Know the Current

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Published On
Dec 13, 2019

Last month’s article covered the technical requirements of power over ethernet (PoE), limited-power (LP) cable, regular Class 2 and 3 cables and the temperature limitations of Class 2 and Class 3 cables and installations. But I didn’t cover the hazards involved in these relatively new systems and the care that must be taken to limit the amount of current in large bundles of cables that often accompany these installations. A relatively small amount of current on each cable, when bundled in large quantities, can result in limited heat dissipation and destruction of the cables’ insulations.

Quality components, appropriate wiring methods and qualified installers who understand the hazards are a necessity for these new systems to be used without introducing fire hazards. Probably the most important part of these three issues are the qualified contractors, engineers and electricians who will design, install and maintain these systems, and the qualified inspectors who will ensure the systems are properly installed in accordance with the NEC .

It is imperative for all involved in new electrical systems to understand the difference between the old Class 2 and Class 3 power-limited systems that have been around for more than 100 years, and new systems that use the same limited-power supplies for the old systems. It is extremely important to remember the old, limited-power system with a maximum power supply nameplate of 100 volts (V)-amperes (A) and a maximum amperage of 5A at 20V, 3.3A at 30V and 0.005A above 30V up to 150V for alternating current systems.

Table 11(A) in Chapter 9 provides levels of power for power-limited AC systems and Table 11(B) provides the same for DC systems. The limitations in these tables have been used for doorbells, low-voltage lighting, nurse call systems, motor control circuits and similar applications for many years. Many municipalities, counties and states do not require permits and inspections on these systems because the voltage and amperage aren’t considered high enough to be a safety concern. Electrical contractors can continue to treat these power-limited circuits the same as in the past.

These new PoE systems used Table 11(B) in Chapter 9 of the NEC with the power supply permitted to be a maximum of 100 VA, can operate at a maximum 60V DC and can have a maximum current of 150V ÷ 60V = 2.5A. These power supplies are inherently limited, meaning there isn’t any overcurrent protection required.

The difference between the limited usage and minimum number of cables or conductors involved in a normal Class 2 or Class 3 system used for nurse calls, motor controls and doorbells is the number of cables that are bundled together from the source, such as a PoE server or module to the various loads. Where these cables are supplying individual lighting loads, there could be hundreds of cables bundled together. With large bundles of cables, some of the four-conductor data may be limited to a maximum of a half an ampere or smaller depending on the size of the conductors. For example, a Class 2 plenum cable (CL2P) with 1A of current in a bundle of 192 CL2P cables could have a temperature rise of more than 180°C with a maximum cable insulation of 60°C (140°F) or three times the temperature that it was rated to carry. Any cable marked as 60°C (140°F), will not last very long before the insulation is badly compromised when operated at 180°C (356°F).

The electrical contractor and electrician must be able to determine the amount of current the cables are rated to carry, and the power source must be adequately marked to provide the installer with a method to match the output current from the source to the conductor ampacity in the cables.

Based on 725.179(G), the Class 2 and Class 3 conductors must be marked for a temperature rating of not less than 60°C (140°F). The power source must be marked with a label at the points of termination to indicate the maximum voltage and the rated current output per conductor. Where multiple connection points have the same rating, a single label can be used.

Qualified design personnel must recognize the hazards and design the systems appropriately. In addition, qualified and knowledgeable personnel must be used to install these systems and authorities having jurisdiction must take notice to ensure the systems are safely installed and used.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and

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