Understanding Wire & Cable Specifications

Would you believe that not everyone I encounter daily has a detailed understanding of cable specs? Okay, so I’ll admit, it takes a special kind of commitment (read: madness) to retain an encyclopaedic knowledge of wire and cable terminology. It’s a fact – manufacturers love their acronyms and abbreviations. Good luck trying to understand a suppliers’ technical data spec! Thankfully, you don’t have to rely on luck when purchasing wire and cable (Pro tip: NEVER rely on luck!). Here’s my whistle-stop guide to understanding manufacturers’ technical data sheets:  

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Wire & Cable - Spec Sheet Cheat Sheet

If you’re tasked with making manufacturing decisions, it’s vitally important that you understand what the technical data sheet is telling you. You DO NOT want to be the person who bought a wire with an insufficient temperature rating for its intended purpose! Manufacturers produce technical data spec sheets to allow buyers to accurately assess a product’s quality and suitability for meeting their requirements. It summarizes the performance, technical and physical characteristics in sufficient detail for a design engineer to fully understand its role in the electronic system. Broadly speaking, different manufacturers will supply their own self-styled spec sheets. Some will include more information than others, and the information contained therein can be shown under different headings. While the formatting can vary, wire and cable manufacturers are thankfully among the more detailed exponents. When it comes to electronic wire and cable, details matter. For the purposes of this article, we’ll use a Belden technical spec sheet as our template. Belden is a global leader in end-to-end signal transmission solutions and a respected authority in the enterprise, industrial and broadcast sectors.  

Physical Characteristics

As the name implies, this section of the spec sheet details the notable physical elements of the product: Conductor - This is the metallic component of cables through which electrical power or electrical signals are transmitted. Conductor size impacts DC resistance, current carrying capacity and breaking strength requirements, and is specified by either AWG, Circular Mil Area, or Square Millimeters:

AWG – Conductor size is usually specified by the American Wire Gauge (AWG). Used mostly in North America, AWG is a standardized measurement of physical size (diameter) of copper and aluminum wire conductors. The larger the gauge number, the smaller the wire diameter.

Circular Mil Area – Circular mil area defines the cross-sectional areas of conductors. Size is specified in terms of the total area of a cross-section of the copper in circular mils (cmil). A circular mil is a unit of area equal to the area of a circle one mil in diameter.

Square Millimeters - Metric sizes are given in terms of square millimeters (mm2).

Stranding – Simply refers to a conductor which is made up of a group of wires. Stranded wire is composed of small wires bundled or wrapped together to form a larger conductor and is more flexible than a solid wire with the same cross-sectional area. Wire strands are typically either compact, compressed, or concentric, and constructions vary in the size, number and configuration of the individual strands. AWG designations for stranded constructions usually refer to their AWG size or its stranding detail, often detailed on the spec sheet as two numbers – such as “20 AWG 19/32”. In this example, 20 represents the overall gauge size, 19 is the number of strands, 32 is the gauge size of each of the 19 wires. Material – Strictly speaking, conduction material can include metals, electrolytes, superconductors, semiconductors, plasmas and non-metallic conductors such as graphite. Commercial and industrial wire most commonly uses copper due to its high conductivity. In fact, it’s the second most conductive metal in the world behind silver, but the significantly higher cost of silver makes it impractical in most cases. High quality manufacturers will use tinned copper (TC), a type of copper wire coated in a thin layer of tin to protect the copper from corrosion that would decrease the wire’s efficiency in humid or high-heat environments. No of Pairs – This refers to two insulated wires of a single circuit associated together; sometimes known as a “balanced” transmission line. Conductor Count – Here your spec sheet details the number of conductors in a multiple-conductor cable. I.e. the number of conductors cabled together and insulated from one another. Insulation Material – One of the most important components of a wire. Your selection of insulation material depends on several factors, such as its required life, dielectric properties, resistance to temperature, resistance to moisture, mechanical strength and flexibility. Its vitally important to select the right material for the required use. Insulation material is typically either a thermoplastic compound, such as Polyvinyl Chloride (PVC), or a thermoset compound (such as Silicone). Nominal Wall Thickness – This simply describes the thickness of the applied insulation or jacket. Generally, the wall thickness increases as required voltage increases. Outer Shield Material – The outer shield is a sheet, screen, or braid of metal that’s placed around the conductors to contain any unwanted radiation or to prevent unwanted interferences from external sources. For example, a “Taped” style is helically wound over the conductors, and commonly made from aluminum or polyester. Some large manufacturers may use proprietary materials here. Belden for example supply their patented ‘Beldfoil’ electrostatic shield, which is made from reinforced metallic foil. Coverage – This percentage represents the total physical area of a cable covered by the shielding material. Braided and foil shields have their advantage and disadvantages, so the ideal coverage depends entirely on your requirements. Drain Wire – A drain wire is the bare, stranded wire you find interleaved with the wrapping foil inside shielded cables. This wire plays an important part in facilitating the cable's operation, by removing unwanted electrical noise and allowing for effective grounding. The drain wire remains in contact with the metallic side of the shielding tape all through the body of the cable, placing it in an ideal position to connect a shielded cable to its ground terminal. Outer Jacket Material – The outer jacket is a sheath that protects the wire or cable core from mechanical, moisture and chemical issues. The outer jacket helps with flame resistance and protects against sunlight. Jackets come in a variety of types and styles and are mainly plastic or rubber based. It’s common here for the spec sheet to detail a thermoplastic, such as PVC, or a thermoset compound such as silicone. Rip cord – These are industrial yarns that sit beneath the jacket. The purpose of a rip cord is to help cut back the jacket to expose the insulated wires within, minimizing the risk of cutting any of the wires inside. This helps make splicing cables easier during installations.  

