Electrical Wires and Cables: The Complete Guide
Electrical wiring is used in every circuit, from power transmission, to household electric service and electronics. At some point, every professional or hobbyist will need to select a wire or cable, or at least understand the basic requirements of wiring for a particular situation.
Wiring codes and regulations are designed to protect people and property. Before embarking on your next project, ensure that you have a good understanding of the wiring needed to ensure operational safety.
Choosing the Right Electrical Wire or Cable
There are a few steps that make choosing the correct wire or cable easy:
- Determine if you need a wire or a cable. Use a wire where you need a simple, single conduction path between two points. Use a cable when you need multiple conductors, such as a standard ‘hot’, ‘neutral’, and ‘ground’ wire setup to complete a grounded circuit.
- Determine the voltage, current, and power of the system that the wire will be used in. Wires and cables are rated for these factors so ensure that your chosen wire or cable is rated for higher parameters than what you expect to see in the complete circuit. Factor in resistive losses for longer wire or cable runs. Use these parameters to determine the right type and gauge of wire or cable to use. It’s often recommended to choose a thicker (lower gauge #) wire or cable to ensure lower losses and greater current carrying ability (ampacity).
- If selecting a cable for a special application such as outdoor or underground use, make sure you select the right type of cable. Standard nonmetallic (NM) cable, also called Romex, is designed for indoor residential use. UF, or underground feeder cables, are common for running power to a shed or garage, and metal sheathed cables such as BX are common for outdoor use.
- Use the right color wire for any permanent applications. It’s okay to have an assortment of colors in a breadboard or prototype electronics circuit, but for home wiring or any permanent installation, stick to the designated colors. Wire color standards vary from one country to another so ensure you are using the right colors for your location.
Electrical Wires and Cables
Although the terms ‘wire’ and ‘cable’ are often used interchangeably, they have distinctive meanings. A wire is composed of metal in solid, stranded, or braided form.
A solid core wire is made of a single piece of metal, whereas stranded and braided wires are made of smaller strands of metal that are bundled or braided together. Solid core wires are cheaper, more mechanically rugged, and have lower electrical resistance than stranded or braided wires. However, stranded or braided wires offer superior flexibility and more resistance to fatigue cracking over time.
Wires are manufactured in specific diameters, called gauges. The lower the gauge, the thicker the wire. Larger diameter wires can carry more current while also having less resistance and thereby reducing power losses. Standardized gauges help improve safety while decreasing manufacturing costs. We’ll cover gauges and specifications in this article.
Wires are often insulated to prevent accidental contact with the conductor. Polyethylene, PVC, Kapton, and Teflon are common insulators.
Side Note: Wire is used in many applications other than electrical service. Wire is used structurally, such as in buildings and on bridges. It is also commonly integrated into jewelry and furniture.
Cable vs. Wire
Wires are frequently defined as single conductors, with cables distinguished as assemblies that contain multiple conductors.
However, according to the most commonly accepted definition (such as Encyclopedia Britannica), a cable consists of one or more conductors that are packaged together.
In other words, every wire is a cable, but not every cable is a wire.
Most of the time, anything that isn’t a single conductor with or without insulation is referred to as a cable. A good example is a coaxial cable, which has a single conductor but is referred to as a cable due to its’ shielding.
The important thing is that the wire or cable is built to the specification you need.
Wire Gauge Sizes
Wire gauge is the most common measurement of the size, either diameter or cross-sectional area, of a wire. A wire’s gauge determines not only the maximum rated current for the wire, but also the resistive losses as current travels through it. Gauge represents only the size of the conductor and does not include the thickness of the wire’s insulation.
Different wire gauge systems have been developed over time. There are three dominant systems that are used across the world: AWG, SWG, and IEC 60228.
In the US, the most commonly used wire gauge system is AWG, or American Wire Gauge. The higher the gauge number, the smaller the wire. The largest AWG size is 0000 (4/0) which is pronounced ‘four aught’, representing a diameter of .46″. The smallest is 32 AWG, representing .00795″.
AWG should not be confused with SWG, the Standard Wire Gauge developed in Britain during the 19th century. SWG has largely fallen into disuse but is still used for certain applications like guitar strings. SWG ranges from 7/0, representing .500″, to 50 SWG, representing .0010″.
The most common system outside the US is IEC 60228, which defines a standardized set of wires based on their cross-sectional area in mm2. IEC 60228 sizes range from .5 mm2 to 2500 mm2.
Wire Color Coding
Countries have different standards for color coding (based on the color of the wire’s insulation).
In the US, black and red are typically designated as ‘hot’ wires, white or silver are used for neutral lines, and green or green with yellow stripes for ground wires.
It’s always important to be careful and not rely solely on color coding, particularly with older installations or those that have not been confirmed.
Wire Labelling: THHN, THWN and Others
Wires are labelled with letters that represent properties of the wire, such as heat resistance or wet-use application.
In residential applications, THHN and THWN letters are the most common types of electrical wires. The letter code labelled on the wire can be deciphered as such:
T – Thermoplastic insulation
H – Heat resistant
HH – High heat resistance up to 194-degrees Fahrenheit
W – Rated for wet locations
N – Nylon-coated to resist damage from oil or gasoline
X – Synthetic polymer, flame-resistant
Many other wire designations exist.
Reading Cable Classifications (i.e. 14-3)
Electrical cables are typically identified by two numbers that are separated by a hyphen or slash, such as 14-3 or 14/3. The first number represents the gauge of the conductor of each wire in the cable. The second number represents the number of conductors in the cable.
However, most cables also include a bare copper wire to function as a ground wire- so in effect, there is an extra conductor in each cable.
A 14-3 (or 14/3) cable contains a total of four (4) 14 gauge wires, which would generally be used as two (2) hot wires, one neutral, and a ground wire. A 12-2 (or 12/2) cable contains three (3) 12 gauge wires; one hot, one neutral, and one ground.
Manufacturers do also sell cables without a bare ground wire, so it’s always best to confirm before purchasing.
Metallic vs. Non-Metallic Sheathed Cable
Common cables can be sheathed in metal or an insulating polymer. Metallic sheathed cables are used for conduit wiring, and exposed or outdoor applications, where the metal sheath offers greater protection for the wires inside as well as the ability to ground through the sheath itself.
NM Cable: NM, or nonmetallic cable, is the most common cable for residential use. Commonly called Romex, these cables are color coded based on their gauge size.
BX Cable: The most common metallic sheathed cables are referred to with the designation ‘BX’.
MC Cable: Another common metal sheathed cable, designated ‘MC’, also offers metallic cladding but the outer sheath cannot be used as ground; these cables carry an extra wire with green insulation to be used as the ground wire.
NM Cable Sheath Color Coding
Sheaths are typically color coded and labelled for specification. Adherence to the standardized color coding is optional, but has been adopted by most manufacturers.
BLACK = used for 6 AND 8-gauge wire. 6-gauge wire is typically rated for 60 amp circuits, while 8 gauge is usually rated for 45 amps. Check sheath labeling for gauge and circuit specifics.
ORANGE = 10-gauge wire, 30-amp circuit
YELLOW = 12-gauge wire, 20-amp circuit
WHITE = 14-gauge wire, 15-amp circuit
GRAY = Underground cable. Since all UF (underground feeder) cable is gray, check the sheath labeling for gauge and circuit specifics.