DOE’s cable tests indicate that PoE lighting can hold cable losses to about 5% of PoE-switch output when a project’s average cable length is no more than 50 m and the installation follows the conductor guidance in ANSI C137.3-2017. That is a conditional design result, not a guaranteed loss figure for every cable, luminaire or building. DOE also warned that poorly designed systems can lose more than 15%. A second test series found that larger conductors reduced losses and that bending or bundling had little added effect under its limited test conditions: 44 W luminaires and ambient temperature below 30 °C.
The studies matter because a PoE system moves power and data over the same copper cable. Resistance in that cable consumes part of the switch’s output before the luminaire receives it. Cable gauge, run length, connected wattage, temperature, installation configuration and connector compatibility therefore belong in the lighting design—not as an afterthought.
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What DOE actually measured
The U.S. Department of Energy investigated energy losses in Ethernet cable used between PoE switches and lighting fixtures at the Pacific Northwest National Laboratory Connected Lighting Test Bed. Part 1 was conducted in September 2017. The test arrangement used a PoE switch, luminaires as the load and a reference meter. Investigators compared a cable run near 50 m with a run near 0 m and examined nine cable models alongside the recommendations in ANSI C137.3-2017.
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These were controlled, exploratory studies rather than a survey of every cable, switch or luminaire on the market. The reports describe what happened with the tested combinations and conditions; they do not establish a market-wide average loss.
DOE’s general explanation of the technology is available in its Power-over-Ethernet Technology overview. The detailed methods and measurements are in the Part 1 report and the Part 2 publication record.
The headline result: 5% under a defined project condition
The Part 1 report’s key finding was that ANSI C137.3-2017 guidance appeared effective at limiting cable energy losses to 5% in PoE lighting applications provided that the average cable length on a project does not exceed 50 m. The percentage refers to the PoE switch’s output, not to a universal percentage of a building’s lighting energy.
DOE’s Part 2 announcement similarly says the guidance appeared effective at limiting cable power losses to less than 5% “in the cables tested.” That wording matters. A designer cannot take 5% as an unconditional allowance for a 75 m average run, a warmer environment, a higher-power fixture or an unverified cable-and-connector combination.
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DOE also stated that losses in poorly designed systems could exceed 15%. That is a warning about what an unsuitable design can permit, not a typical measured loss and not a prediction for every project. The safe interpretation is to design and verify the cable path so the actual installation stays within the applicable guidance and project performance target.
Part 1 and Part 2 at a glance
| Study | When and where | What was tested | Reported finding | Important qualification |
|---|---|---|---|---|
| Part 1 | September 2017, PNNL Connected Lighting Test Bed | PoE switch, luminaires and reference meter; nine cable models; run near 50 m compared with one near 0 m | ANSI C137.3-2017 guidance appeared to limit losses to 5% of switch output | Applies when the project’s average cable length is no more than 50 m; exploratory test |
| Part 2 | July–August 2018, same test bed | PoE switch, luminaires and reference meter; additional cable-selection and installation tests, including three cables excluded from Part 1 for compatibility | Larger conductor diameter reduced losses; bending or bundling in uninsulated conduit did not substantially increase losses in the reported setup | Bending/bundling result used 44 W luminaires and ambient temperature below 30 °C; DOE says higher temperatures and higher loads can change the result |
The DOE Part 2 announcement provides the published context for those observations, while the DOE Part 1 announcement summarizes the earlier work.
Why conductor size changes cable loss
Increasing conductor diameter generally lowers a copper conductor’s DC resistance. With less resistance, less of the switch’s delivered power is dissipated as heat along the cable, so the luminaire receives a larger share. DOE observed lower losses as conductor diameter increased in the tested cables.
For specifications, DOE says a designer can state a minimum American Wire Gauge (AWG) consistent with the relevant ANSI C137.3 guidance when lower losses are desired. AWG is more useful here than a broad category label alone: two cables carrying the same Ethernet category designation can differ in conductor construction and resistance. Use the manufacturer’s published DC-resistance data and the exact cable type proposed for the installation.
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The DOE tests do not establish a universal ranking of cable brands, shielding arrangements, fire ratings or every cable construction. They also do not show that one Ethernet category always loses less power than another. Treat those properties as separate compliance and installation decisions unless product-specific resistance data demonstrate otherwise.
Five variables that determine how much power is lost
Average and individual run length
Longer copper paths have more resistance and therefore more opportunity for voltage drop and power dissipation. The 5% DOE result is tied to an average project cable length of no more than 50 m, not a promise that every individual run may be any length without review. Map the actual routes, calculate the project average and identify the longest runs before selecting the switch and cable.
Connected luminaire wattage
The load matters because higher-power devices draw more current through the same conductors. DOE highlighted a contemporary design concern as IEEE 802.3bt enabled devices approaching 90 W input power per powered device: cable selection and installation would become more important. That statement records the report’s 2018 context; it is not a current market-status claim or a guarantee that a given 90 W installation will meet a particular loss target.
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Higher temperature increases a conductor’s DC resistance. A cable route in a hot ceiling void, roof space or crowded enclosure can therefore lose more power than the same route at a lower temperature. Keep the temperature assumption attached to calculations and verify the cable’s permitted operating range.
