e large strand (solid-cored cable, such as household Romex). These cables are relatively cheap to produce but are awkward to run (they are stiff and kink easily), and have little resistance to vibration (flexing of the copper causes it to work-harden and fracture). Minimum strands
To improve flexibility, increasingly large numbers of ever-finer strands are combined to form a finished conductor. UL has three main stranding categories: Type 1, with a minimum of seven strands per conductor, and Types 2 and 3, with higher numbers of strands. The SAE, on the other hand, does not have stranding categories.
For boat cable, UL 1426 requires a minimum number of 19 strands per conductor for 16 gauge and larger cables. This stranding requirement has been adopted by the American Boat and Yacht Council (ABYC)an organization that writes boatbuilding standardsfor all boat wiring. This automatically rules out solid-cored and Type 1 cables and also many SAE-rated cables that otherwise comply with the ABYC standards.
Type 2 stranding has a minimum of 19 strands per conductor in smaller cable sizes (16-gauge to 8-gauge), and then higher numbers of strands as cables get larger. Type 3 stranding is a little different since it is based upon a constant strand size of 30 AWG (0.010") for all cables. It is interesting to note that every inch of 5/16-inch copper rod at the start of the wire-making process produces approximately 1,000 feet of Type 3 stranding.
In cable sizes larger than 18-gauge, a Type 3 strand is considerably smaller than any Type 2 strand. As a result, Type 3 cables have a far higher number of strands for any given cable size than do Type 2 cables, with the number of strands increasing disproportionately as the cable size gets larger. For example, a 2/0 Type 2 cable may have as few as 127 strands, whereas a 2/0 Type 3 cable will have a minimum of 1,323 strands. The net result is that, all other things being equal, a Type 3 cable is considerably more flexible (which aids installation) and less prone to fracture from vibration (which is important in marine applications) than a Type 2 cable. The trade-off, of course, is that the Type 3 cable is also considerably more expensive to produce.
The UL 1426 standard has been written in such a way that Type 2 stranding is acceptable for all boat cable. Nevertheless, some manufacturers use Type 3 stranding for their boat cable. Given that the standard does not differentiate between Type 2 and Type 3 cables, it is up to the customer to make the distinction when buying a cable and to factor this into the purchasing decision. Assembling a conductor
Once the copper, whether tinned or not, has been drawn down to its final strand size, it is time to start putting the strands back together to form the cable’s conductor. The technology here has been borrowed from the cloth and rope-weaving industries. The individual strands are wound onto bobbins and then fed to machines that combine the strands in a variety of different patterns and lays, much in the way that wire rope is formed for rigging purposes.
Just as with wire rope, the manner in which a cable is put together has a significant impact on its flexibility and also its tendency to kink when being pulled off a spool. In the simplest cables, the strands are laid parallel to one another and given a twistthis is known as a bunch lay. More sophisticated cables have the strands combined in one of three patterns: 1×19, 7×7, and 7×19. These are known as rope lay and correspond to similar combinations used in the wire rope industry. Rope-laid cable is more flexible and less likely to kink than bunch-laid cable. Of the rope lays, the 1×19 lay is the least flexible and the 7x 19 the most flexible. The more complex the lay, the higher the price.
Besides the particular pattern in which the cable is laid up, there are other obscure construction issues that impact the quality of the finished productfor example, the pitch. The pitch determines to what extent strands are twisted around each other during lay-up. The longer the pitch, the fewer times the strands are twisted in a given length; the tighter the pitch, the more times they are twisted. "It is cheaper to run a longer pitch," Jon Wiig, vice-president of Kalas, said. "This is particularly done with SAE wiring. We run a tighter pitch that gives our cables greater flexibility, with less likelihood of kinking, and also results in more copper per inch for a given cable size. It is also, of course, more expensive." Thermoplastics versus thermosets
The conductor is now ready to have its insulating sheath added. Two broad types of insulation are availablethermoplastic and thermoset. The principal difference between them is that the thermoplastics soften when heated, whereas thermoset materials undergo a chemical change once they are set and will not soften. Thermosetting materials have excellent insulating properties. In addition, in a circuit-overload condition thermosets char rather than melt. They resist burning longer than thermoplastics, even when a direct flame is applied. The charred material becomes brittle and will crumble, but until dislodged they will still provide some insulation. This is in contrast to the thermoplastics, which melt down with the result that, if a short occurs in one cable in a bundle, the heat often melts the insulation on neighboring cables, resulting in additional shorts.
