An Introduction to Wire and Cable

The wire and cable industry and the Milspec Direct segment of it, the flexible cord, cable and extension cord market, has its own language, trade names and sets of initials to describe its products. The following will give you a basic understanding the different types of wires and cables, with emphasis on flexible cord for extension cord sets and power cords.

To begin, the main purpose of various types of wires and cables is to move or conduct electricity from one place to another; for example, from the generating station to your office to power a computer. Unfortunately, electricity won't pour like water or oil, so we can't push it through a hollow pipe to move it from here to there. We have to provide a material along which electricity will flow.

Most materials permit electricity to flow through them, even the materials we call insulations. It's just a matter of how much electricity will flow through. Obviously we need a material with very low resistance to the flow of electricity, 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 material must be strong and able to be easily bent and flexed without breaking.

The best materials for all types of wires and cables found were the metals, and copper had the best combination of characteristics. For instance, silver conducts electricity best, but is too scarce and expensive for ordinary wire use, whereas aluminum is cheap, light and plentiful but is a much poorer conductor than copper. Materials such as steel or nickel are high in resistance to the flow of electricity compared to copper, and so are seldom used except in very special applications.

To contain electricity within the conductor, we must use a material whose resistance to the flow of electricity is so high that only very tiny amounts of electricity will leak out of the copper conductor. This material also must be easy to apply, strong, able to be bent or flexed and must also resist environmental conditions such as heat, cold, the sun's rays, ozone from smog, gas and oil, mechanical abuse in use, and so on. These materials are called "insulations".

Insulations in the early years were made up of layers of braided cotton covering the copper wire, each layer being coated with a material such as lacquer or asphalt. These were not very good insulations, as they tended to dry out, become brittle and crack in cold weather, etc. A newer approach was to use rubber, which was found to be a major improvement. The shortage of natural rubber during World War II led to a search for substitute materials, among which was synthetic rubber, which is still in wide use today as an insulation material. Invention of such plastics as polyethylene and polyvinyl chloride (or vinyl), led to many other plastics for use as insulation. Given the number of good insulations available to cover copper conductors, discussion of construction of flexible cord and cable can begin.


The first thing to consider is how large the copper must be to handle the electrical current. To help understanding, 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, and is always accompanied by a number which specifies the actual size of the copper conductor; for example, 18 AWG. These numbers range from 1 to 50 and each number equals a particular size. The common numbers in this market are No. 2 through No. 18 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 must be considered, meaning 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 equals a smaller size wire. Finally, the AWG number and stranding are usually written as: 14-41 x 30 = Final AWG #14 size; 41 = Number of strands; 30 = Strand gauge size.

These various sizes of copper are made from 5,000 pound packages of heavy copper rod at speeds of more than a mile a minute in our copper mill.

The proper AWG size to use depends on the power required, usually shown as watts or horsepower, for the unit to which the wire must supply current. Underwriters Laboratories Inc. has calculated ratings for size and length and has issued tables which give proper AWG sizes, Amps and Wattage requirements for extension cords.


The conductors are now covered with one of many synthetic insulations. Then 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 which might expose the copper conductor to abrasion, cutting, burning, melting, or crushing.

Milspec insulates with thermoplastic materials only, the most commonly used being PVC or "vinyl" which makes up a high percentage of production.

Growing in use is a newer material called TPE, "Thermoplastic Elastomer" or thermoplastic rubber, used for insulating and jacketing flexible cord. There is also usage of PE, or Polyethylene 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 calls all flexible service cord by the type letter "S" if insulated and jacketed with rubber, with other designations, like "C" for cotton and "A" for asbestos. Type "S" is the heaviest construction, made to resist the hardest service requirements. These cords have individually heavily insulated conductors twisted together, with a heavy jacket overall.

Service cords for less hard use are designated type "SJ", with lighter construction. Intended to power vacuum cleaners, these cord constructions are too stiff and heavy to be used in the household, so an even lighter construction is used, designated Type "SV". All of these constructions are still too rugged to be used as cords for lamps, radios etc., so very light duty types, with the conductors held parallel (instead of twisted) and without individual insulation over each conductor were 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" indicated "Heat Resistant" cords, "E" means thermoplastic elastomer, and special uses such as "Water Resistant" also are printed on the cord. To avoid confusion, we have additional information to clarify the uses and types of flexible cord.

There are also some 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".

The following explains how cords designated "Extra Hard Service" or "Hard Service" may be used. "Extra Hard Service" describes Types "ST", "SE" or "SO". Extension cord sets in this category may be used for industrial plants, construction work, marine use and in mobile homes. They are designed to resist extremely rough abuse, abrasion, crushing and cutting in various environments.

"Hard Service", describes Types "SJT", "SJE" and "SJO" cord. This cord is used in light industrial work, office spaces, etc., where the Type "ST" is too heavy and stiff and its extreme toughness is not needed. "SJT" cords are used on hand-held appliances such as electric drills or for office appliances, such as typewriters and for extension cords to be used with such tools. It is also used for outdoor use in extension cords.

The lightest among the types of wires and cables are types "SVT", "SVE" and "SVO". These are intended for indoor use, using thin walls of insulation and jacket material. 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 lighter office appliances.

Parallel cords for light duty are limited to use indoors, which they are subject to limited abuse and flexing. The lightest duty cord is Type "SPT-1", used for lamps, clocks and other appliances of this nature. However, television sets and other heavier appliances require a somewhat heavier light duty cord: Type "SPT-2" is required for parallel-wire indoor-use extension cords. Type "SPT-3" has the heaviest insulation in the parallel light duty category and is used for power supply cords for refrigerators and air conditioners.

There are hundreds of other types of wires and cables, all designed for different end uses, some that Milspec offers are Landscape Lighting Cables and Speaker and Sound Cables. Our facilities allow us to make many other types of wires and cables, as demand requires.

Milspec also makes other types of wires and cables which, while not listed by Underwriters Laboratories, have been designed by the Milspec Engineering Department to be safe, reliable, quality products when properly used.

To help identify some of the cable types available, please refer to the glossary page, which shows some of the common types of wires and cables.