Evaluate PoE resistance unbalance and bundle considerations

What you should be able to do

Distinguish resistance unbalance within one pair from unbalance between pairs, explain unequal current sharing, and identify the information needed before evaluating a powered cable bundle.

Two different resistance comparisons

Within-pair unbalance compares the DC resistance of the two conductors belonging to one twisted pair. Pair-to-pair unbalance compares effective pair resistances. For PoE DC analysis the two conductors in a pair act as parallel paths; the effective resistance is not simply the sum used for a loop-resistance result. Use the instrument's defined parameter and applicable limit, rather than treating similarly named readings as interchangeable.

Fluke explains that excessive unbalance can interfere with simultaneous power and data operation. The test report must identify the actual unbalance measurements. A generic PASS does not establish that all requested PoE-related parameters were measured.

Worked through

The drawing is a simplified DC sharing model, not an Ethernet pinout or a complete PSE/PD circuit. The source, return path, magnetics and negotiation circuitry are intentionally absent. Do not build this as a PoE wiring arrangement.

For this mathematical example alone, the two paths have the same 2.4 V longitudinal voltage drop: Conductor A: 8 ohms. Conductor B: 12 ohms. I_A = 2.4 / 8 = 0.30 A. I_B = 2.4 / 12 = 0.20 A. Total = 0.50 A. The current is unequal even though the paths experience equal voltage drop. The 2.4 V figure is not a PoE source voltage or an allowed cable drop.

The effective resistance of those parallel paths is: R_eq = (8 x 12) / (8 + 12) = 96 / 20 = 4.8 ohms. This is different from their 20-ohm series sum. Do not use this intentionally exaggerated example to select an unbalance pass/fail threshold. Percentage definitions and absolute-resistance provisions must come from the selected test method.

Separate heating from current sharing

For a resistive path, P = I²R. In the example: P_A = 0.30² x 8 = 0.72 W. P_B = 0.20² x 12 = 0.48 W. Total resistive power = 1.20 W. Check: 2.4 V x 0.50 A = 1.20 W.

These values describe the invented parallel segment only. They do not predict a real cable's temperature rise. At fixed current, doubling resistance doubles I²R heating; at fixed voltage, current changes. Always state what is held constant before comparing heating scenarios.

Bundle considerations

Heat removal depends on installation conditions. Cable construction, conductor size, ambient temperature, loading and pathway type influence the thermal situation. Bundling can restrict heat dissipation. Temperature can also affect transmission performance, so an ampacity or temperature check does not replace the required data-performance assessment.

The colored center of the poster bundle is illustrative, not a measured thermal image. Its circle count is not a permitted bundle size. Product-specific tables have assumptions; do not extract a cable count while discarding cable type, ambient, PoE loading or containment conditions.

Original planning exercise

A work order proposes adding powered access points to an existing cable route. The file contains a data certification report but no remote-power design record. Before approving the change, assemble:

• Exact cable and cord products, conductor size/material and temperature ratings.
• Intended PoE type/class and supported device demand.
• Number of cables, expected simultaneous powered loading and bundle arrangement.
• Ambient conditions, conduit/tray/other containment and any insulation or heat sources.
• Applicable manufacturer guidance, project criteria and locally adopted requirements.
• Whether within-pair and pair-to-pair unbalance tests are required and included in the saved reports.

This worksheet identifies missing evidence; it does not authorize an apprentice to approve a thermal design. Escalate unresolved design or acceptance questions to the responsible qualified person.

Supervised test planning

For disconnected cabling, select the supported tester method and adapters from Lesson 236. Confirm that the configured test actually includes the requested unbalance parameters. Retain the exact limit and full results. If a reading fails, inspect the documented setup and use diagnostic evidence before deciding whether a termination, component or cable needs correction. Follow authorized procedures and retest after corrective work.

Knowledge check

Where beginners go wrong

Sources