When working with electrical cables, the markings on a cable sheath includes a set of two numbers separated by either ”-” or “/”, such as 12-2, 12/2, 14-2, 14/2, or any like combinations, the number on the left of the punctuation specifies the AWG gauge of the conductors inside the cable while the one on the right indicates how many conductors are inside the cable, but excluding the ground wire. Figures 1a, 1b, and 1c show those markings for three different types of cables.
The AWG wire sizing—The wire size or AWG gauge number has nothing to do with the amount of voltage applied through it but does specify the maximum amps or current that can be submitted to it without heating up, but there is another marking on the cable’s sheathing, however, that specifies the maximum voltage at which the cable is safe to operate—usually 300V or 600V. So if you’re wiring a line of 120 volts outlets requiring 20 amps distribution—around the kitchen counter, for instance where many appliances can run at the same time during peak periods—you’ll need 12/2 cables and 20 amps rated outlets—all outlets are not created equal. The 12 AWG wires will withstand that amount of current being of a larger diameter, where the 14 AWG wire being smaller will heat up over 15 amps and cause the wire or a 15 amp outlet to melt or catch fire.
Amps rating by wire size—Note that the AWG number decreases, the wire diameter increases.
14 AWG gauge wires can handle a load of no more than 15 amps.
12 AWG gauge wires can handle up to 20 amps at 120 volts if one wire is white for the neutral return line. If neither of the two wires is white, being black. red, blue, or another color, each should carry one side of the 240 volts line. So even with only two wires, beware if you come across a circuit with a feed that doesn’t have a white neutral present.
10 AWG gauge wires are for loads up to 30 amps, usually at 240 volts. Where the appliances they feed have internal circuitry running on 120 volts—light bulbs or controls—10/3 cables with one wire white should be used, but if 120 volts isn’t required, a 10/2 cable would be ideal and cheaper.
8 AWG gauge wires can handle up to 45 amps. Whether it’s going to be 8/2 or 8/3 here again goes by the same recommendations as above.
A 6 AWG gauge wire capable of handling 60 amps is better suited for feeding a sub-panel, a range, a double oven, depending on the amperage rating listed on the appliance. For any of these situations, it should be a 6/3 cable to also supply the 120 volts.
The number of conductors—As indicated in figures 1a, b, & c, there is always the number on the right of the AWG wire size that indicates how many conductors are inside a cable. That number of conductors does not include the ground wire which is an extra wire with the only purpose of providing a common ground reference to the house’s electrical.
Most of the wiring needed for a house is Romex 14/2 and 12/2 for the 120 volts circuits. For the 240 volts loads, however, it depends on the load’s circuitry if it requires a 12/2, 10/2, 8/2, or a 12/3, 10/3, or 8/3 cable.
A 14/3 and 12/3 cables have a black, a white, and another colored wire (red, blue, or other colors) for a more specialized use when hard wiring smoke and CO detectors, with that extra wire feeding the signal from a tripped alarm to all the other alarms around the house.
Another use of a 14/3 cable is to wire a three-way switch where two different switches can activate the same light fixture in a room. The third wire, called “traveler”, is used to ensure that there is always voltage at one or the other switch to power the light fixture.
“G”, “w/G”, or “with Ground” markings—on the cable’s sheathing mentions the ground wire but is not always used.
Absence of a white neutral wire—There is sometimes indications on jacket’s markings that limit which voltage a cable should carry. Looking at Figure 2 for instance, following the marking “2” is another marking indicating “RED-BLACK” which is the color of its two wires respectively. Knowing that a neutral “white” wire is required for the return path when wiring a circuit on 120 volts, such cables must only be used for 240 volts since colored wires are always for hot power lines.
“NM” or “Romex” markings—The type of cable to use around the homes is normally the NM (meaning non-metallic) and also called and marked as Romex. Figures 1a and 1b are both NM or Romex cables (Figure 3).
”NMD” or “NMD90” markings—The addition of the letter “D” to the NM marking refers to the use of that cable in dry locations only and furthermore, the number “90” refers to its heat rating at 90° C.
“CU” or “AL” markings—are also fully spelled out sometimes as COPPER or ALUMINUM refers to the material the wires are made with (Figure 4).
“U”, “UF” and “UF-Sunlight Resistant” markings—Sometimes when a feed from the house’s electrical panel is wired to an exterior load such as an exterior post lamp, a pump, outside outlets, or an adjoining structure, a UF cable (underground feeder shown in Figure 1c) is used if it’s to be submitted to humidity or water, and for that very reason should always be hooked up through a GFCI circuit breaker. The UF markings sometimes include “Sunlight Resistant”.
A ”B” or ”-B” marking—can be included after the NM and the UF cable markings instead of “D” to indicate a higher heat rating of 194° Fahrenheit.
Date or year marking—also complement the sheathing to give the date of its production.
So in the end, it’s just a matter of understanding what the coding says.