Change Summary

• New 840.160 (Powering Circuits) was added under new Part VI (Premises Powering of Communication Equipment over Communication Cables) with direction to comply with the new 725.144 for power delivery circuits that exceed 60-watts on communications cables.

Expert Analysis

Articles 725.144 and 840.160 both relate to power-overlocal area networking (LAN) cable and because of this relation the analysis for both articles is covered together here.

Over the past decade or so, power-over-local area networking (LAN) cable technologies such as Powerover- Ethernet (PoE) have become a viable powering option for a wide range of applications including premises-powered broadband communications systems. PoE is being widely used today with data and communications circuits. Device manufacturers are designing more sophisticated equipment that demands increased power. As this power is increased, the heat generated within the cable increases as well. This is especially true when the cables are deployed in large bundles. The additional heat generated by the increased current could potentially push the cables beyond their rated temperatures.

To address this concern, table 725.144 and its associated language mentioned earlier in this book were added. These new provisions limit the current carrying capacity of communication cables based on conductor size, bundle size, the cables temperature rating and maximum ambient temperature. In addition, a new optional type of “limited-power” (LP) cable has been introduced to simplify the cable choice and installation considerations. LP cable may be installed “regardless of the number of cables in a bundle.” New language added at 725.179(I) indicates that limited power (LP) cables must be listed as suitable for carrying power and data up to a specified current limit for each conductor without exceeding the temperature rating of the cable where the cable is installed in cable bundles in free air or installed within a raceway, cable tray, or cable routing assembly. The cables must also be marked with the suffix “-LP” with the ampere limit located immediately following the suffix LP, where the current limit is in amperes per conductor. For example, a 1-ampere Class 2 limited power cables could be marked “CL2-LP (1.0A).” The ampere limit located immediately following the suffix LP is the ampacity of each conductor in a cable. As with the current ratings for non-LP cable given in table 725.144, the “-LP” current rating only applies up to a maximum ambient temperature of 30C, independent of the cables temperature rating. For maximum ambient temperatures greater than 30C, all cables ampacity ratings must be de-rated using table 310.15(B)(2). The code also states that no conductor (or cable) should be used in such a manner that its operating temperature exceeds its rated maximum temperature. Optical fiber cables and communications cables are required have a temperature rating of not less than 60°C (140°F) by sections 770.179 and 800.179. Where cables carrying communications and power are installed, cables rated for temperatures above 60°C (140°F) may be required. In addition, the code indicates that the current in the power circuit shall not exceed the current limitations of the conductors utilized in the circuit.

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