One cable. Data and power, together. That’s the pitch for Power over Ethernet, and it holds up — if you understand what you’re actually working with. PoE has been in the field long enough that most network gear supports it, but there are still three distinct standards floating around, a handful of “passive” implementations that predate the standards entirely, and a switch budgeting problem that bites people more often than it should.
This article covers the standards, what devices actually need which tier, and how to plan power consumption before you buy a switch — not after you discover half your access points are brownout-cycling at 2am.
What PoE Actually Does
Power over Ethernet delivers DC power alongside data over the same Cat5e/Cat6 cable that would otherwise only carry network traffic. The power source equipment (PSE) — typically a PoE switch or a midspan injector — supplies current over the cable pairs. The device at the other end, called the powered device (PD), draws what it needs.
Before the IEEE standardized this, vendors were doing it their own way. Cisco had a proprietary implementation. Ubiquiti shipped gear for years with “passive PoE” — a fixed voltage (usually 24V or 48V) delivered with no handshake, no negotiation, no protection. You matched the voltage or you fried something. Some of that gear is still in the field, and passive PoE injectors are still sold, so it’s worth understanding the distinction: active PoE negotiates; passive PoE does not. Stick with active (standards-based) PoE for any modern deployment. The handshake protects you from mismatched voltages and overloading.
The IEEE standards are what everyone means today when they say “PoE.” There are three of them, with the latest split into two tiers.
The Standards Table
| Common Name | IEEE Standard | Type | Max Power at PSE | Min Power at PD | Pairs Used |
|---|---|---|---|---|---|
| PoE | 802.3af | Type 1 | 15.4W | 12.95W | 2 |
| PoE+ | 802.3at | Type 2 | 30W | 25.5W | 2 |
| PoE++ | 802.3bt | Type 3 | 60W | 51W | 4 |
| PoE++ | 802.3bt | Type 4 | 100W | 71.3W | 4 |
A few things worth pulling out of that table.
PSE vs. PD wattage: The power available at the PSE (switch port) is always higher than what the PD (your device) actually receives. Cable resistance eats some of it. The PD figures are the guaranteed delivery minimums — what the standard promises will actually arrive at the device. When a manufacturer says a camera or access point needs “up to 25.5W,” they mean PD watts. When a switch spec sheet lists “30W per port,” that’s PSE output. Those numbers are measuring different ends of the same cable.
802.3bt uses all four pairs: The original PoE and PoE+ standards only used two of the four wire pairs in a Cat cable, which capped how much power they could push. 802.3bt (PoE++) uses all four pairs — which means you need Cat5e or better with all pairs intact and terminated. Damaged or partially-punched cable that got away with PoE+ may not hold up for PoE++.
Backward compatibility: All three standards are backward compatible. A PoE++ switch will happily power a PoE device, negotiating down to the appropriate level. The handshake handles this automatically. The reverse doesn’t work — a PoE switch cannot provide enough power for a device that requires PoE++.
What Devices Need What
802.3af / PoE — Up to 12.95W at the Device
The original standard still covers the majority of simple network devices deployed today. If a device needs less than 13W, it likely lands here.
- VoIP phones — Standard desk phones with a small screen typically draw 3–8W. This is the original use case PoE was built around.
- Basic IP cameras — Fixed-angle cameras with no moving parts, no heater, no IR spotlight pull 5–12W. Bullet and dome cameras from most vendors fall in this range.
- Access control readers — Door readers, card scanners, keypads: 2–5W is typical.
- IoT sensors and environmental monitors — Anything that’s mostly just broadcasting data periodically will live comfortably on PoE.
- Simple wireless access points — Lower-end single-band or older 802.11n APs. Worth noting that modern Wi-Fi 6 and Wi-Fi 6E APs typically need PoE+ or higher; check the spec sheet.
802.3at / PoE+ — Up to 25.5W at the Device
PoE+ is the workhorse tier. Most of what gets deployed in a modern small business or prosumer home lab lands here.
