GigE Vision PoE Budget: The Camera Datasheet Is Not the Load
Quick answer / TL;DR: Short answer: budget from the power the camera needs at its connector, add the cable loss, and compare to what the PSE port can deliver. At 100 metres, 24 AWG loses noticeably more than 23 AWG. IR illuminators can double a camera's draw, and 802.3af at 15.4 watts often will not cover it, so specify 802.3at.
Three PoE adapters: injector adds power, splitter removes it, midspan sits in the run.A camera rated at 12 watts does not draw 12 watts from the switch. It draws 12 watts at its own connector, and the cable consumes the difference, which at 100 metres through thin conductors is not a rounding error.
PoE budgets are usually calculated by adding datasheet figures and comparing to the switch total. That method fails on long runs and on cameras with IR illuminators, which are the two cases where it matters most.
This article gives the arithmetic, the gauge effect, and the negotiation behaviour that determines what a camera actually receives.
PoE delivers power over the same pairs that carry data, which means the cable's conductor resistance is in series with the load. Loss rises with current squared, so a camera that draws twice the current loses four times the power in the cable.
The standard power tiers are the starting point: 802.3af provides 15.4 watts at the source with 12.95 watts guaranteed at the device, and 802.3at provides 30 watts with 25.5 watts guaranteed.
The Budget Arithmetic
| Item | 802.3af Type 1 | 802.3at Type 2 |
|---|
| PSE output per port | 15.4 W | 30 W |
| Guaranteed at device | 12.95 W | 25.5 W |
| Reserved for cable loss | 2.45 W | 4.5 W |
| Typical camera draw | 4 to 8 W | 8 to 20 W |
| IR illuminator added | often exceeds budget | usually fits |
The reserve figure is what the standard allows for cable loss at the maximum channel length with the specified conductor size. A cable with thinner conductors, or a channel longer than the assumption, exceeds it.
That is why a camera that works on a two metre bench cable can fail to start on a 90 metre installed run: the same camera, the same port, a different loss.
The IR Trap
Illuminators are switched loads. A camera may draw 6 watts in daylight and 14 watts at night, and the port budget is set for the higher figure.
Systems commissioned during the day are frequently commissioned at the lower figure, and the first night produces a camera that reboots or drops off the network.
- Budget from the camera's maximum draw including illuminators, not its nominal figure
- Prefer 23 AWG over 24 AWG for long PoE runs, because the loss difference compounds
- Check the PSE total budget, not only the per-port budget, since a switch can be port-compliant and total-limited
- Commission with the illuminator on, or note the night-time figure explicitly
- Measure the voltage at the camera, not only at the switch
Real-World Applications
A perimeter installation runs cameras at 95 metres with IR illuminators. The initial design used 802.3af and 24 AWG, and half the cameras rebooted at dusk. Moving to 802.3at resolved it without changing cable.
A production line uses 802.3af cameras at 30 metres, where the loss is negligible. The short runs mean the cheaper tier is adequate, and the budget has headroom for a camera upgrade.
A multi-camera inspection station draws 60 watts total across four cameras. The switch has a 120 watt total budget, so the per-port figures pass but the total is the constraint that decides whether a fifth camera can be added.
An outdoor gantry uses cameras with heaters as well as illuminators. The heater draws continuously, so the budget is calculated on the cold-start condition rather than the running condition.
Standards and Verification
PoE is specified in IEEE 802.3af, 802.3at and 802.3bt, with the last adding Type 3 and Type 4 higher-power tiers. Cable channel performance follows ISO/IEC 11801 and TIA-568, and bundle thermal derating follows TIA TSB-184-A.
Verification should include the negotiated power class at each port, the measured voltage at the camera under load, and a check of the switch total budget against the sum of allocated port budgets.
For cameras with illuminators, repeat the measurement with the illuminator active. A voltage that is adequate in daylight is not evidence of anything.
Negotiation and Fallback
A PSE and a camera negotiate a power class at startup. If the class the camera requests is not available, a compliant PSE refuses power rather than delivering a lower amount.
That behaviour is useful: a camera that does not start is reporting a budget problem rather than failing silently later.
Where a camera starts and then reboots under load, the negotiation succeeded and the loss is in the cable, which points at length or gauge rather than at the switch.
Related reading: GigE Vision multi-camera bandwidth — blogs/news/gige-vision-multi-camera
Related reading: PoE++ cable requirements — blogs/news/poe-plus-cable-requirements
Related reading: GigE Vision cable — blogs/news/gige-vision-cable
A camera that reboots at dusk is not a firmware problem. It is a power budget that was calculated in daylight.
Frequently Asked Questions
How much power does a GigE Vision camera need?
Typically 4 to 8 watts for a machine vision camera, but 8 to 20 watts once IR illuminators or heaters are included. Budget the maximum, not the nominal.
Is 802.3af enough for a PoE camera?
For short runs and cameras without illuminators, usually. For 100 metre runs or cameras with IR, 802.3at is the safer specification.
Does cable gauge affect PoE?
Yes. Thinner conductors have higher resistance, so more of the budget is lost in the cable. The difference between 23 and 24 AWG matters on long runs at higher currents.
Why does a camera work on the bench but not installed?
Cable loss. The bench cable is short, so almost all the budget reaches the camera. The installed run consumes the reserve the standard allows.
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