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What size wire makes the most sense?

April 25, 2018
And how temperature and power requirements affect your decision-making

As the Industrial Internet of Things (IIoT) continues to gain mindshare and acceptance, the roles of the cables, connectors and wires that move the data become more important. Our panel of experts addresses what you need to keep in mind when deciding wire size, including the effects of temperature and power.

Wire size can be dictated by ampacity, but is often prescribed by the end user. Since today's PLCs need only a few mA to change an input state, does it still make sense to use 14 AWG signal wire, or are there other options? How do temperature and power requirements affect this decision?

Jennifer Grace, product technical manager—connectivity at Balluff: The short answer is no. Copper is expensive, and 22 gauge or even 24 gauge is more than sufficient as it pertains to sensors and sensing applications. A gauge that’s too light will result in signal power being wasted due to the series resistance of the cable, while a gauge that is too heavy is unnecessarily awkward and costly.

The energy dissipated in the wire will cause a temperature rise, but the heat will also be given to the environment. The type of insulation determines the amount of thermal isolation. It also determines the maximum temperature allowable. Variables such as cable length, whether or not the cable is shielded, where and how the cable is routed, what other types of devices are nearby, ambient temperature and price need to be considered in choosing the right gauge and cable jacket for the application.

Allen Bennett, field application engineer at Allied Electronics & Automation: Bigger wire size is better to a point. More importantly, a shield around the wire or two twisted pairs with shields is best. It not only reduces EMI on control signals, it physically strengthens the run length and keeps impedance low.

Resistance increases with temperature and exhibits the possibility of limiting small control currents. In addition, if the run is extremely long, the impedance will increase, potentially causing EMI-induced faults. The same happens when signal wires are run next to high-voltage ac conductors.

Alex Dzatko, proposals specialist at Pepperl+Fuchs: Higher AWG wire, such as 16, 18 and even 20 AWG, is being used for signal wires used with PLCs. All wires have temperature ratings, so in a very high or very low temperature environment, the temperature specifications on the wire need to be confirmed to be able to operate properly in that environment. High power requirements often mean a low AWG wire is needed; a larger conductor can carry more Amperage. If a high AWG wire was to be used for a power circuit, the temperature of that wire will rise more rapidly and to a higher peak temperature. Mitigating heat build-up is a necessity in PLC cabinet applications. High and sustained heat will rapidly decrease component lifetime.
Scott Byrne, senior manager, power, instrumentation & controls, at Matrix Technologies, a Control System Integrators Association (CSIA) member: Under some applications it does make sense to use something smaller, such as a 16 AWG. An example of this would be internal panel wiring between PLC I/O modules and related terminal strips. These would typically be installed in a clean controlled environment with proper lighting. However, with smaller wire comes more difficulty stripping, applying labels and terminating, especially in factory-floor environments. For these reasons alone, a 14 AWG wire is preferred for field wiring. There isn’t typically a huge economic justification in terms of reduced cost for smaller wire and conduit to overcome the efficiency using larger wire.
Jack Zurick, senior control systems engineer at Thermo Systems, a Control System Integrators Association (CSIA) member: For the control-panel designer, 14 AWG is not commonly used for signals that are on the order of a few milliAmperes. Standards such as UL 508A, NFPA 79 and JIC, have been developed which allow the usage of smaller-gauge wiring for this purpose. Even 20 AWG is allowed as long as the wire is adequately protected against overcurrents and is used where PLCs are concerned. Larger-gauge wiring is often mandated in situations where more power is required by devices, both in the field or in a control panel. UL 508A does require the use of 14 AWG for power circuits. A distinction should be made between control power to a digital input module and power circuits to a heater or motor or variable-frequency drive (VFD). Temperature has an impact on the ampacity of wire, but, at the ambient temperatures we usually encounter in a control panel, this impact is minimal. Power requirements should always be considered when sizing wires. As the wire gauge increases, the ampacity of the wire will increase at any given temperature.

ALSO READ: Don't let poor wire and cable decisions slow down your next project

About the author: Mike Bacidore
About the Author

Mike Bacidore | Editor in Chief

Mike Bacidore is chief editor of Control Design and has been an integral part of the Endeavor Business Media editorial team since 2007. Previously, he was editorial director at Hughes Communications and a portfolio manager of the human resources and labor law areas at Wolters Kluwer. Bacidore holds a BA from the University of Illinois and an MBA from Lake Forest Graduate School of Management. He is an award-winning columnist, earning multiple regional and national awards from the American Society of Business Publication Editors. He may be reached at [email protected] 

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