We've all done it. Walk into a building supply store and head for the electrical department to buy some electrical wire for a project that we have at home. In most cases, we have no idea what one type of wire is from the other. Even more, we couldn't tell the difference from 100-amp wiring and 200-amp wire, let alone the difference between 20-amp and 30-amp wiring. This simple set of facts is enough to make my hair stand up on the back of my neck, fearing the dangers of having an electrical fire at some point.
Maybe it's just to extend a circuit for an additional outlet or two, or maybe it's an underground feed to a swimming pool you just added? In any case, the point is that now you're at the store looking at the wire, which one is the right wire for you? What size wire do I need? Is color important?
Not only are there different types of wiring, both indoor and outdoor wiring, but also underground wiring and specific types, like heat resistant and corrosion-proof wiring. If that isn't enough to distinguish the differences, there is also copper and aluminum wiring. Each has its own unique wire sizes and ampacity-carrying capabilities.
Although aluminum wiring was used in the past, today's homes use copper wiring. Aluminum wiring often heated up under load, causing it to become loose in the connections and sometimes caused electrical fires. Those connections already in homes may work forever untouched and not overloaded, but I would advise an electrical upgrade by replacing the wiring to copper.
Wire coloring makes the wires look pretty, right? Well maybe, but the real reason for wire colors is to identify them for their intended uses. The green wire is used for grounding, always! In the past, and I've personally seen this while working on an old school project. The "hot" wires, feeding branch circuits, were actually green in color. I was taken aback! Everything I had been taught about wire colors was now a red flag, and caution was the word working on the circuitry.
When dealing with NM wiring, the colors are normally black and white for two conductor wiring and red, black and white for three conductor wiring. In most cases, the black or black and red are used as hot wires for feeds or traveling conductors. The white is usually neutral, but not in all cases. You see, sometimes the third wire, being the white wire, is used as a "hot' or "switch leg" in the circuit.
In these cases, you need to wrap the end of the wire with electrical tape to identify it as such. This will ensure that no one confuses it use and ties it into the neutral wires. Because just looking at wires is hard if not impossible to distinguish between the different sizes and types, wires have something special on its coating to tell us what the wiring really is.
To help you understand the correct choice, manufacturers have labeled the outer coating of the wire with types and gauges of wire. You see, the insulation covering the wire tells the story about the wire itself. Along with different sizes of wire, there are many types of wire used around the home. Knowing which type of wire to use is just as important as determining the proper gauge, amperage limit, and the maximum wattage load limit of the electrical wire you choose.