Cable Power

iStock/ CSA-Printstock
iStock/ CSA-Printstock

With the addition of new Section 725.144 and the accompanying Table 725.144 in the 2017 NEC covering transmission of power and data to a powered device comes the requirement for a special marking of the cable used for these installations. The important part is recognizing new types of insulation and the requirements in limiting the possibility of increased current flow that affects the temperature of individual conductors within cable bundles.

One of the most common applications of powered devices is video cameras, powered from a Class 2 DC power supply that uses two conductors for power to the camera, and two conductors for data transmission back to the video display and recording device. The video camera with DC Class 2 power can be operated manually by an operator using a joystick or by the monitoring equipment automatically providing random or systematic sweeps of the area. The use of Class 2 power to a device is not limited just to that application. It is now being used for an increasing number of loads and varying types of systems, such as Class 2 power to LED lighting systems that are being controlled at a remote location.

With the increased usage of power over ethernet and other applications, individual cables are now supplying lighting and other loads with the source originating at information technology servers and similar sources.

These individual cables to each individual load results in large numbers of cables being installed in bundles with each individual cable carrying a varying amount of current and generating a certain amount of heat. The current in each of these cables is bundled, and the source of power will often cause an increase in temperature within the cable bundle itself. This can cause failure of insulation within the cable assemblies. For this reason, 725.144 and the accompanying Table 725.144 limits the amount of current on each conductor based on the size, type and rating of the conductor’s insulation within the bundle.

For example, the smallest size conductor in Table 725.144, a No. 26 AWG, is limited to 1 ampere for bundles containing one to seven cables for 60°, 75° and 90°C insulation. As the number of individual cables in the bundle increases, the amount of current permitted for each individual conductor decreases. Correspondingly, as the size of the conductor increases, the current on each conductor also increases but is limited as more conductors are added.

Since 725.144 and the accompanying Table 725.144 are dealing with insulation ratings of the conductors of 60°, 75° and 90°C, Section 725.179(G) has been changed. This section in previous NEC cycles only dealt with the required voltage rating of Class 2 cables at 150-volt (V) insulation and Class 3 cables rated at 300V insulation. For the 2017 NEC cycle, 725.179(G) now also requires the temperature ratings of the individual conductors within the cable assemblies to be a minimum rating of 60°C.

In addition, a new Section 725.179(I) has been added to deal with a new type of limited power cable called Type LP cable. Limited power cable must be listed as suitable for carrying power and data circuits up to a specified current limit for each conductor but without exceeding the temperature rating of the cable if it is in free air or installed within a raceway, cable tray or cable routing assembly. These LP cables must be marked with the suffix -LP and the ampere limit located immediately following, where the current limit is in amperes per conductor. For example, the ampere limit located is the ampacity of each conductor in the cable. For a 1 ampere Class 2 limited-power cable, the marking would be CL2P-LP (1A) for plenum rated cable, CL2R-LP (1A) for riser cable and CL2-LP (1A) for general purpose cable. This ampacity rating of the LP cable will correspond to the marking requirements in 725.121(C) that requires the power sources for limited power circuits in 725.121(A)(3), or other listed equipment marked to identify a Class 2 power source. For listed audio, video and industrial equipment, limited power circuits have a label indicating the maximum voltage and current output for each connection point. The marking took effect on Jan. 1, 2018.

All of these changes indicate that our electrical low-voltage systems have become much more complex, difficult to understand and more of a challenge to install.

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 Mark.C.Ode@ul.com.

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