“Science is but a perversion of itself unless it has as its ultimate goal the betterment of humanity.” – Nikola Tesla
Imagine being one of the first people to hook up a cable to an electric generator and flipping the switch. How would you know how much current a cable could handle?
Apparently you don’t. That might explain why there were 23 fires in six months at the 65 mills in the New England area where electric lighting was installed in the year 1881.
The current carrying capacity, or ampacity, of a cable depends on a number of things, and one of the biggest factors is the material from which it is made. Silver is actually a better conductor than copper, but at about 70 times the expense, you’re not likely to find it in the feeder cable of your typical rental house. Aluminum is cheaper than copper, but it’s not as good a conductor. To match the current carrying capacity, you need a larger sized cable. During the late 1960s and early 1970s, copper was in short supply, and a lot of houses were wired with aluminum conductors. But because most of the devices these aluminum conductors were wired to, like breakers, switches, and outlets, were designed for copper wiring, they made poor connections, and ended up as inspiration for the Talking Heads song Burning Down the House.
Aside from the type of material, the physical geometry of a conductor also has a big impact on its ampacity. Obviously, the larger the diameter, the more current carrying capability it has. Wire gauges are structured on a series of increasing cross-sectional diameters of wire. The American Wire Gauge, or AWG, is the standard in the United States and some other countries. It’s a counter-intuitive system where the number associated with a wire size goes up as the diameter of the wire goes down. A #1AWG wire, for example, is larger than a #22AWG wire. It’s done that way to confuse those who might think of logic as a good thing, and also because the gauge is based on the way wire is drawn. In particular, the number of times it has to be drawn in order to manufacture a particular size corresponds to the gauge number. A #1AWG wire, for example, is drawn, or pulled, through a die once, while a #10AWG is drawn 10 times. Still, the first time you encounter this wire-size numbering system, you kind of have to stand on your head to figure it out.
The material and geometry are not the only things that have a bearing on the ampacity of a wire or cable. Much of it has to do with the insulation around it. If you look at AWG gauges, you’ll see that many times it doesn’t give you any information about the ampacity of copper wire, only the resistivity, diameter, and the area of a cross section. Sometimes it might list the ampacity for a given condition, such as when it is clad in a temperature-rated insulation. This is because the ampacity of wire and cable is dependent upon the type of insulation it has. When wire conducts electricity it heats up. If too much current flows through it, the heat can melt the insulation. Given the choice between logic and insulation, I’ll take insulation every time. Therefore, wire and cable is rated according to the gauge and the type of insulation it has.
The production industry on the West Coast learned these lessons the hard way. When the Olympics came to Los Angeles in 1984, the only existing code addressing single-conductor feeder cable had been written into the National Electrical Code® (NEC®) that year, and it was for type S and type SO cable. Ironically, there was no such thing as single-conductor type S or type SO cable. Most production houses used welding cable with single pole welding or cam-type connectors. But it wasn’t really suited for such use because it was 90-volt, intermittent-duty cable. Manufacturers interested in capturing some of the entertainment market started making 600-volt welding cable, and most production houses on the West Coast bought it.
Along came the 1984 Olympics. That spring, a Los Angeles county inspector red-tagged a temporary building being used for live broadcast television production because it was powered via welding cable and terminated in cam-style connectors at the building. A letter to city officials soon followed, asking permission to use a particular brand of continuous-duty cable that was rated for 600 volts. Up until that time, the City of Los Angeles had never officially addressed the issue. It had never come up, and they simply ignored it. But the letter asking permission to use it forced the city to formally address the issue and make new rules.
The city granted permission to use the cable, but only during the Olympics, not afterward. They completely disallowed the use of welding cable — period. Any production company that had purchased this particular type of cable would have to trash it. Without the weight of the NEC behind it, there was no other choice. But the problem was that the NEC wouldn’t write it into the code unless it was UL listed for that particular purpose. In 1987, type W and type G was added to Table 400.5(B), the ampacity table for feeder cable. But it didn’t list ampacities for single conductors, only two- and three-conductor cables. Also, these cables were rated for 2000 volts, they were double-jacketed, and they weighed a ton. No one wanted to use them.
Over time, type W and type G cables were used as the basis to get single conductor feeder into the ampacity table. Once that hurdle was cleared, a cable manufacturer convinced UL to list their cable, albeit not as a particular cable type, but marked “for use in accordance with Articles 520 and 530 of the NEC.” This cable has blue insulation and it was as flexible as welding cable. It seemed to be a success. But the issue was far from resolved.
The blue insulation on this cable was made of thermoplastic, not thermoset, like the entertainment cable we currently use. As the name implies, thermoset is “set” by heat, and then it stays set under the worst of conditions. Thermoplastic, on the other hand, retains its plasticity as it heats, cools, and reheats, over and over again. Since this cable insulation was made of thermoplastic, a cigarette would burn it, and the hot city streets of L.A. would melt it. The infamous blue cable that had permeated the industry back then had to be thrown away.
But in the meantime, a precedent was created. Over the next revision cycle of the NEC, a USITT committee with Mitch Hefter, Ken Vannice, Randy Davidson, Mike Lanni, Mike Skinner, Dick Thompson, Mark Bauserman, Steve Terry and others wrote proposals to allow for other single conductor cables that were listed and met other necessary requirements. The result was type SC, SCT, and SCE cable, which is now the de facto industry standard.
I’ve overheard several production electricians say that a certain feeder cable is good for a certain number of amps. In other words, “Four-aught is good for 400 amps,” or “One-aught is good for 260 amps.” The truth is, it depends on a number of factors, all of which are spelled out in the National Electrical Code, and in particular Table 400.5(B), for single conductor feeder cable.
We no longer have to guess at sizing cables, and as a result we’ve reduced many of the hazards associated with power distribution. But that doesn’t mean they aren’t still there. They’re just waiting for a slip up. Don’t let it happen on your watch.
