Every integrator has lived this one: you install 24 cameras on a Tuesday, test everything in daylight, the picture is gorgeous, the client signs off. At 9:47 PM the security guard calls — half the cameras dropped "on their own." It's not a ghost. It's the infrared LEDs waking up with the dark: consumption climbed a few watts per camera, and the switch, already running on fumes, started shutting down ports like a captain throwing cargo overboard. PoE is genuinely modern magic, but it runs a ruthless little budget office: when the watts run out, somebody goes hungry. This guide makes sure that somebody is never your project.
48 volts down the same cable: the trick explained
Power over Ethernet injects DC — nominally 48 V, anywhere from 44 to 57 V depending on the type — over the very same twisted pairs that carry your data. Don't they fight? No: data travels as the difference in voltage between the two wires of each pair, while power rides as the common level of both wires, coupled through the transformers at each end. It's like whistling while you walk: two things at once, zero interference.
The two players have proper names: the PSE (power sourcing equipment — the switch or injector, the one writing checks) and the PD (powered device — the camera, AP or phone, the one cashing them). And here's the first number worth tattooing somewhere: the PSE always sends more than the PD receives, because the cable charges a toll along the way:
- 802.3af (PoE, Type 1): the PSE outputs 15.4 W; the PD is guaranteed 12.95 W.
- 802.3at (PoE+, Type 2): 30 W out of the PSE, 25.5 W at the PD.
- 802.3bt (PoE++): Type 3 at 60 W / 51 W, and Type 4 at 90 W / 71.3 W.
That gap isn't marketing rounding — it's straight physics. Cable loss is I²·R, and the standard's numbers check out to the watt. In 802.3af, up to 350 mA flows through a worst-case 20 Ω loop: 0.35² × 20 = exactly 2.45 W, which is precisely 15.4 − 12.95. In 802.3at it's 600 mA over 12.5 Ω: 0.6² × 12.5 = 4.5 W — that is, 30 − 25.5. And in Type 4, with nearly 1.73 A in play, 18.7 W go missing: one watt out of every five stays behind heating the cable. Which is why — spoiler for the voltage-drop section — 802.3bt spreads the current across all four pairs.
Classes 0–8: the switch reserves the worst case
When you plug in a PD, a silent negotiation happens in milliseconds. First the PSE probes for a 25 kΩ resistance signature ("there's a real PoE device here, not a wet laptop"). Then the PD declares its class — a number from 0 to 8 that says how much power it could possibly need. And here lies the detail behind half the field drama out there: the switch reserves the class maximum, not what the device actually draws.
It's like booking a restaurant table for 8 "just in case": even if only 3 people show up, the whole table stays blocked all evening. A Class 4 camera that sips 8 W in practice still locks 30 W out of the switch budget, because the switch has no way to predict when the IR LEDs, the housing heater, or the PTZ motor will kick in.
| Class | Standard | PSE reserves | PD receives | Typically |
|---|---|---|---|---|
| 0 | af | 15.4 W | 12.95 W | "default" class when the PD doesn't identify itself |
| 1 | af | 4 W | 3.84 W | sensors, basic IP phones |
| 2 | af | 7 W | 6.49 W | IP phones, simple fixed cameras |
| 3 | af | 15.4 W | 12.95 W | fixed cameras with IR, Wi-Fi 5 APs |
| 4 | at | 30 W | 25.5 W | cameras with IR/heater, Wi-Fi 6 APs |
| 5 | bt | 45 W | 40 W | small PTZs, Wi-Fi 6E/7 APs |
| 6 | bt | 60 W | 51 W | PTZs, small remote switches |
| 7 | bt | 75 W | 62 W | displays, thin clients |
| 8 | bt | 90 W | 71.3 W | large PTZs, lighting, laptops |
The 720-watt trap: your switch's power budget
Now let's assemble the pieces with the industry's most classic gotcha. You bought a "24-port PoE+" switch for your 24 Class 4 cameras. Sounds perfect: 24 ports, 24 cameras. But down in the datasheet's fine print it says PoE budget: 370 W. Let's do the arithmetic the salesperson skipped:
- Each Class 4 camera forces the switch to reserve 30 W, even if it idles at 8 W. Reservation goes by class, not by consumption.
