The wire and cable industry has its own lingo, trade names and sets of initials to identity various products and their respective applications. Here's an overview of different types of wire and cable. The emphasis is on the evolution of flexible cable used for extension cord sets and power cords.
From this perspective, the main purpose of various types of wire and cable is to move (conduct) electricity from one place to another; for example, from the generating station to your office or job site. Unfortunately, electricity doesn't pour like water or oil, so we can't push it through a hollow pipe to move it from here to there. We need a material that electricity can flow along.
Electricity flows through most material, even material used for insulation. It's just a matter of how much electricity can flow through. Obviously we need a material with very low resistance, in order to hold down the size of the conductor and make it economically feasible to use. In addition to low resistance and low cost, the conducting material must be strong and able to easily bend and flex without breaking.
Metal is the best conductor for all types of wire and cable; copper is the best of all metals. This is because copper has the best combination of desirable characteristics. Silver, for example, is a superior conductor of electricity, but it is too scarce and expensive for ordinary wire use. Aluminum, on the other hand, is cheap, light and plentiful but it's a much poorer conductor than copper. Metals such as steel or nickel are high in resistance to the flow of electricity compared to copper, so they're seldom used except in very special applications.
Containing electricity within the copper conductor requires a material with high resistance so that minimal amounts of electricity can "leak". The material must be easy to apply, strong and highly flexible. It must also resist environmental conditions such as heat, cold, UV rays, ozone from smog, gas and oil, mechanical abuse, and so on. This material is called "insulation".
Originally, insulation consisted of braided cotton layered over the copper wire. Each layer was coated with a material such as lacquer or asphalt. This early insulation wasn't the best because it had a tendency to dry out, become brittle, crack in cold weather, etc. A newer approach was to use rubber, which was a major improvement. The shortage of natural rubber during World War II led to a search for substitute materials and the development of synthetic rubber, which is still in wide use today. The invention of materials such as polyethylene and polyvinyl chloride (or vinyl), led to the further development of many other plastics now used as insulation.
Copper
The first thing to consider is how much copper is needed to handle the electrical current. To illustrate, compare electrical current to water flowing through a garden hose. If the hose is 1/4 inch in size, a certain amount of water will pass through it, depending on the water pressure behind it. If more water is wanted at the hose nozzle, the pressure must be increased. If the pressure can't be increased because the hose might burst, a larger size hose is needed.
At equal pressure, there will be more water at the end of a 1/2 inch size hose than there will be at the end of a 1/4 inch size hose. Electricity works the same way. A larger wire will allow more current flow than a smaller wire because it has less "resistance" to the flow.
This leads to the first set of letters in the language of the industry. Copper size in the U.S. and Canada is referred to as AWG, which means American Wire Gauge. It is always accompanied by a number that specifies the actual size of the copper conductor; for example, 18 AWG. These numbers range from 1 to 50 and each number relates to a particular size. The common numbers in our market are No. 2 through No. 16 AWG.
To explain why the smallest numbers are the largest sizes: each number represents one wire-drawing step in reducing the size of the wire. So #14 AWG (.064" diameter) represents 14 drawing steps down from #1 AWG. Obviously, the smaller the wire size, the more drawing steps are involved, and the larger the AWG number.Of course, the AWG number does not completely describe the copper wire. Flexibility is a key factor. "Flexibility" is defined as how easily the wire can be bent, and how many times it can be bent without breaking. If the wire is going to be permanently installed, as is the wiring inside the walls of a house, it isn't important for it to resist repeated bending, so for permanent wiring such as Type "TW" or Type "NM" cable, a solid copper wire is used. On the other hand, a wire to be used as the cord on a vacuum cleaner must be able to resist thousands of bends and flexes.
The best way to accomplish this is to make the conductor out of many fine strands of copper, twisted together to make up the equivalent of the AWG number needed. For instance, a No. 18 AWG stranded wire may be made up of 16 strands of No. 30 AWG, or for even greater flexibility, may be 41 strands of No. 34 AWG, or for even more flexibility from 64 strands of No. 36 AWG. Remember that a larger AWG number indicates a smaller size wire.
The proper AWG size to use depends on the power requirements of the equipment, tool or appliance you’re plugging into. This is usually shown as watts or horsepower. Underwriters Laboratories has calculated ratings for size and length and issued tables that define AWG sizes, amperage and wattage requirements for extension cords.
