Derating factor, a term that is easily forgotten but very important if we want our electrical installations to function and endure safely. Derating is a simple matter of reducing the rated capacity of something such as a cable or a machine. Why? Because actual working conditions under which a cable works are seldom ideal. When we’re picking cables, we always have derating factors in mind to ensure that we’re not overloading the cable under real site conditions. This is safe and efficient.

The biggest reason that we derate is heat. Current flowing through a cable sees resistance, and resistance generates heat. The greater the current, the more heat you have. If you’ve got a bunch of cables bunched together, or they’re buried, or installed in tight conduits, that heat doesn’t have anywhere to go. It builds up fast. By derating, we’re accounting for that buildup and ensuring the cable runs within its safe temperature range. It’s not about being overly cautious—it’s about being realistic.

Although the resistance of one meter of wire may be small, high currents in long cable lengths and prolonged cables can make the resistance large enough to matter. The cable becomes warm, and as it warms, the resistance goes even higher. That additional resistance is responsible for added voltage drop and power loss, and it mounts up to an inefficient system. Derating puts you in command of these losses before they result in a problem.

Why Cables Overheat More Than Predicted

There are several important reasons why cables overheat more than predicted. One of the first factors we consider is the load factor. The load factor explains to us how heavily the cable is being utilized on a long-term basis. When the load factor is low, the cable has time to recover between times of heavy usage. But if the load factor is nearly 1, then the cable is operating near full capacity most of the time, and that continuous load causes the temperature to rise. More temperature means you have to derate more harshly.

The other factor we always take into account is the ambient soil temperature. When the cables are being buried underground, the soil temperature plays a significant role. Hotter soil retains heat, which reduces the ability of the cable to cool itself. Here, we use a higher derating factor to ensure that the cable can safely carry the load without heating up. Cold soil, however, assists in absorbing and dissipating the heat, so the cable works better.

Along with that comes the soil’s thermal resistivity. Soils are not created equal. Wet soil has high heat conductance and tends to keep the cables cooler, while dry or sandy soil contains heat and makes the cable hotter to run. In large jobs, we might even send the soils to a laboratory to determine their thermal resistivity, particularly if we’re running in soils that are unknown as to ground conditions. Dry earth has greater resistivity and makes it more difficult for the cable to dispose of heat, and thus derating is necessary.

Installation of the cable also plays an important part. The AS/NZS 3008 standard establishes a number of installation methods—such as cables in open air, buried in earth, in conduits, or exposed to the sun—and each of them will have an impact on the cable’s capacity to let go of heat. If a cable is open and spaced, it can dissipate well and requires very little derating. But if it’s buried or enclosed, particularly with other cables, the heat is trapped. Then we ensure the derating factor represents actual conditions so we’re not overestimating the ampacity of the cable.

Joule heating, or Ohmic heating, is still another natural source of heat within cables. It’s where electricity flows through the cable and strikes atoms in between, transferring energy and producing heat. Joule’s Law dictates that the resulting heat produced varies as the square of the current times the resistance. Thus as current goes up, heat increases even more dramatically. High-current circuits, then, must undergo more stringent derating.

Cable burial depth is another issue that’s easily forgotten. The deeper you put the cable underground, the more soil it needs to conduct heat through to emerge at the surface. That extra soil insulates, trapping the heat. When we’re laying cables more than half a metre deep, we use a higher derating factor to compensate for the decreased cooling. And if cables are in enclosures below ground, there’s frequently air stuck around them, creating still more thermal resistance and needing even more derating.

Other Important Derating Considerations

The type of insulation you use for your cable also influences its current carrying capacity. If we know the installation will be exposed to hot environments or high loads, we select a cable with insulation with a higher temperature rating. In this way, the cable can withstand greater heat without degrading, and you can keep performance with reduced derating. Selecting the appropriate insulation is a small thing that makes a significant difference.

And then there’s the proximity effect. If cables that transmit alternating current are put near one another, their magnetic fields interfere with each other. This tends to make the current focus in particular areas of the conductor, leading to increased resistance and heat buildup. The greater the number of cables you lay side by side, the bigger a problem it becomes. Something we always consider when designing parallel runs or multiple conduits to share.

Lastly, the skin effect has a role to play in what portion of the cable’s cross-section is effectively being utilized. At greater frequencies, alternating current travels closer towards the conductor surface, reserving the center region unused. In effect, it decreases available surface area for the current to flow through, increases resistance, and generates more heat. The frequency, the more significant the skin effect will be. With DC, you don’t have this problem, but with AC—particularly at industrial frequencies or on long runs—it can’t be neglected.

How to Calculate the Derating Factor

When we are derating, we always begin with the base ampacity of the cable from AS/NZS 3008. Then we multiply that figure by each applicable derating factor—whether it’s for soil, grouping, ambient temperature, installation type, or burial depth. For instance, if we have a 100-amp cable and apply group derating factors, soil derating, and ambient temperature derating factors of 0.8, 0.9, and 0.95, respectively, the ultimate current carrying capability ends up being slightly more than 68 amps. That’s the number we actually design to.”.

The important point to take from this is that paper cable ratings don’t always translate to reality. Real world conditions are seldom perfect, and if you don’t factor those in, you’re asking for heat damage, system losses, or worse. Derating is how we ensure our designs are safe, efficient, and will last. It’s not a matter of box-ticking—it’s about doing it right first time.