The primary disadvantage of thermosetting materials is their cost, which has, until recently, been prohibitive. This picture may be changing since Berkshire Electric Cable Company, in Leeds, Mass., has recently introduced a line of UL 1426 thermoset cables into the market at a more competitive price.
Of the thermoplastics, polyvinylchloride (PVC) is almost universally used for boat cable. This might lead the casual observer to conclude that the insulation on one cable is much like the insulation on another, but nothing, according to Crail Gordon, president and CEO of Sylvin Technologies, could be further from the truth. "PVC is the most versatile polymer on Earth," Gordon said. "It can be made to perform all kinds of functions. As a result, we have to design a particular compound for a specific purpose; how we work with the compound has a huge effect on the quality of the finished product." It turns out that differences in the compounding of the insulation are probably the most important factors in distinguishing a premium boat cable from that of a lesser quality.Compounding
Creating the compound for cable insulation is very much akin to baking a cake. The process starts with a measured amount of resin, in powder form, being dumped into the equivalent of a large, industrial cake mixer. The mixer is turned on, and a liquid plasticizer is poured in. The friction generated by the mixing process raises the temperature. At critical points various other ingredients (fillers, stabilizers, and substances that alter the properties of the finished product) are added. When ready, the mixture is dropped from the bottom of the mixer into a cooler, from where it passes into an extruder in which an elongated helical screw drives the hardening compound through a set of small holes in just the same manner as grandma’s meat grinder used to grind meat. The spaghetti-like extrusions are chopped into small pellets that are shipped to the wire-making factory.
The quality of this compound and its physical properties are affected by numerous variables. First and foremost is the resin, which can range all the way from prime to what might be termed dregs, with an impact on such things as the abrasion resistance of the insulation. Plasticizers, too, come in different grades, with a significant impact on such things as the hardness of the finished product.
Fillers are added to thicken the mix. When price is a consideration, calcium carbonate is used, but this degrades the compound’s electrical properties. Better-quality compounds are filled with a special clayat four times the price. Intermediate compounds might have a mix of clay and calcium carbonate. And then there are various additives that can be used to enhance flexibility; increase moisture, oil, and UV resistance; and improve flame retardancy. All add cost.
Finally, given two compounds that have been mixed to the same "recipe," the manufacturing process itself can produce differing results. As an example, Crail Gordon mentioned a situation in which uneven pellet size caused production problems.
The net result is that many different compounds can be made to meet the requirements of UL 1426, but there will be significant differences in the insulating qualities of these compounds. Some, but by no means all, of these quality differences are identifiable under the standard since UL 1426 gives different heat ratings (which translates to different current-carrying capabilities) to different cables. At one end of the scale are those cables rated for temperatures up to 75° C (167° F) in a dry environment, and 60° C (140° F) in a wet environment (these are listed as 1W1); at the other end of the scale are cables rated for 105° C (221° F) in a dry environment, and 75° C (167° F) in a wet environment (these are listed as 5W2). The cable jacket will be marked with the appropriate temperature rating.
At the end of the day, some UL 1426-rated boat cables will barely comply with the minimum requirements at the lower heat ratings, while others will considerably exceed the requirements at the highest heat ratings. The cable may, or may not, be oil resistant. This is optional under UL 1426. The standard simply says that if the cable is marked as oil resistant it "shall comply with the requirements for 60° C (140° F) oil resistance in UL 83." Then there are other properties not covered in the standard at all, such as a high resistance to UV degradation. The bottom line is that quality requires expensive raw materials and additives; it costs money. As with so many other things, you get what you pay for.Fitting the insulation
As mentioned, the compound leaves the compounding plant in the form of pellets. Inside the wire-making factory, for both thermoplastic and thermoset insulation these pellets are dumped into a hopper and fed into a heated chamber where the compound more-or-less liquefies once again. The melt is driven, by another helical screw, into an extrusion head through which the finished copper conductor is being fed. The conductor is sealed at its point of entry into this chamber, but where it emerges on the other side it passes through the center of an enlarged hole. The semi-liquid compound is squeezed out of this hole, with the conductor at its center, and immediately begins to set. Thermoplastic cable, with its freshly applied jacket, now passes through a long, refrigerated water trough in which the jacket fully hardens. Thermoset cable goes through an additional process that changes its molecular structure.
