Power over Ethernet (PoE) delivers data and DC power over one cable, and the three standards differ in how much power they can deliver: 802.3af (PoE) offers up to 15.4W, 802.3at (PoE+) up to 30W, and 802.3bt (PoE++) up to 60W or 90W. This guide explains the budgets, when each makes sense, and how PoE compares to traditional power supplies.
What Is Power over Ethernet (PoE)?
PoE sends both network data and DC power over the same Ethernet cable, so devices like cameras, access points and sensors need only a network drop, not a wall socket. A PoE switch or injector supplies the power, and the powered device (PD) draws what it needs up to the standard’s limit. The power budgets are defined by the IEEE 802.3 standards.
802.3af vs 802.3at vs 802.3bt: Power Budgets Compared
The budgets decide which devices each standard can serve:
| Standard | Name | Max power per port | Typical devices |
|---|---|---|---|
| 802.3af | PoE | 15.4W | IP phones, basic cameras |
| 802.3at | PoE+ | 30W | Pan-tilt cameras, access points |
| 802.3bt | PoE++ | 60W / 90W | LED lighting, PTZ cameras, terminals |
Note that the port budget is at the switch; the device receives slightly less after cable loss. The IEEE standards are the reference — see IEEE 802.3 for the specification.
PoE vs Traditional Power Supplies: When to Use Which
Choose PoE when running a power cable is the hard part — cameras on ceilings, access points, sensors in walls — and when central backup power is a benefit. Choose a traditional power supply when the device needs more than 90W, when distance exceeds PoE’s cable limits, or when the installation already has sockets. Many devices support both, letting installers pick per site.
Advantages of PoE in Modern IT Environments
PoE’s advantages are operational: one cable for data and power, central UPS protection at the switch, remote power cycling, and simplified planning for edge devices. The trade-offs are the 100m cable limit and the port budget being shared across the switch’s power budget. For OEM projects, the decision is whether the device should be PoE-powered, adapter-powered, or both.
PoE Planning for OEM Projects
When you design a PoE-powered product, define the class and power draw early, confirm the PSE (switch/injector) compatibility, and plan the input stage to accept 48V DC. If your product line also needs adapter-powered variants, a factory that handles both power paths — like ours — simplifies the BOM. The OEM/ODM page covers mixed power projects, and the GaN charger product line shows the adapter side.
PoE Power Budget: How Many Devices Per Switch
The number of devices a PoE switch can power is not the number of ports — it is the total budget divided by each device’s draw.
A 24-port PoE+ switch with a 370W budget can power roughly 12–18 typical cameras or access points, not 24, because each 30W-capable port draws its actual load and the budget is shared. When you plan a deployment, total the expected draw of every device, add 20% headroom, and pick the switch class that fits — the same budgeting logic you would use for any power supply.
PoE for Cameras and Access Points: Real-World Draw
Device draw varies more than the port rating suggests, and real numbers prevent an undersized deployment.
- IP cameras — basic models draw 4–7W; pan-tilt-zoom cameras pull 15–25W with heaters and motors.
- Wi-Fi access points — 8–15W typical; multi-radio and tri-band units can reach 25W+.
- Sensors and IoT gateways — 2–10W, often the easiest to power.
Spec the camera or AP first, read its actual draw, then size the switch class — 802.3at for most, 802.3bt for the heavy units.
PoE Injectors vs PoE Switches: What’s the Difference
Both deliver power over Ethernet, but they fit different situations and the choice affects cost and management.
- PoE injector — adds power to one cable between a normal switch and the device; cheapest way to power a single camera or AP.
- PoE switch — integrates power into every port; simpler cabling, central management and one power budget for the whole rack.
- Midspan vs endspan — the same distinction from the vendor’s side: injectors are midspan, PoE switches are endspan.
For one or two devices, an injector is fine; beyond that, a PoE switch pays for itself in wiring and management.
FAQ
Can a PoE++ switch power an 802.3af device? Yes — the standards are backward compatible; the device draws only what it needs.
How far can PoE run? The Ethernet limit is 100m for data; power follows the same cable run.
Do I need a special cable for PoE? Cat5e or better is recommended; higher-power PoE++ benefits from thicker copper.
Can a PoE device also accept a wall adapter? Yes, many support both, with the adapter taking over when present.
Can you build adapter-powered variants for my PoE device? Yes — mixed power paths are a standard OEM project. See the OEM/ODM page or contact us.
References: IEEE, Wikipedia’s Power over Ethernet article, Microchip for PoE ICs, and Texas Instruments for PoE app notes.