A recent technical brief from the Communications Cable & Connectivity Association (CCCA) discusses changes to the 2017 National Electrical Code (NEC) and their impact on the ICT industry:
2 Simple Ways to Comply with the 2017 National Electrical Code
Changes to the 2017 National Electrical Code and its Impact on the ICT Industry
As recently as 2015, it’s likely that a network cable installer’s only concern was system performance - installing a compliant system that would meet the bandwidth and transmission requirements requested by the customer. Now, as the number of applications utilizing Power over Ethernet (PoE) technologies continues to grow, 4-pair cables are increasingly being used to transmit both data and power. This convergence of previously separate cabling systems is adding new dimensions to cabling performance and safety requirements, as is evidenced by the 2017 revisions to the National Electrical Code (NEC).
Predictably, a 4-pair cable carrying up to 100W of power is prone to generating heat. Heat generation in a single cable is not generally a problem on its own, as most compliant category cables are rated to withstand above-ambient conditions. However, when these cables are bundled together, that heat rise can become a very significant safety issue. Depending on variables like wire gauge, the amount of current being carried, and the number of cables in a bundle, it’s very easy to see how certain bundles could be prone to overheating, exceeding the max temperature rating of the cable. In fact, a 2015 Underwriters Laboratories (UL) fact finding study confirmed the reality of this issue and developed a roadmap to ensure safety in use.
This is where the NFPA got involved and the 2017 revisions to the NEC come into play. The 2017 revision of the NEC (Article 725) features an “Ampacity Table” for Class 2 & 3 data cabling in applications involving power over 4-pair cables (Table 1). The ampacity table provides the necessary guidance of maximum allowable number of cables in a bundle for users and installers who intend to carry power to connected devices (accounting for wire gauge and maximum current).
Following the guidance provided by the ampacity table ensures cables in a given bundle will not exceed their temperature ratings.
In addition to following the guidance of the ampacity table, the 2017 NEC offers an alternative path to compliance through something called an “LP” or “limited power” designation. “LP” is an optional designation and cable marking that may be added to listed cable at a given amperage to attest that the cable fully complies with the requirements of the NEC, regardless of cable bundle size (provided that the rated current limit per conductor is not exceeded.) To achieve “LP” designation, cables are further tested to ensure they will not exceed their temperature rating at the maximum current limit of each conductor.
The “LP” designation allows installers and designers a simple installation option by not restricting bundle sizes, and eliminating the need to count cables.