Extruders can generate varying amounts of pressure (squeeze) when applying the jacket to a conductor. A light squeeze results in something known as tubed-on insulation, which has a high degree of flexibility but is susceptible to moisture penetration. A hard squeeze has higher moisture resistance but has less flexibility.
For the extrusion process to be a success, tight control is needed over both the centering of the conductor in its insulation (concentricity), and also the thickness and uniformity (no flaws) of the insulation. An off-center conductor results in a thin, easily damaged wall of insulation on one side of the cable, while uneven thickness and flaws will compromise the cable’s electrical integrity. UL 1426 does not have specific requirements for concentricity as such, but it does set parameters for minimum average insulation thickness (the total thickness of the insulation on both sides of the conductor, divided by two), and for minimum wall thickness at any given point.
At Kalas, both minimum average insulation thickness and minimum wall thickness deviations are held to well within allowable UL deviations. In common with much of the industry, the company’s newest equipment uses laser-based micrometers that not only measure these tolerances to within a thousandth of an inch on a near-continuous basis, but also output a printed record that identifies the location of any substandard sections of cable, enabling these sections to be cut out (in contrast to older inspection methods based on spot testing, which can result in substandard sections of cable being overlooked). This is just another way in which a quality manufacturer is differentiated from one of a lesser quality, but in a manner that is not easily detected by the consumer. To quote Dick Witwer: "If there is inconsistency in the concentricity or insulation thickness, somewhere along the line it will be below specifications, but you will never pick it up until you have a field failure. With our techniques, when compared to cables made to less precise tolerances, we are able to give our cables more of a cushion, both in terms of resistance to abrasion and damage such as nicking, and also in their ability to withstand overloads."
After manufacture, the cable is run through a spark tester that subjects it to a high voltage (from 7,500 to 10,000 volts, depending on cable size) designed to discover any pinholes and other flaws in the insulation. Assuming it passes this test, it is ready to be labeled. It is stamped by a print wheel that adds, at a minimum, the name of the manufacturer, the cable size, its voltage rating, and the fact that it is boat cable. If this information is not on the cable, it does not comply with UL 1426. Additional information may be included, such as the temperature-rating of the cable (e.g., 5W2), the fact that it is oil resistant, the fact that it is rated for 600 volts (which means it is based on AWG cable sizes and not SAE), and so on. Finally, the cable is packaged and shipped to the customer.Making choices
By now it should be clear that UL 1426, and "boat cable," are useful designations that help to distinguish high-quality marine-rated cables from lesser cables and from others that are sometimes promoted as suitable for marine applications when in fact they are not. An example of the latter is tinned copper wire built to UL 1015 and often called Machine Tool Wire (MTW) or Appliance Wiring Material (AWM). In spite of being tinned, this cable is not tested for water resistance, and as a result does not comply with ABYC standards. Note, however, that it is often dual-listed with AWM 1230, in which case it has been tested for moisture resistance and does comply with ABYC standards (as you can see, this can get quite complicated; you have to read the fine print on a cable to see what you are getting).
Returning to UL 1426, it is clear that it encompasses a broad array of cables with markedly different properties. At one end of the scale are SAE-sized cables (low voltage) constructed from untinned copper with Type 2 stranding, a bunch lay, and an insulating jacket that complies with UL 1426 BC1W1. At the other end of the scale are AWG-sized cables (600 volts) constructed from tinned copper with Type 3 stranding, a 7×19 rope lay, and an insulating jacket that complies with UL 1426 BC5W2. Within this latter category are cables that exceed the requirements of UL 1426 BC5W2 by some measure, and that have other desirable properties that are either optional (e.g., oil resistance) or are not covered by the UL standard at all (for example, UV resistance). These are the Cadillacs of the marine wiring world, and they command a premium price. Unfortunately, there is currently no designation that enables such cables to be distinguished from other UL 1426 BC5W2 cables of a lesser quality, and often there is no visual distinction by which the customer can tell them apart. But hopefully readers of this article will at least be armed with some questions to be asked of vendors that will help to differentiate one cable from another, so that more informed choices can be made when it comes to selecting cables for a given application.
