IEEE 802.3bt Test Procedures for PoE++ Installations

What 802.3bt Changes for the Tester

Three fundamental changes from 802.3af/at testing:

1. Four-pair power delivery

802.3af and 802.3at deliver power on two pairs (Mode A or Mode B). 802.3bt delivers power on all four pairs simultaneously. Every conductor in the cable must be terminated correctly and free of damage. A wiremap fault on the spare pairs that did not affect basic PoE will break 802.3bt -- the device negotiates as Type 3 or Type 4, attempts to draw current on all four pairs, and fails when one pair has no continuity.

2. Two-event classification

802.3af and 802.3at use a single classification voltage event to declare classes 0-4. 802.3bt extends this with a second classification event. The PSE drops the classification voltage, then re-applies it -- a 2-event-aware PD signals classes 5-8 during this second event. PoE testers that only handle 1-event classification cannot fully validate 802.3bt-capable hardware.

3. Higher current and tighter voltage budget

802.3bt Type 4 delivers up to 960 mA per pair at full load. Across four pairs, that is much more current than 802.3at's 600 mA on two pairs. Higher current means more voltage drop on the same cable, which means tighter voltage budget at the PD. A run that comfortably delivered 802.3at may struggle with 802.3bt.

Pre-Test Checklist for 802.3bt Installs

Before applying any PoE test procedure, validate the cable infrastructure:

Skip any of these and the PoE test that follows may pass under benign conditions but fail in production. The pre-test is not optional for 802.3bt.

The 802.3bt Test Procedure

Step 1: Verify wiremap with all 4 pairs

Use a VDV MapMaster 3.0 or LanSeeker wiremap tester. Confirm all 8 conductors are correctly terminated (1-2, 3-6, 4-5, 7-8 pairs continuous to the same pins on the far end). Any miswire, open, or short on any pair fails the test.

Step 2: Certify the cable to Cat6/6A spec

Use a Net Chaser or equivalent certifier to confirm the run meets the cable category specification: NEXT, return loss, insertion loss, and length all within Cat6 (or Cat6A for 10Gbps) margins. The cable's TIA category certification is independent of PoE support, but a cable that fails certification will not reliably support 802.3bt at full power.

Step 3: Test PoE class detection at the switch

Connect a 802.3bt-capable PoE tester to the switch port via a short patch cable. The tester should declare Class 8 and verify the switch responds with successful 2-event classification. If the switch only acknowledges Class 4 even when the tester declares Class 8, the per-port configuration is limiting the class. Check the switch CLI to enable higher-class operation.

Step 4: Move to the device end and test under load

Repeat the test at the far end of the installed cable run. The tester should report:

• Successful Class 8 (or appropriate class) negotiation
• Power present on all four pairs
• Voltage at the device end above 42.5V (52-57V at the switch minus voltage drop)
• Wattage delivered matching the declared class allocation

Step 5: Document voltage drop

Compare voltage at the switch end versus voltage at the device end. Drop should be below 9V on a 90-meter Cat6 run for Class 8. Higher drop indicates termination issues, marginal cable, or a run that should be Cat6A.

Step 6: Verify under simulated load

If the tester supports load simulation, apply a load matching the device's full power draw and verify voltage stays above the device threshold under load. Static voltage measurements can be misleading -- voltage may sag under load enough to cause the device to brown out even though no-load voltage looked acceptable.

Common 802.3bt Test Failures

Class declared but power only on 2 pairs

The PSE recognizes Class 5-8 but only delivers power on the data pairs. Cause: per-port configuration has 4-pair PoE disabled, or the switch firmware does not properly implement 4-pair power despite advertising 802.3bt support. Workaround: try a different port, update firmware, or verify the switch model number against the manufacturer's PoE compatibility matrix.

Voltage in spec at switch, out of spec at device

Cable resistance is dropping more voltage than expected. Causes: poor termination at one end, CCA cable, runs longer than the cable category supports for the load, or thermal heating in dense cable bundles. Re-terminate, replace if CCA, or move the device to a shorter run.

Type 3 succeeds, Type 4 fails

The cable can support 51W but not 71.3W. Often caused by termination resistance that is fine for moderate current but fails at higher current loads. Re-terminate both ends and retest. If Type 4 still fails on Cat6, upgrade the run to Cat6A.

Test passes static, device fails under load

Static voltage looks fine, but the device reboots when it powers up its full load (PTZ motors, IR illuminators, multiple radios). Cause: voltage sags under load to below the device's brown-out threshold. Solution: load-test with a tester that simulates the actual device current, or temporarily place the device on a known-good short run to confirm device health, then address the cable.

Class downgraded by LLDP

The PSE initially negotiates Class 8 via classification, then LLDP-MED renegotiates the device down to a lower class because the device declares it only needs less power. This is correct behavior, not a fault, but can confuse testers that read the initial classification rather than the post-LLDP state. Verify against the device's actual operational power draw.

Tools That Properly Test 802.3bt

Documentation: What to Record per Run

For commissioning records, warranty claims, and future troubleshooting, document the following per port:

• Date and tester operator
• Switch make, model, and firmware version
• Switch port number and configured PoE class limit
• Cable category, brand, and run length
• Wiremap test result (pass/fail, all 8 conductors)
• Cable certification result (Cat6 or Cat6A pass/fail)
• PoE class negotiated
• Voltage at switch end (no-load and under load)
• Voltage at device end (no-load and under load)
• Wattage delivered
• Pairs carrying power (verify all four for Type 3/4)

The tester output is the primary documentation. Many modern testers export reports as PDF or CSV for archive. See our guide on reading certification reports for what each parameter actually means.