- Wireless access points (Wi-Fi 5 / Wi-Fi 6) — Dual-band and tri-band APs with multiple spatial streams need more headroom. A UniFi U6 Pro, for example, draws up to 13W in use but can burst higher during startup; PoE+ gives the margin needed.
- PTZ cameras — Pan-tilt-zoom cameras have motors. Moving parts cost watts. Count on 15–25W depending on whether the camera has IR or a heated housing.
- Video IP phones — Phones with large color touchscreens, built-in video cameras, and Bluetooth can approach the PoE+ ceiling.
- Small outdoor enclosures with radios — Wireless bridges, sector antennas, and outdoor APs in heated/cooled enclosures pull more than their indoor counterparts.
802.3bt Type 3 / PoE++ — Up to 51W at the Device
This is where things get interesting. 60W at the PSE means you can power devices that would have previously needed a local power brick and a separate data cable.
- Wi-Fi 6E and Wi-Fi 7 access points — Multi-radio APs with 6GHz support can draw 30–45W. Enterprise-grade Aruba, Cisco, or Meraki APs often require Type 3.
- LED lighting systems — PoE-powered luminaires for office environments. Not a niche use case anymore — PoE lighting eliminates low-voltage wiring runs and allows software-controlled dimming and scheduling.
- Video conferencing room endpoints — Dedicated conferencing bars and camera/speaker units with local processing.
- Digital signage displays — Small-format screens for wayfinding, menu boards, or lobby displays.
802.3bt Type 4 / PoE++ — Up to 71.3W at the Device
The high end. 100W at the PSE pushes into territory where you’re essentially replacing a laptop power adapter or powering something with significant compute.
- Thin clients and small form-factor PCs — A low-power workstation or terminal that can run entirely off a single PoE cable, with no local power supply. Some vendors specifically market PoE-powered mini PCs for digital signage or kiosk deployments.
- Laptops — Several laptop manufacturers have begun supporting USB-C PoE adapters. A laptop that charges at 65W is well within Type 4’s delivery capability.
- PTZ cameras with environmental controls — High-end PTZ units in outdoor or industrial enclosures with integrated heaters and high-powered IR illuminators.
- Industrial automation and robotics — Sensors and actuators with enough draw to need the full budget.
PoE Power Classes
The IEEE standards also define a class system (Class 0 through Class 8) that devices use during negotiation to declare how much power they need. The switch uses this to allocate its budget. You don’t normally configure this — the device declares its class automatically — but it’s useful to understand when reading switch logs or troubleshooting allocation issues.
| Class | Standard | Max PD Power |
|---|---|---|
| 0 | 802.3af | 12.95W (default, unclassified) |
| 1 | 802.3af | 3.84W |
| 2 | 802.3af | 6.49W |
| 3 | 802.3af | 12.95W |
| 4 | 802.3at | 25.5W |
| 5 | 802.3bt | 40W |
| 6 | 802.3bt | 51W |
| 7 | 802.3bt | 62W |
| 8 | 802.3bt | 71.3W |
A Class 0 device is one that doesn’t declare a class — the switch has to assume worst case and allocate the full 15.4W for it. If you have older, non-classifying devices on a congested switch, this is one of the places your budget disappears without obvious explanation.
Switch PoE Budgeting
This is where people get into trouble.
When a switch is marketed as “24-port PoE+,” that does not mean it can simultaneously deliver 30W to all 24 ports. The math would be 720W, and a switch that could actually do that would cost significantly more, weigh more, and need more cooling than most of the switches in that market segment. The total PoE budget — the power envelope the switch’s internal supply is rated to deliver across all ports combined — is a separate, smaller number.
A representative example: A 24-port PoE+ switch with a 185W total budget. Each port can deliver up to 30W individually, but the sum of all active ports is capped at 185W. If you tried to drive every port at maximum, you’d hit the wall at around 6–7 ports. What actually happens depends on the switch: some queue power allocation by port priority, others simply stop powering new devices when the budget is exhausted, and some will shut down lower-priority ports to accommodate a new device.
How to Calculate Your Budget
The calculation itself is straightforward. The discipline is in using actual device consumption, not the per-port maximums.
