Learning The American Wire Gauge System
Wires are sized according to the American Wire Gauge System, AWG. Thicker, heavier wires can carry more current and have less resistance, and the thicker the wire, the lower the gauge number. For example, a number 8 wire is thicker than a number 12 wire and has less resistance to current.
The size of the wire chosen for a given application depends on the distance from the voltage source to the load as well as the current draw of the load. See the accompanying table.
Current can fail to flow if the conductor is the wrong size, material, length, and temperature.
The following factors cause variation in a wire’s resistance:
• Material - The material is important because some materials are better conductors than others. For example, silver is one of the best conductors available because it naturally has a very low resistivity (1.59 x 10-8 Ω·m at 20 °C) but it is never used because of its cost. Copper and aluminum are not nearly as expensive with resistivity values that allow for use in a household circuit (1.7 x 10 -8 Ω·m, and 2.8 x 10-8 Ω·m at 20 °C respectively.)
• Size — The size of the wire matters because the larger the diameter of the wire, the lower the resistance. More electrons can travel in a larger space. When a wire is too small, it’s a little like the electrons bottleneck, and the resistance can cause overheating, potentially resulting in burned wires or a fire.
• Length — The length is important because the longer the wire, the higher the resistance. That’s why larger wires are chosen for longer distances. In fact, the resistance of the wire is directly proportional to its length, so a 50-foot wire has 5 times more resistance than a 10-foot wire. Resistance occurs as the result of collisions between charge carriers and the atoms of the wire. There’s likely to be more of these collisions on a longer wire. If the wire is too long, the voltage will drop and the load will not have the power it needs to operate.
The wire needs to have a low enough resistance to carry the current to the load while maintaining the proper voltage, with no greater than 10 percent fluctuation in the voltage.
• Temperature — The temperature of the material matters because hotter materials cause greater resistance. That is because at higher temperature, the atoms in the wire vibrate more. Electrons flowing through this wire are more likely to collide, which makes the resistance higher. For this reason, circuits require proper ventilation because heat buildups can cause malfunctions.
Wires are covered by insulators to prevent electrical shock from accidental contact with live wires. In the United States, these insulators are color coded to indicate which wires are which. The power wire is the wire from the voltage source, and is generally red or black. The neutral wire is the wire that runs from the load back to the source and is usually white, grey, or any color with 3 white stripes. Protective ground wires must be green. Bonding wires are bare No. 8 copper wires.
There are different classes of insulation that are suitable for conduits and the temperature the material can withstand when the wire heats up. If the wrong class of insulation is used, it can melt, a dangerous situation.
Another level of protection, conduit, or raceway for wiring is piping or tubing designed to enclose and protect the wires. It comes in both rigid and flexible forms and is made from different materials depending on the particular application. For example, it can be made of plastic or corrosion-resistant metal.
Local building and electrical codes regulate the type of conduit that is used for specific applications as well as how it should be installed. When work has been completed, it should be inspected for safety as well as compliance to local codes.
Figure 1: Wire gauge for measuring thickness of wires