• • • CABLES • • •


CABLE SIZING IN POWER QUALITY APPLICATIONS Why getting it right the first time matters


BY JOHN MITCHELL,


GLOBAL SALES AND MARKETING DIRECTOR, CP AUTOMATION


T


ypically, cable sizing is treated as a routine electrical task: calculate the load current, select a higher-rated cable and


move on.


In conventional linear loads, that approach has worked for decades. However, many modern industrial facilities feature variable speed drives (VSDs), harmonic filters and various power quality equipment, making this traditional thinking problematic. Here, John Mitchell explores some of these challenges.


Incorrect cable sizing doesn’t just harm efficiency, it can cause other issues like overheating, nuisance tripping, premature component failure and expensive rework that’s only discovered at commissioning. When working with different electrical components, cable sizing must be treated as a critical design decision, not an afterthought.


Cable sizing challenges


in VSD systems Variable speed drives (VSDs) are non-linear loads that draw current in pulses, introducing harmonic distortion into the supply. These harmonics alter the way current is distributed within a conductor. At the fundamental frequency, current flows relatively uniformly through the conductor’s cross-section, with a significant portion travelling near the centre. As frequency increases, however, current is forced toward the outer surface of the conductor, and this is known as the skin effect. Consequently, higher-frequency harmonic currents are increasingly confined to the conductor’s surface, reducing the effective cross-section available for current flow.


Standard cable ratings assume even current distribution across the conductor, but harmonics


electricalengineeringmagazine.co.uk


break that assumption. As a result, only a portion of the conductor carries significant harmonic current, increasing resistance and heat. Oversizing cables traditionally mitigated this, but the addition of harmonic mitigation equipment has made careful sizing even more critical.


Harmonic filters, which actively inject currents to cancel distortion, may carry predominantly harmonic current themselves. This makes cable derating essential, typically by a factor of 1.35 to 1.5 depending on the application and harmonic content. Without appropriate derating, cables can easily overheat and filter performance can be reduced.


For example, electricians are rarely trained in harmonic behaviour, so unless the system is designed holistically, errors become inevitable.


Sizing for CTs


Cable sizing errors are particularly problematic in current transformer (CT) circuits. Because CT secondary circuits operate at low voltage, typically 1A or 5A, they are often treated more casually. Some installers may assume “any small cable will do”. However, CTs are highly sensitive to burden, the total load imposed by cable length, cable size and connected devices.


If the burden exceeds the CT’s rating, the CT could overheat or fail. This impacts metering accuracy, power factor readings and harmonic measurements. In extreme cases, CTs can burn out.


Finally, protection devices, such as circuit breakers and miniature circuit breakers (MCBs), must also be considered. These devices respond to thermal effects, and if sized only for the fundamental current, they risk nuisance tripping or providing inadequate protection in the presence of harmonic heating.


The hidden cost of harmonics High harmonic currents add thermal stress on cables, terminations and protective devices. Over time, this can lead to insulation breakdown, melted conductors and hard-to-detect intermittent faults. Usually, these issues only become visible during commissioning, when downtime is most expensive. We often find that the root cause of these issues is a disconnect between stakeholders. One party may specify the harmonic filter, another supplies it and a third installs it. None of these fully accounts for cable derating, installation method or protection coordination.


Incorrect CT cable sizing can also directly distort power quality measurements and lead to financial penalties. We recently saw the impact of this when visiting a customer who had been penalised for having a power factor of 0.56, despite on-site measurements showing near unity. The cause was traced to the CT wiring installed using thin bell wire over a long loop. Simply replacing the cable with correctly sized conductors immediately restored accurate readings, no equipment changeouts were needed.


Correct CT cabling is important for safety reasons too. CT secondaries must never be left open-circuited. Appropriate shorting terminals should be installed close to the CT, enabling the circuit to be safely shorted before any downstream disconnection. Without proper shorting, dangerously high voltages can develop, damaging equipment and posing a risk to people on site. For more information on industrial power quality, including measurement and mitigation options, including harmonic filters, surge suppression and power factor correction, visit the CP Automation website or speak to a member of the engineering team to get the advice you need.


https://www.cpaltd.net ELECTRICAL ENGINEERING • FEBRUARY 2026 31


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