Power Over Ethernet Study Examines Cable Energy Loss

A new study conducted in the Department of Energy’s Pacific Northwest National Laboratory (PNNL) Connected Test Bed looked into the use of Power over Ethernet (PoE) and whether increased voltage drops can happen in poorly designed systems. The study used a PoE switch, a set of luminaires, and a reference meter to test various designs. The longest cable length was 50 meters.

The study found that “guidance offered in ANSI C137.3-2017 does appear to be effective in limiting cable energy losses to 5% in PoE lighting applications, provided that the average cable length on a project does not exceed 50 meters.”

Here are some of the findings:

• Cable losses were found to decrease with increasing conductor diameter (i.e., numerically smaller wire gauge), as would be expected. No such trend was observed for cable Category, fire rating, or manufacturer; however, considering the study limitations (e.g., the set of cables tested), this does not mean these parameters do not affect cable losses.
• Cable power loss can be accurately determined using values reported by the power sourcing equipment (PSE) if the powered device (PD) input power does not vary with cable length. Notably, of the two luminaires used as a lighting load in this study, one did not hold input power constant in this manner, and neither model reported its own power use.

The researchers also had recommendations for lighting designers, manufacturers, and installers. They include:

• PoE lighting system designers should specify that minimum American Wire Gauge (AWG) must be per ANSI C137.3 guidance, or specify minimum AWG directly if even lower losses are desired.
• PoE lighting system designers/suppliers/installers should publish statistics on PoE cable lengths used for each project (e.g., minimum, maximum, mean, median), along with information on each model used (e.g., wire gauge, Category, fire rating, shielding).
• Manufacturers of Ethernet cables and connectors (RJ45 plugs) should publish lists of compatible cabling products or parameters relevant to compatibility (e.g., tolerances for overall diameters of cable and insulated conductors) in product documentation.
• Given the growth of high-power PoE applications, more Ethernet cable manufacturers should publish rated DCR values specific to each product. Although these values would be expected to fall between standard nominal and maximum values, knowledge of actual representative DCR values would enable selection of cables that minimize energy losses.
• PoE switch manufacturers should state measurement accuracy for switch-reported PSE output power in product documentation. In addition, PSE output voltage should be reported by the PoE switch.

You can read the entire DoE study here