Insulation
Today, manufacturers can choose from an array of synthetic insulations to cover conductors. A number of these insulated wires (two, three, four or more) are cabled together, a protective jacket is put over them and a flexible cord or cable product is produced.
Insulation, as previously mentioned, must be highly resistant to the flow of electricity, flexible, strong and protective against anything which will permit electricity to leak through it. This includes water, oil, chemicals and environmental conditions which may rupture the insulation, such as excessive heat or cold, ultra-violet rays, or smog. It must also protect against mechanical abuse that might expose the copper conductor to abrasion, cutting, burning, melting, or crushing.
A newer material used for cold weather cords is called TPE, "Thermoplastic Elastomer" or thermoplastic rubber. Polyethylene, or PE, is used for antenna lead wire and multi-conductor cables.
Flexible Cord is the industry name for the types of wires and cables from which power supply cords and extension cord sets are made. "Flexible" means that the copper conductors are made of fine strands of wire to permit easy flexing for thousands of times without breaking. "Cord" is just a conventional name for small cables with a limited number of conductors, usually not more than four. Because there are so many different kinds of flexible cords, all designed to meet varying service conditions, Underwriters Laboratories issued a standard which listed the cords for different service condition by Type Letters and specified sizes, thickness of insulation and protective jacked, performance requirements including tests and marking and labeling requirements.
Wire Types
Underwriters Laboratories identifies all flexible service cord insulated and jacketed with rubbe by the type letter "S". Type "S" is the heaviest construction and is designed to meet the hardest service requirements. These cords have individually heavily insulated conductors twisted together, with a heavy jacket overall.
Service cords for less hard use in the construction trade are designated type "SJ”. These cords are too stiff and heavy to be used in the home, so an even lighter construction, designated Type "SV", is used,
All of these constructions are still too rugged to be used as cords for lamps, radios etc., so a very light duty type, with the conductors held parallel (instead of twisted) and without individual insulation over each conductor was designed. This is called Type "SP".
Over the years, many changes had to be made as better insulating materials became available. There are now dozens of separate classes of insulations in the Underwriters Laboratories Standards. Since many of these are thermoplastic materials, UL decided to add another letter, "T", to indicate that the cord is insulated with thermoplastic, not rubber, resulting in SPT, SVT, SJT and ST. With the tremendous increase in the use of oils as lubricants, it became necessary to develop oil resistant cords. To identify these types, an "O" was added; later weather resistant cords were constructed, so "W" was added. "H" indicates "Heat Resistant" cords, "E" means thermoplastic elastomer, and special uses such as "Water Resistant" also are printed on the cord.
Typically, there are also special markings printed on the cord, such as temperature rating of 75 degrees Celsius (C), 90C or 105C, "WATER-RESISTANT" and usages such as "FOR MOBILE HOME USE".
Here are examples of suitable applications for each designation:
"Extra Hard Service" describes cords with Types "ST", "SE" or "SO" designations. Extension cords in this category are used for industrial plants, construction work, marine use. RV and motor homes, etc. They are designed to resist extremely rough abuse, abrasion, crushing and cutting in various environments.
"Hard Service" refers to cords with Types "SJT", "SJE" and "SJO" designations. These cords are used when Type "ST" is too heavy and stiff and its extreme toughness is not needed in light industrial work, workshops, office spaces, etc. "SJT" cords are used on hand-held appliances such as electric drills or for office equipment, such as computers and copiers. It is also used indoors and outside as extension cords.
The lightest among the types of wires and cables are types "SVT", "SVE" and "SVO". These, ,with thin walls of insulation and jacket material, are intended for indoor use,. They are the most flexible of the jacketed cords and were originally designed as the power supply cords for electric vacuum cleaners or for extension cords for such equipment. They are now also used for light duty office equipment.
Parallel (flat) cords for light duty are limited to use indoors, where there is less abuse and flexing. The lightest duty cord, Type "SPT-1", is used for lamps, clocks and similar appliances. Television sets and other heavier appliances require the somewhat heavier Type "SPT-2" light duty cord. Type "SPT-3" has the heaviest insulation in this category. It is used for power supply cords for refrigerators, air conditioners and similar large appliances.
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