Power over Ethernet (PoE) technology offers the ability to provide both low-voltage direct-current (DC) power and communication over a standard Ethernet cable-also referred to as a local area network (LAN) cable or Category cable. Light-emitting diode (LED) technology has reduced the power required for lighting applications, while advances in PoE standards and technology have yielded substantial increases in the amount of power that can be delivered to a networked device over a single cable. As a result, PoE technology is emerging in lighting and many other applications beyond its historical foothold in telephony and networking equipment. Several major LED luminaire manufacturers have introduced PoE connected lighting systems in recent years, making this a potentially disruptive technology. PoE lighting systems can offer improved efficiency relative to traditional line voltage alternating current (AC) systems, because AC-DC power conversion losses can be reduced if this work is consolidated among one or more PoE switches, rather than being distributed among a greater number of smaller LED drivers. However, this effect can be offset to some extent by increased losses associated with increased voltage drop in the lowvoltage Ethernet cabling. In fact, these losses could exceed 15% in poorly designed systems. Aspects of cable design that can affect cable energy performance include wire gauge, Category (e.g., 5e), fire rating, and shielding. Unfortunately, most cable manufacturers only state maximum DC resistance (DCR) or reference standards that specify DCR limits; few publish nominal DCR values. This report summarizes the results of an exploratory study investigating power losses in Ethernet cables used between PoE switches and luminaires in PoE connected lighting systems. Testing was conducted at the Pacific Northwest National Laboratory (PNNL) Connected Lighting Test Bed in September 2017. A test setup comprising a PoE switch, a set of luminaires, and a reference meter was used to test nine cable models of varying design. Power measurements for two widely differing cable lengths-one near 50 m and another near 0 m-were used to determine the portion of PoE switch output power dissipated by each cable model. The results were analyzed to explore the impact of cable selection on PoE lighting system energy efficiency, as well as the effectiveness of guidelines recently introduced by the American National Standards Institute (ANSI) C137 Lighting Systems Committee. The key study finding is that the 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 m.