ProcirelPower over Ethernet: One Cable for Data and Power
PoE sends DC power over the same Cat5e/Cat6 cable as Ethernet. Standards (802. 3af/at/bt) deliver from 15 to 90 W, eliminating wall warts wherever a network drop exists.
> At a glance: 6 minute guide · part 10 of 10 in the power supplies and batteries complete guide track · includes a worked example and a quick-reference table.
## Standards and power levels
802. 3af (Type 1) delivers up to 15. 4 W at the port; 802. 3at (PoE+, Type 2) 30 W; 802. 3bt (Type 3/4) reaches 60/90 W. After cable losses, powered devices see somewhat less budgets are always quoted at both ends for this reason.
| Standard | Port power | Delivered at device | Typical loads |
| — | — | — | — |
| 802.3af | 15.4 W | ~12.95 W | Cameras, sensors |
| 802.3at (PoE+) | 30 W | ~25.5 W | APs, phones |
| 802.3bt Type 3 | 60 W | ~51 W | Pan-tilt cameras |
| 802.3bt Type 4 | 90 W | ~71 W | Laptops, lighting |
| Passive | Fixed, no negotiation | Depends | Vendor-matched gear |
## Classes and negotiation
Powered devices declare a class; the switch (or midspan injector) budgets accordingly. Passive PoE skips negotiation common in hobby gear and demands voltage-matched care. 24 V passive into an 802. 3af device is a smell event.
## Deploying with sense
Switches carry a total PoE budget across ports eight 30 W devices need a 240 W class switch, not a 150 W one. Cable quality and length (100 m limit) set losses; injectors rescue one-off devices cheaply.
## How to apply this in your build
Work through the sequence below each step assumes the previous one passed. For numbers that need calculating, the linked tools at the end of this guide do the arithmetic instantly.
1. Sum PD classes against the switch PoE budget
2. Match standard-to-standard; never mix passive blindly
3. Respect the 100 m channel and cable quality
4. Use injectors for single devices outside switch reach
### Worked example
Six 802. 3af cameras (each ~8 W) total 48 W comfortably within a 120 W PoE switch budget with headroom for a future access point. Run the numbers yourself with the Wire Gauge (AWG) and the result should agree to within rounding.
> Practical note from the bench. Our network builds tabulate every PD and its class against the switch budget the spreadsheet outlives memory.
## Common mistakes to avoid
– Assuming every network port supplies power
– Passive 24 V injectors into standards-based cameras
– Ignoring total switch budget while adding devices
## Key takeaways
– Standards and power levels the foundation of this guide; revisit it if any measurement here surprises you.
– Classes and negotiation the foundation of this guide; revisit it if any measurement here surprises you.
– Deploying with sense the foundation of this guide; revisit it if any measurement here surprises you.
## Prerequisites and preparation
Before starting: sum pd classes against the switch poe budget and match standard-to-standard; never mix passive blindly. Keep the [Wire Gauge (AWG)](/tools/wire-gauge-awg) open every number in the worked example is reproducible. Total time including the bench steps: about 6–6 minutes.
## Who benefits most
Hobbyists meeting this topic for the first time, students who want the version with real numbers instead of abstract symbols. Returning engineers refreshing a corner of the craft. The mistake list alone justifies the visit every entry in it was learned the expensive way.
### Quick reference card
| Aspect | Where to find it in this guide |
| — | — |
| Core theory | Standards and power levels |
| Application steps | How to apply this in your build |
| Worked numbers | Worked example |
| Failure modes | Common mistakes to avoid |
## How this fits the power supplies and batteries complete guide track
This guide is one stop in the structured learning path. Start from the [power supplies and batteries complete guide](/tutorial/power-batteries-complete-guide) pillar page for the full map, or continue with [backup power](/tutorial/ups-inverter-basics) and [network segmentation](/tutorial/iot-security-best-practices). For the arithmetic, open the [Wire Gauge (AWG)](/tools/wire-gauge-awg).
## Frequently asked questions
Do I need a PoE switch or an injector?
One device → injector; several or growing → switch economics take over quickly.
Can PoE damage non-PoE equipment?
Standards negotiation prevents it; passive systems can know which world you are in.
Is there a calculator for this?
Yes the [Wire Gauge (AWG)](/tools/wire-gauge-awg) tool runs the formulas from this guide instantly, client-side, with no signup.
## What to read after this
– The complete power & batteries guide: [Power & Batteries complete guide](/tutorial/power-batteries-complete-guide)
– Read next: [bms design tutorial: battery management systems explained](/tutorial/bms-design-tutorial)
– Also in this track: [solar mppt explained: maximum power point tracking](/tutorial/solar-mppt-explained)
– Continue with: [linear vs switching regulators: choosing correctly](/tutorial/linear-vs-switching-regulators)
– Calculate as you go: [runtime estimator](/tools/battery-life) · [regulator designer](/tools/lm317-regulator) · [capacitor code tool](/tools/capacitor-code)
– From here, the natural continuation is the next guide in the track index. It assumes exactly the vocabulary this page built and adds the next layer of practice.
## Field notes
The fastest way to internalise this topic is to change one variable deliberately and predict the result before measuring. Wrong predictions are the curriculum, they show exactly which mental model needs revisiting, and the bench grades honestly.
Component substitution is a legitimate experiment as long as it is deliberate. Swap one part, predict the effect, measure, and record. That single habit converts a parts bin into a teaching lab and makes every future guide in this track faster to absorb.
## Formulas and checks from this guide
Verification checklist for this track. Verify regulation under load, not just open-circuit, measure inrush where it matters. Treat every lithium cell as energetic chemistry that has earned its protection chain.
Bookmark this page against your next build in the track. The checklist above is the same one used across 18 guides in this series.
## Field lessons worth keeping
Any lithium pack of 2S or more, without exception.
No. Oversizing hammers the diodes with inrush and buys ripple you no longer need once regulation follows.
## How to revisit this guide
Second readings work best with a purpose. Pick one section from Standards and power levels,Classes and negotiation,Deploying with sense and rebuild only that part at the bench, predicting each value before measuring. Prediction errors mark exactly which concept needs the next pass, and the linked power calculators resolve any arithmetic doubt in seconds. Keep the marked sections in your notebook: after a month of builds, that list becomes your personal Power syllabus.