Posted by: Henry Franc on April 07, 2017
The use of category cables for power delivery has been getting lots of attention lately – especially given the changes in NEC (2017), NFPA 70 (2017) and potentially CEC C22.1 (2017 proposed revisions). This attention is related to potential safety issues that may arise when high power, high temperature and high cabling density are present.
The National Fire Protection Association (NFPA), Chapter 3, Table 725.144, “Transmission of Power and Data,” contains information about the ampacity rating of conductors at various temperature ratings based on gauge and bundle size. UL has created LP certifications (optional – not required by code) to identify cables that are designed and tested to carry the marked current under reasonable worst-case installation scenarios without exceeding the cable’s temperature rating.
This arose through an allowance in the older version of NEC, which allowed electricians to substitute Class 2 and Class 3 data cables (category cables) for 18 AWG wire in certain instances.
In theory, it all sounds well and good. But, as current increases, so does temperature (and, from a data perspective, performance drops), so we should be concerned about it. If nothing else, power delivery – and the safe use of power – is important. The negative implications in the built environment, however, may be somewhat fewer than what we have feared. Much of this fear was generated by uncertainty and a resistance to change when the information was published by NFPA without much forewarning.
Many, including myself, didn’t know much about the issue; oftentimes, we fear what we don’t know. Furthermore, there was a great deal of uncertainty about how it should be applied. The definition of a bundle was even questioned, along with things like the impacts of bundle separation. Neither the United States’ National Electrical Code nor the Canadian Electrical Code currently define what they consider to be a bundle. International standards-developing organizations (SDOs) generally define a bundle as “a group of cables tied together or in a closely packed configuration for at least 1 m.”
Rather than rehash the fear, and let various organizations firefight with different points of view, why don’t we talk about the guidance that exists already, or is in the final stages of completion? Whether it’s BICSI, TIA or IEC, there are many guides from global SDOs that can help demystify the practicalities.
BICSI is working on BICSI-007 (its “intelligent buildings” standard), and expects to publish it later this year. The BICSI document will offer guidance on dealing with cable bundles in various scenarios and multiple applications, as well as techniques to address any issues that may arise through the use of increasing power levels over category cables.
TIA updated TSB-184-A, “Guidelines for Supporting Power Delivery Over Balanced Twisted-Pair Cabling.” TIA TSB-184-A provides tables based on a maximum 15°C temperature rise for various ambient temperatures based on conductor size, current and whether the bundles are run in free air or in a conduit.
It also provides considerations based on category types, as well as installation conditions. This includes the use of sub-bundles and separation of bundles – both of which reduce maximum temperature rise.
From a practical perspective, in most cases, due to industry trends and the recommendations of various SDOs, LP ratings are not generally required. The majority of installations deal with power delivery below 60W (IEEE PoE Type 3 or lower) and Category 6 or higher rated cabling. Also, the ambient temperature is often in the 20°C range, and would have to reach 45°C ambient temperature before it became an issue.
There are some cases, however, where you may need to consider using LP-rated cables:
For the Canadians in the audience, the public consultation period for the CEC closed on Feb. 27, 2017, and the expected changes to C22.1 are expected to closely resemble those adopted in NEC 2017.
Keeping Up to Date on Power Delivery
I would encourage all of you to read up on the wealth of information in the marketplace. A good starting point is BICSI-007, as well as TIA TSB-184-A. All major cabling manufacturers also have resources available to end-users, consultants and contractors.
Remember to not panic about ampacity tables or LP ratings – they are there for our protection. I see a bright future for PoE using category cables. It is a great way to achieve safer power delivery, centralize power distribution redundancy and monitoring, increase energy efficiency and provide real savings to the building owner/operator. It also provides power delivery on demand (auto sensing – only provided when connected), flexibility and ubiquitous connectivity through a standard Ethernet port.
Learn more about Belden’s enterprise network solutions, which are designed to safely deliver power, save time, reduce downtime and help you prepare for the future.