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Connectors, patch cords and transitions
The reports caution that losses may be greater when cables are connected to patch cords, bundled in conduit and loaded near 90 W. Every transition adds another interface whose compatibility and resistance must be acceptable. Include permanent links, patch cords, jacks, plugs and any intermediate hardware in the design review rather than testing only the horizontal cable.
Installation arrangement
Part 2 found no substantial increase from bending or bundling in uninsulated conduit under its reported 44 W and below-30 °C conditions. That result does not remove the need to follow bend-radius, fill, separation and thermal-installation requirements. A different conduit, temperature or load can produce a different outcome.
How to design a PoE lighting cable run
- List every powered device. Record each luminaire’s required input power, the PoE class or budget used by the system and any startup or control equipment that shares the port.
- Draw the real cable routes. Measure or estimate each permanent link, patch cord and transition. Calculate the project’s average length and flag the longest paths; do not substitute a nominal floor-to-floor distance.
- Set a conductor requirement. Use the applicable ANSI C137.3 guidance and state a minimum AWG or an equivalent maximum DC-resistance value in the specification. If reducing losses is a priority, choose the larger conductor supported by the connector system and installation method.
- Check temperature. Establish the highest credible ambient temperature for the route, including ceiling plenums, risers and equipment rooms. Reassess resistance and allowable loading at that condition.
- Validate the complete channel. Confirm that cable, plugs, jacks, patch cords and the PoE switch are compatible with the intended IEEE 802.3 operation. A permanent-link cable that works in isolation is not proof that the assembled channel will work at the planned power.
- Review bundling and bends. Apply the cable manufacturer’s bend-radius and bundle limits, account for conduit fill and avoid assuming that the Part 2 result covers hotter or more heavily loaded installations.
- Verify before turnover. Measure or document delivered power, link operation and any device-specific diagnostics on representative longest and most heavily loaded runs. Record the cable construction, temperature assumption and test configuration so later changes can be evaluated.
Compatibility is part of the efficiency specification
Part 1 excluded three cable models because of compatibility issues, which is a practical reminder that low resistance alone is not enough. DOE recommends that buyers and specifiers consider products independently certified as compliant with IEEE 802.3, citing Ethernet Alliance certification as an example. Certification does not replace project engineering, but it gives the design team a documented basis for interoperability.
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Require manufacturers to identify compatible cables and connectors or to publish the relevant compatibility parameters. Ask for the conductor size, DC resistance, temperature rating, connector family and any restrictions on patch-cord use. Keep those records with the lighting submittal so substitutions can be checked against the same criteria.
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What the DOE results do—and do not—say about AC lighting
PoE lighting can consolidate AC-to-DC conversion at PoE switches, creating an opportunity to reduce duplicated conversion equipment. Cable losses consume part of that potential benefit, however. The DOE cable study measured losses in Ethernet runs; it did not establish a universal whole-building energy comparison between PoE lighting and line-voltage AC lighting.
A fair project comparison must include the complete electrical path: source conversion, switch or driver efficiency, cable losses, controls, standby consumption and the actual operating schedule. Use the DOE result as a cable-design constraint, not as proof that one architecture will always use less energy.
Limits every specification should acknowledge
- Part 1 was exploratory and covered nine unshielded cable models. Because three other models were excluded for compatibility reasons, the effect of shielding was not evaluated in that study.
- One of the two luminaires did not hold input power constant as cable length changed, and neither luminaire model reported its own power draw. This complicates direct interpretation of every length comparison.
- The tested cable models and installation practices do not represent every product or installation.
- The 5% figure depends on the cited ANSI guidance and the condition that average project cable length does not exceed 50 m. It is not an unconditional allowance for any load, cable or environment.
- The test work took place in 2017 and 2018. The reports do not independently establish the current edition of a standard or present-day product availability.
For those reasons, a project that is longer, hotter, more heavily loaded or assembled with different connectors should be engineered and validated on its own terms rather than labeled “5% loss” by default.
A practical way to read a cable submittal
Start with the values that directly affect resistance and loading. Compare the proposed conductor gauge or published DC resistance, the longest and average run lengths, the luminaire input wattage, the highest route temperature and the complete connector path. Then check the manufacturer’s IEEE 802.3 compatibility documentation and installation limits.
Do not use a brand name or a Category label as a proxy for measured loss. If a submittal omits resistance, temperature or connector information, request those data before approval. If the project’s average run approaches or exceeds 50 m, or if fixtures approach the higher-power range discussed by DOE, require a project-specific loss and voltage-drop review instead of relying on the headline percentage.
Bottom line for designers
DOE’s evidence supports a focused design rule: with ANSI C137.3-2017-oriented cable selection and an average project run of no more than 50 m, the tested PoE lighting arrangements kept cable losses to roughly 5% of switch output. Larger conductors reduce loss, while temperature, load, connectors and routing can move the result. The Part 2 bending-and-bundling finding is reassuring only for the tested 44 W luminaires at ambient temperatures below 30 °C. Treat the percentage as a qualified engineering benchmark, document the assumptions and verify the complete installed channel.
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