How to Test PoE Injector Output Voltage and Current

The Short Answer

PoE Standards Reference

The PSE (Power Sourcing Equipment) is the injector or switch port. The PD (Powered Device) is the camera, AP, phone, or whatever consumes the power. PSE voltage is what the injector outputs; PD voltage is what the device sees after cable losses. Maximum power at the PD is lower than at the PSE because some power dissipates as heat in the cable.

What You Need to Test PoE Output

Dedicated PoE tester (recommended)

A purpose-built PoE tester safely simulates a PD signature, allows the injector to energize, and reads voltage and current under load. It identifies which pairs are powered (Mode A or Mode B), reports the negotiated PoE class, and shows whether the injector is delivering its rated power. Browse PoE testers for current options.

Multimeter plus RJ45 breakout

A digital multimeter set to DC voltage, paired with an RJ45 breakout adapter that exposes each of the 8 conductors as a labeled test point. This works for measuring voltage but cannot simulate a PD signature -- so 802.3-compliant injectors will refuse to energize. Use this method only with passive (always-on) injectors or with a load already connected.

Known-good PoE device

A PoE camera, AP, or splitter that you know is working. Plug it into the injector to trigger the energize sequence, then measure voltages at the device end with the multimeter. This is a common field workaround when no PoE tester is available.

Cable patch cord

A short, known-good Cat5e or Cat6 patch cord between the injector and the breakout. Bad patch cords are a frequent source of "the injector is broken" misdiagnoses.

Step-by-Step PoE Output Test

Step 1: Identify the PoE standard

Read the injector label. It will state 802.3af, 802.3at, or 802.3bt and may specify a PoE class (Class 0 through 8). The standard tells you the expected voltage range and which pairs may carry power. If the device is a switch with PoE ports rather than a standalone injector, check the switch documentation -- some switches are PoE on every port, some only on selected ports.

Step 2: Power the injector

Plug the injector's DC adapter (or AC line cord on larger units) into a known-good outlet. Most injectors have a green status LED when powered. If the LED stays off, the injector itself is dead before you test anything else.

Step 3: Connect to PoE tester or PD

Run a patch cord from the injector's PoE output to your PoE tester or known-good device. The injector should detect the load, run its detection sequence (a few hundred milliseconds), and energize the output. A second LED on the injector typically lights to indicate "PoE active." Without this LED, the injector did not detect a valid PD and is not delivering power.

Step 4: Measure voltage at the injector

If using a PoE tester, the tester displays voltage directly. If using a multimeter and breakout, with the PD connected through the breakout in pass-through mode, measure DC voltage between the appropriate pin pairs.

For Mode A (end-span injectors and 802.3af switches): measure between pin 1 (or pin 2) as one polarity and pin 3 (or pin 6) as the other. The polarity of which side is positive is determined by the PSE and is not standardized -- some injectors put positive on 1-2, others on 3-6. Either is valid.

For Mode B (mid-span injectors): measure between pin 4 (or pin 5) and pin 7 (or pin 8).

For 802.3bt Type 3/4 (4-pair PoE): power is on all four pairs simultaneously. Measure between any two of the four pairs.

Step 5: Measure voltage at the PD

Run a long patch cable (the actual installation cable, ideally) from the injector to the PD location. Measure voltage at the PD end of the cable (between the same pin pairs you tested at the injector).

Calculate the voltage drop: PSE voltage minus PD voltage. A few hundred millivolts is normal. More than 3 V drop on a Cat5e or Cat6 run under 100 meters indicates excessive cable resistance, undersized conductors (CCA cable), or a high-resistance termination.

Step 6: Measure current under load

With the PD connected and operating, the PoE tester displays current draw in mA. The reading should match the PD's specification (cameras typically draw 200-600 mA, APs 200-1000 mA, multi-radio gear up to 2 A on 802.3bt Type 4). A current draw far below the device's nameplate rating may indicate the device is in low-power mode or has not fully booted; far above suggests a failing PD.

Step 7: Confirm pair selection

The PoE tester reports which pairs carry power: Mode A (1-2/3-6), Mode B (4-5/7-8), or all four pairs. Verify this matches the PD's expected mode. Most modern PDs accept either, but some legacy devices only accept Mode A or only Mode B.

Voltage Drop Calculation

Voltage drop on a PoE link is governed by the cable's DC resistance and the current flowing. The formula is V_drop = I x R x 2 (the 2x accounts for the round trip: power flows out on one pair, returns on another).

Cable resistances per 100 meters:

• Cat5e (24 AWG solid copper): ~9.4 ohms per 100 m
• Cat6 (23 AWG solid copper): ~7.5 ohms per 100 m
• Cat6A (23 AWG): ~7.5 ohms per 100 m
• Cat5e CCA (copper-clad aluminum): ~14 ohms per 100 m

Example: A 250 mA load on a 90 m Cat5e link uses 2 conductors (Mode A or B), so each conductor carries 125 mA (split across the two wires of a pair). Resistance is 9.4 x 0.9 = 8.5 ohms per pair (round trip). Drop = 0.25 x 8.5 = 2.1 V. The PD sees PSE voltage minus 2.1 V.

For 802.3bt 4-pair PoE, the load is split across all four pairs, halving the per-pair current and quartering the loss. This is one of the reasons 802.3bt can deliver 90 W where 802.3af can only deliver 15 W on the same cable.

Common PoE Injector Faults

No voltage at injector output, even with PD connected

The injector is not detecting the PD signature. Possible causes: dead injector, faulty patch cable, PD without proper signature resistance, or wrong PoE standard (a 802.3af injector will not power an 802.3bt-only device that requires 4-pair detection).

Voltage at injector but not at PD

Cable break or open conductor. Run a wiremap test on the cable. See our continuity testing guide and our multimeter procedure.

Voltage drops to zero when PD draws current

Injector overcurrent protection has tripped. Either the PD is exceeding the injector's rated power, or there is a fault inside the PD drawing excessive current.

Excessive voltage drop along cable

Undersized conductors, copper-clad aluminum cable, excessive run length, or high-resistance terminations. Re-terminate, or replace the cable with proper Cat5e/Cat6 solid copper.

Power on wrong pairs

Some PoE devices only accept Mode A or Mode B. If the injector delivers Mode B and the device expects Mode A, the device will not power up. Most modern PDs are compatible with both modes.

Related Reading

For a broader procedure on PoE fault isolation and pinout reference, see our complete PoE testing guide and the PoE testing best practices. To compare PoE testers, see the 2026 best PoE testers roundup. The Net Chaser also detects PoE presence alongside its core throughput testing.