Step 1: Inventory your devices and their real draw.
Pull the spec sheet for every PoE device you’re connecting. Look for the actual rated power consumption (or “PoE power consumption”), not the port maximum the standard allows. A UniFi U6 Lite access point is rated for up to 10W of actual draw, even though it’s technically a PoE+ device that could draw up to 25.5W. Using 25.5W in your budget calculation for that device wastes 15.5W of headroom on paper.
Step 2: Sum your realistic consumption.
Add up the expected watts for each device based on its spec sheet. If you have devices with variable draw (PTZ cameras that use more when moving, APs that use more under heavy client load), use the higher operational figure, not the idle figure.
Step 3: Add a headroom buffer.
A common rule of thumb is 20–25% overhead above your calculated sum. This accounts for startup current spikes (many PoE devices draw peak power during boot), unclassified devices that cause the switch to allocate worst-case headroom, and future devices you haven’t deployed yet.
Budget Required = (Sum of Device Watts) × 1.25
Step 4: Compare to the switch’s rated PoE budget.
The switch’s total PoE budget needs to exceed your calculated requirement. This number is in the switch’s datasheet — look for “PoE budget,” “total PoE power,” or “maximum PoE power.” Do not confuse this with the maximum per-port wattage, which is a different number.
A Worked Example
Say you’re deploying the following on a single switch:
- 6× Wi-Fi 6 access points at 13W each → 78W
- 4× IP cameras (fixed, no PTZ) at 8W each → 32W
- 8× VoIP phones at 5W each → 40W
- 2× PTZ cameras at 20W each → 40W
Subtotal: 190W
With 25% headroom: 238W minimum PoE budget required
A 24-port switch with a 185W budget doesn’t cut it. You’re looking for something in the 250–370W range, or you split the deployment across two switches.
Budget Allocation Policies
Different switches handle budget exhaustion differently, and it’s worth knowing how yours behaves before you deploy — not during an outage.
First-come, first-served: The switch powers devices as they come online, in order. When the budget is full, new devices don’t get power. Ports aren’t prioritized.
Port priority: The switch lets you assign priority levels (high, medium, low) to individual ports. When the budget is constrained, high-priority ports are maintained and low-priority ports are shed first. This is how you protect your access points and cameras from being knocked offline when someone plugs in a device on a low-priority port.
Static allocation: Some managed switches let you configure a fixed wattage reservation per port, regardless of what device is connected. This gives you predictable budgeting but wastes headroom on ports where devices draw less than the reservation.
On any managed switch worth running, you should configure port priorities explicitly rather than relying on defaults. Know which ports are critical, set them high, and let the switch protect them automatically.
Cable Requirements
PoE works over Cat5e and above for all standards through 802.3bt. The practical considerations:
- Cable length: PoE is rated to the same 100-meter maximum as standard Ethernet. Longer runs have higher resistance, which means more power is lost as heat in the cable, and less arrives at the device. For 802.3bt Type 4 deployments at full power, shorter runs are better.
- Cable quality: Cheap, out-of-spec patch cables that pass data fine can have resistance high enough to cause problems under PoE load. If you’re troubleshooting a device that powers up unreliably, the cable is a legitimate suspect.
- All four pairs: For 802.3bt, all four pairs need to be intact and properly terminated. A partially-punched keystone jack that’s been working for years on standard PoE may fail under 4-pair power delivery.
Practical Takeaways
PoE is one of those infrastructure layers that’s invisible when it works and genuinely annoying when it doesn’t. The problems almost always trace back to one of three things: wrong standard for the device (PoE where PoE+ was needed), budget exhaustion at the switch (more devices than the supply can support), or a cable that’s marginal under load.
Before buying a switch, know your devices, know their real draw, add 25% overhead, and match that to the switch’s published PoE budget — not the per-port wattage number that’s usually what gets listed first in marketing copy. A 30W-per-port switch with a 185W total budget is a different product than a 30W-per-port switch with a 370W total budget, and the spec sheets will tell you which is which if you know to look.
The next person trying to figure out why half the APs went dark after adding four cameras should not have to start from zero.