- Total demand: 24 × 30 = 720 W requested from the budget.
- Available budget: 370 W. Shortfall: 720 − 370 = 350 W. The switch covers barely 51% of what's being asked.
- Ports it can actually power at once: 370 ÷ 30 = 12.3, so 12 ports (12 × 30 = 360 W, with 10 W to spare). Port 13 would push the total to 390 W, and there's nowhere to get it.
- Net result: 12 cameras running and 12 begging for watts. The datasheet didn't lie — "24 PoE+ ports" means any port can deliver PoE+, not that all of them can at the same time.
- To feed all 24 Class 4 cameras with 20% headroom you need 720 × 1.2 = 864 W of budget — or two switches, or cameras that negotiate less over LLDP.
Before buying the switch (or promising ports), run your device list through the PoE budget calculator: it totals by class, applies headroom, and tells you on the spot whether the budget holds.
Distance charges too: voltage drop
The second silent toll collector is cable length. Copper has resistance, and Ohm's law doesn't haggle: push 600 mA through a worst-case channel (12.5 Ω loop over 100 m) and 0.6 × 12.5 = 7.5 V vanish along the way. A Type 2 PSE outputs at least 50 V, so the device receives 42.5 V — which is exactly the minimum a Type 2 PD accepts. The standard is engineered to the millimeter: at 100 m with everything against you, you survive on fumes. At 110 m "because it was just a bit further," you don't.
Decent cable buys you slack: real 23 AWG Cat6 runs about 6.68 Ω of loop per 100 m over two pairs, so at full 802.3at load you lose around 4 V and 2.4 W — better than worst case, but still energy you paid for that ends up as heat inside the raceway. And this is where 802.3bt's play makes sense: spreading the same current across all four pairs halves the effective resistance, and the loss with it — those 2.4 W drop to 1.2 W. With 90 W on the line that's not elegance, it's thermal survival.
Two practical takeaways: first, respect the structured channel's 100 m (the TIA-568 channel guide builds the run for you, patch cords included) and sanity-check any doubtful stretch with the voltage drop calculator. Second, if you pack many PoE cables into one tray, all that I²R heat adds up: large bundles call for thermal derating — the same concept the conductor ampacity calculator applies on the electrical side.
Field habits that keep your phone quiet at night
Don't mix sketchy injectors. The 24 V "passive PoE" bundled with some radios negotiates nothing: it shoves voltage down the line without asking whether the far end can take it. Plugging an 802.3af device into a passive injector (or vice versa) is a lottery with a switch port as the stake. Simple rule: 802.3 with 802.3, passive with passive, and never a no-name injector "because it was the one in the van."
Measure real draw, but budget by class. Your 24 cameras may well sip 8 W each at idle — 192 W of real consumption, comfortably inside 370 W. But the switch reserves by class (720 W), and real draw climbs at night when the IR kicks in. The switch CLI shows you both columns:
switch# show power inline Available: 370.0 W Allocated: 360.0 W Free: 10.0 W Port State Class Alloc(W) Actual(W) Gi1/0/1 on 4 30.0 7.9 Gi1/0/2 on 4 30.0 8.4 ⋮ Gi1/0/12 on 4 30.0 8.1 Gi1/0/13 off n/a 0.0 0.0 ← no budget left
Keep 20% headroom. Whatever baseline you budget on (class or negotiated draw), add 20%: it covers the IR LEDs, the housing heater, power-supply aging, and the extra camera the client "just remembered" mid-project. With 192 W of measured draw, budget 230.4 W as your floor — and if you reserve by class, the 864 W from the example. Also configure per-port priority: if watts ever run short, let the switch drop the parking-lot camera, not the boardroom AP.
What to do now
List your PDs with their class and measured draw, run them through the PoE budget calculator, and validate any long or doubtful run with the voltage drop calculator. If the numbers come out tight, upsize the switch or downsize the class — but let arithmetic make that call, not the dark at 9:47 PM.