG: The Basics
Wire gauge is easy to confuse, especially because as the number gets smaller, the actual wire gauge gets bigger! AWG stands for American Wire Gauge and is a specification that gives a specific wire size for wire etc. Sizes (specs) range from 0000 ("the largest quarter") to 40 (the smallest). The term "wire gauge" refers to how the wire is made and the resistance of the wire. You can calculate that the size of the wire doubles in size every 6 steps. However, when we wish to explain wires in terms of their electrical uses, we will primarily be dealing with the resistance of wires – more on that later.
Wire Gauge and Current
Since we're talking about wire, for all practical purposes the gauge of the wire determines the amount of current it can safely carry and its resistance. With a simple calculation, the wire gauge also tells you the weight of the wire per unit length. This is because all current wires are primarily made of copper.
What about aluminum?
Aluminum wire is almost never used in residential construction, except for larger gauge stranded aluminum wire (greater than #8 AWG). However, as copper prices skyrocketed, homes built or remodeled in the 60s and 70s made heavy use of it. Aluminum is a good conductor, but has a low ampacity (the point where the current causes the cable to start melting). It also expands and contracts more and corrodes. As you can imagine, those of you with aluminum wiring in your home might want to look at updating their system, or at least get it checked out on a regular basis.
CPSC Steps In
Aluminum wire was used in branch circuits from the early 60's until the mid 70's, mainly because of copper shortages in those years. However, experience has shown that solid aluminum wire is not as reliable as copper wire. Older aluminum wire is softer and more prone to thermal expansion than copper wire. Aluminum wire expands and changes size as it is heated by electric current. When the current stops and the wire cools, it contracts and leaves a gap between the conductor and the terminal. This process is called "cold flow". It is usually the cause of arcing and overheating.
Aluminum wire is also more prone to oxidation than copper wire. Aluminum oxide used as an insulator can also cause bad connections and cause arcing and overheating. It is always a good idea to apply an anti-oxidant paste to the ends of aluminum wire.
American Wire Gauge, Current and Cable Diameter
Ok, back to copper wire and wire gauge measurements. Here's how gauge ratings reflect cable size and determine the amount of current that can flow through the wire:
As you can see, larger wires (smaller gauge numbers) can support more current, hence the larger breaker size.
AWG (American Wire Gauge) vs. SWG (Standard Wire Gauge)
The wire gauge system we use here is called American Wire Gauge (AWG) or "Browne & Sharpe". It was developed in the United States for the electrical industry and is designed for use on non-ferrous metals (non-magnetic metals that do not contain iron). The number on the meter corresponds to the resistance of the wire. A thicker wire can have more electrons running along it, so it has a lower resistance and a lower gauge number. Conversely, thinner wires carry fewer electrons and have higher resistance and larger gauge numbers.
SWG is a system developed in the UK, but before you write it off, note that it is also used in the US – only for ferrous and non-electrical applications. Remember how we defined gauge in the first place? Well, the SWG gauge numbers (as they don't apply to cables) correspond specifically to how many times the metal needs to be pulled through the formwork to reduce it to the desired diameter. As the machine pulls the wire through it, it tapers. This results in a higher SWG number.
The Brits made wires long before we did…but we figured out how to regularly transmit electricity through wires and turned it into an industry first.
Determine the correct size of the circuit
Determining the correct wire gauge for your application includes checking the amperage (current) you expect to be pulling on the wire. The electrical circuit for a standard home usually has a circuit breaker rated at 20 amps (15 amps or 14 gauge for older homes and certain uses). If you draw more power than the circuit or cable is designed for, the breaker "trips" or shuts off the power so you don't reach the ampacity (melting) point of the wire.
A rule of thumb is the 20% rule. Use only 80% of wire capacity. This is a great way to compensate for variables like wire length, swelling, surges, etc. If you're wiring your house or adding a new circuit, you'll most likely have 2/12 cable and a 20A breaker. Now, if you're adding a hair dryer, electric oven, or some other high current appliance, then you'll probably want to use a product that's compliant specifications of compatible wires.
Hope this helps. Is there a problem? Post them in the comments below!