Electrical Characteristics

Conductor DCR - This refers to the direct current resistance of a conductor, derived from the resistance of the wound wire. The resistance of a long wire is greater than the resistance of a short wire because electrons collide with more ions as they pass through. The relationship between resistance and wire length is proportional and measured in ohm/1000ft. Capacitance – This refers to the ability of dielectric material between conductors to store electricity when a difference of potential exists between the conductors. The unit of measurement is the farad, which is the capacitance value that will store a charge of one coulomb when a 1-volt potential difference exists between the conductors. One farad is the capacitance value that will permit one ampere of current, when the voltage across the capacitor changes at the rate of one volt, per second. Inductance - The property of an electric conductor or circuit that resists a change in current, thus causing an electromotive force to be generated by a change in the current flowing. It causes current changes to lag behind voltage changes and is measured in henrys. Voltage – This section details the Underwriters Laboratory (UL) voltage ratings for a given wire or cable. It represents the highest voltage that may be continuously applied to a wire or cable in conformance with prescribed standards or specifications. These ratings will be marked and visible on the wire jacket.  

Temperature Range

UL Temperature Rating – The temperature rating of wire and cable products are key operating parameters. The use of wire and cable products outside their design temperature range can result in premature and often expensive failures in service. This section of the spec sheet designates the Underwriters Laboratory (UL) certified temperature rating for the product. Operating Temperature Range – This displays the maximum and minimum temperature tolerances for the product to be used in continuous operation without the loss of its basic properties.  

Mechanical Characteristics

Bulk Cable Weight – Specifies the weight of the product in lbs per 1000 feet. Max Recommended Pulling Tension – Wires and cable damaged during installation can cause service failures. This figure refers to the maximum load per unit of original cross-sectional area of a conductor. Always ensure that your function does not exceed the allowable tension stated by the manufacturer. Minimum Bend Radius/Minor Axis – This refers to the minimum radius that a cable can be bent without the possibility of causing structural or electrical damage to the cable.  

Suitability

As the name suggests, this section denotes the products suitability for indoor and outdoor use, along with a designation for resistance to direct sunlight. Flammability, LS0H, Toxicity Testing - Here the manufacturer details the product’s adherence to strict certifications across a range of recognized domestic and international industry bodies. The spec sheet will always include the requirements unique code for ease of reference. Plenum - This denotes the products suitability for use in the plenum spaces of buildings - used to circulate air back to the heating or cooling system. Plenum cable has insulated conductors often jacketed with Teflon or Halar to give them low flame and low smoke producing properties.  

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  There we have it. A wire and cable spec sheet, demystified. This guide is designed to equip you with the basic knowledge needed to interpret what manufacturers are sharing about their products. Remember, with wire and cable, there’s more than meets the eye – so always ensure you are fully informed before making any purchase decision. Here at WAVE, we’re always happy to answer with your wire and cable enquiries. Simply drop me a line at sales@audiovideoelectric.com or call the team on 1-888-792-9283 Until next time, happy spec’ing. Jeff Bova CEO, WAVE