How to work out the PoE power budget for a camera site

A PoE budget is not the sum of the port ratings printed on the switch. It is three separate numbers — a total supply figure, a per-port allocation rule, and a reserve — and treating them as one figure is the most common reason a camera installation browns out at dusk, when every IR illuminator switches on within a few minutes of each other.

How is a PoE power budget actually calculated?

Begin with the nameplate figure: a 24-port switch may state 370 W total PoE output. Divide by 24 and you get 15.4 W per port, which happens to equal 802.3af exactly. That coincidence misleads people into reading the switch as "af only". In reality the port capability is set by the PSE controller — a port can be negotiated at 30 W — while the 370 W total is set by the internal power supply. Capability per port and capacity per chassis are different constraints, and the smaller of the two governs.

The allocation rule decides how the total is spent. Under static class-based reservation the PSE reserves the class maximum for each port as soon as it classifies the device: 25.5 W for every Class 4 camera, whether that camera draws 9 W or 24 W. On 370 W that stops you at twelve ports even if the real load is 110 W. Under dynamic budgeting the PSE tracks live consumption and grants each new port whatever remains, so the same 370 W can power all 24 ports at 11 W each. Two switches with identical wattage behave completely differently; ask which model is used before comparing datasheets.

Then convert. 370 W of PoE output at 90% internal efficiency draws about 411 W from the internal rail and roughly 440 W at the AC inlet. Most supplies are derated above 40–45 °C, so the realistic maximum is lower than the label in any cabinet that sees afternoon sun.

What separates 802.3af, 802.3at and 802.3bt in budget terms?

Each generation raises the current limit and the guaranteed power at the device. The gap between the PSE and PD figures is not design margin — it is the standard's allowance for loss in the cable.

For cameras the bands are pragmatic. A fixed 4 MP dome or bullet with IR sits inside 12.95 W without effort. A PTZ with a 30× zoom block fits 25.5 W only while the heater is idle; below about −10 °C a heater can pull 15–20 W on its own, which exceeds the Class 4 envelope unless the camera throttles. Type 4 at 71.3 W covers a large PTZ running heater, wiper and defroster at once, or a multisensor unit with several imagers and one shared heater.

Higher classes are not free. Type 3 and Type 4 put power on all four pairs, which requires 4-pair magnetics and 4-pair current sensing in the PSE, cable rated for 960 mA per pair on Type 4, and more heat dissipated in the same chassis volume. Choose the class from measured night-time draw per port, not from a blanket "headroom is safer" instinct.

How many cameras can a 24-port PoE switch really power?

The honest answer depends on which figure you feed into the division: the class maximum or the measured draw.

The first row is the number most quotes use, and it is the number that leaves ten ports dark. Measured draw is the number to design with — 11–13 W for a 4 MP IR bullet with the illuminator at full, 15–22 W for a mid-range PTZ before heaters, 30–45 W for a large PTZ at −25 °C. The fourth row shows why 740 W units exist: they are the only way to populate all 24 ports with cameras that genuinely need Class 4.

Note that the mixed case passes or fails depending on the allocation rule. At 308 W of a 370 W budget with dynamic tracking, the last four PTZ units will power up in sequence and stay up. With static reservation the same hardware stops at fourteen ports, because the PSE reserved 25.5 W for every camera it saw.

How much headroom should you leave on the budget?

Twenty-five to thirty percent above measured peak load is the working figure, and it has four jobs: the IR boost that arrives with dusk, heater cycles in cold weather, the inrush of a camera booting (1.5–2× steady state for one to two seconds), and the ports you will want for an access point or a replacement camera two years from now.

Worked through for a 24-camera site: 24 × 11 W = 264 W measured peak. Add 30% reserve for 343 W. Add two 12 W ports held spare for future access points and you are at 367 W, which is exactly at the edge of a 370 W chassis. The defensible selection is 480 W or a 740 W model with fewer populated ports — not because the arithmetic fails today, but because it fails in the third summer, when the supply has aged and the cabinet runs at 50 °C.

What does the budget calculation usually omit?

Splitter efficiency comes first. A 48 V to 12 V splitter running at 88% needs 13.6 W at the port to deliver 12 W to the device. On a 12.95 W af port the usable output is about 11 W, so a 12 V/1 A splitter rated at 12 W cannot reach its own rating on an af link. The same applies to 5 V and 24 V outputs, and to splitters that feed a 12 V camera with a heater — in that case the splitter has to be on an at port with the heater load added in.

Uplink and management consumption comes second. SFP modules, the management CPU and the fans draw from the same supply on many chassis, so a switch labelled 370 W may deliver 340 W of port power. Third, passthrough devices: a PD-powered switch or an extender consumes 2–4 W from the upstream port, which comes straight out of that port's class envelope rather than out of thin air.

Cold-cable resistance is the one nobody budgets. Copper rises about 0.4% per °C, so a cable pulled through a sun-exposed conduit at 60 °C has roughly 16% more resistance than the 20 °C figure the standard's loss allowance was built around. At 600 mA and 100 m that is another 0.5 W of loss — small, but it is the fifth percent that turns a 96% loaded port into a resetting camera.

FAQ

Does a 30 W port always deliver 30 W to the camera?

No. The port delivers what the camera requests through classification, capped by the PSE. A Class 3 camera on a Type 2 port draws up to 12.95 W at the device and the remaining capacity stays unused. The 30 W figure is a ceiling, not an output.

What happens if the total budget is exceeded while the site is running?

Behaviour depends on the PSE. Better units deny power to newly connected ports and log the event, leaving established links alone. Simpler units shed the highest-numbered ports or reset the whole PSE group. Either way the result is cameras that come back with the illuminator disabled, or that do not come back at all.

Can I mix af, at and bt devices on one switch?

Yes, if the switch classifies per port, which managed units do as standard. Budget each port at its own class maximum when using static reservation and confirm the sum against the chassis total — mixed sites are where the total runs out while every individual port looks healthy.

Is 12.95 W enough for an outdoor camera?

For a fixed camera with IR, usually yes. For anything with a heater, pan-tilt mechanics or a wiper, no. Read the camera's maximum power under its worst specified condition rather than its typical figure, and note that the typical figure is normally quoted at 25 °C with the illuminator off.

Should the reserve cover future ports or future power per port?

Both, and they are not the same reserve. Spare ports need allocation headroom; a camera replacement that adds a heater needs wattage headroom on a port that already exists. Thirty percent above measured peak covers the second case on a 24-port site and leaves roughly two ports' worth of allocation for the first.