X-PoE companion

Voltage drop, dimming, and what people can actually see.

Model lighting runs with 36V and 48V fixtures, 23AWG or 22AWG conductors, and the 1115mA per-channel X-PoE output limit. The chart shows why full-output drop numbers can overstate what happens when scenes are dimmed.

Port context

X-PoE has 2 channels per port. This estimator calculates per channel so fixture current and run length stay easy to reason about.

Inputs

Defaults represent a typical 36V lighting load on 23AWG cable.

Fixture forward voltage?The minimum fixture terminal voltage needed to keep the LED load operating at the commanded current. If cable drop pulls terminal voltage below this threshold, the model treats the fixture as dropout/off.

X-PoE switch input voltage?The DC voltage feeding the X-PoE switch or channel power stage. Default is 54V.

Internal switch voltage drop?Voltage lost inside the X-PoE switch before the cable run. The default assumption is 1.5V.

Available before cable: 52.5V?This is switch input voltage minus the internal switch drop. Cable drop is subtracted after this, then the result is compared with fixture forward voltage.

Rated lumens?The fixture's rated light output from its datasheet. This sets the starting light level before dimming or dropout.

Load current model?The calculator derives fixture current from load power: watts divided by fixture forward voltage. The channel current limit is only a cap; it is not a second load input.

Current used: 1,111 mA = min(40.0W / 36.0V, 1,115mA cap)

Fixture load power?The fixture input power used to estimate current. Current is calculated as watts divided by fixture forward voltage, then capped by the channel current limit.

Channel current limit?The maximum current this channel can provide. X-PoE defaults here to 1115mA. If the fixture power would require more current than this, calculations use this cap.

Wire gauge?Wire gauge sets conductor resistance. 23AWG is the default; 22AWG has lower resistance and therefore less cable drop.

Selected dimming level?The dimming command scales current in this estimator. Lower current means less cable voltage drop and less cable loss.100%

Run lengths?Enter one-way cable length from the X-PoE output to the fixture. The model uses round-trip conductor length for voltage drop.

70 ft?One-way cable length. Electrical resistance is calculated over the outbound and return conductors.

100 ft?One-way cable length. Electrical resistance is calculated over the outbound and return conductors.

300 ft?One-way cable length. Electrical resistance is calculated over the outbound and return conductors.

Fixture summary

Absolute max one-way run

300 ft+

Load current used

1,111 mA

Voltage leaving switch

52.5 V

Power at current limit

40.1 W

Fixture load power

40.0 W

Entered rated output

3,000 lm

Current limit active

At full output, this fixture has enough voltage headroom to stay above forward voltage through the 300 ft planning range. The channel has 16.5V of headroom before cable loss, using a 36.0V forward-voltage threshold and 52.5V available before the cable.

Run

70 ft

23 AWG

Voltage drop: 3.17 V
Fixture terminal voltage: 49.33 V
Forward-voltage headroom: 13.33 V
Drop of fixture voltage: 8.80%
Cable loss: 3.52 W
Estimated delivered lumens: 3,000 lm
Perceived lightness: 100.0 L*

Run

100 ft

23 AWG

Voltage drop: 4.52 V
Fixture terminal voltage: 47.98 V
Forward-voltage headroom: 11.98 V
Drop of fixture voltage: 12.57%
Cable loss: 5.03 W
Estimated delivered lumens: 3,000 lm
Perceived lightness: 100.0 L*

Run

300 ft

23 AWG

Voltage drop: 13.57 V
Fixture terminal voltage: 38.93 V
Forward-voltage headroom: 2.93 V
Drop of fixture voltage: 37.70%
Cable loss: 15.08 W
Estimated delivered lumens: 3,000 lm
Perceived lightness: 100.0 L*

Comparison at 100% dimming

70 ft to 300 ft extra cable drop

10.41 V

Estimated lumen difference

0 lm

Perceived difference

0.0 L*

Forward voltage is a threshold

The fixture does not use every extra volt as more light. It needs enough terminal voltage to stay above forward voltage at the commanded current.

Current drives drop

Voltage drop is I x R. When a scene dims, current falls with the command, so the wire drop falls with it.

Distance matters visually

A 70 ft and 100 ft run are usually not adjacent at the same task plane. Spatial separation reduces direct comparison.

Individual dimming behavior

Each run on its own

These charts remove the side-by-side comparison and show what each cable length does over the dimming range, including perceived lightness.

Estimator notes

This model assumes copper conductors, one-way run length entered by the user, round-trip conductor resistance, and a dropout threshold: switch input voltage minus internal switch drop minus cable drop must remain above fixture forward voltage. Above that threshold, estimated lumens follow the dimming command. Below it, the model marks the fixture as off/dropout. Real fixtures and LED drivers can regulate, fold back, flicker, or fail differently, so use this to explain order-of-magnitude behavior and compare scenarios, not as a compliance certification.