Power over Ethernet is one of those technologies that sounds simple until you start planning a real deployment. You've got a PoE switch, some cameras, a few access points, maybe an IP phone or two. Plug them in and go, right?
Not quite. Every cable run loses power. Every device has a different draw. And if you exceed your switch's power budget, devices start randomly disconnecting — usually at the worst possible time.
Here's how to plan a PoE power budget properly, from the math to the gotchas that catch people off guard.
Understanding PoE Standards
First, let's get the standards straight. There are four main PoE types, and they're not all created equal.
| Standard | IEEE Spec | Max Power (PSE) | Max Power (PD) | Common Use |
|---|---|---|---|---|
| PoE | 802.3af | 15.4 W | 12.95 W | IP phones, basic sensors |
| PoE+ | 802.3at | 30 W | 25.5 W | Access points, PTZ cameras |
| PoE++ Type 3 | 802.3bt (4PPoE) | 60 W | 51 W | PTZ cameras with heaters, small displays |
| PoE++ Type 4 | 802.3bt | 90 W | 71.3 W | Lighting, thin clients |
The gap between "PSE output" and "PD input" is cable loss. A PoE switch delivers 15.4 W, but by the time it travels through 100 meters of Cat5e, the powered device only sees about 12.95 W. That's roughly 2.5 W lost in the cable.
This gap gets bigger with higher power levels. PoE++ Type 4 delivers 90 W at the switch but only 71.3 W at the device — that's nearly 19 W lost over a max-length cable run.
Cable Loss: The Silent Budget Killer
Cable resistance is the main factor. Here's what typical Cat5e/Cat6 looks like:
| Cable Type | Resistance (Ω/100m per pair) | Pairs Used | Notes |
|---|---|---|---|
| Cat5e | ~9.4 | 2 (Alternative A) | Most common |
| Cat5e | ~9.4 | 4 (802.3bt Type 3/4) | Halves effective resistance |
| Cat6 | ~8.0 | 2 | Slightly better |
| Cat6a | ~7.0 | 2 | Best for long runs |
Cable Loss Calculation
For 802.3af/at (2-pair power), the loss calculation looks like this:
Ploss = I² × R_total
Where:
I = Current draw
R_total = 2 × cable_length × (resistance_per_meter) × 2 (out and return)
Example: A camera drawing 12 W at 48V through 70 meters of Cat5e.
Current: 12 W / 48 V = 0.25 A
Cable resistance: 2 × 70m × 0.094 Ω/m × 2 = 26.3 Ω (round-trip for both pairs in parallel)
Wait — let's be more precise. With Alternative A, power goes over 2 pairs.
R_per_pair = 70m × 0.094 Ω/m = 6.58 Ω
Two pairs in parallel: 3.29 Ω each direction
Round trip: 6.58 Ω
Ploss = (0.25)² × 6.58 = 0.41 W
Not bad — about 3.4% loss. But at higher power, it gets worse fast.
Example: A PTZ camera drawing 50 W at 48V through 80 meters of Cat5e:
Current: 50 W / 48 V = 1.04 A
Cable resistance (round trip): ~7.5 Ω
Ploss = (1.04)² × 7.5 = 8.1 W
That's 16% loss. You need 58 W at the switch to deliver 50 W to the camera. Over a full 100-meter run, you'd need even more.
Building Your Power Budget
Here's the step-by-step process for a real deployment.
Step 1: List Every PoE Device
| Device | Qty | Class | Power per Device (W) | Cable Length (m) |
|---|---|---|---|---|
| IP Phone | 10 | 2 | 7 | 15–40 |
| Ceiling AP | 4 | 3 | 15.4 | 25–60 |
| PTZ Camera | 2 | 4 (at) | 25.5 | 30–80 |
| Sensor Hub | 6 | 1 | 4 | 10–25 |
Step 2: Calculate Cable Loss per Device
For each device, estimate the cable loss. A rough rule of thumb for 2-pair PoE:
- Short cables (< 30m): Add 1–2 W headroom
- Medium cables (30–60m): Add 2–4 W headroom
- Long cables (60–100m): Add 4–8 W headroom
For precise numbers, use the actual resistance and current calculations above.
Step 3: Total the Switch Budget
Using our example:
| Device Type | Qty | Power Each | Cable Loss Each | Total per Type |
|---|---|---|---|---|
| IP Phone | 10 | 7 W | ~1 W | 80 W |
| Ceiling AP | 4 | 15.4 W | ~2.5 W | 71.6 W |
| PTZ Camera | 2 | 25.5 W | ~5 W | 61 W |
| Sensor Hub | 6 | 4 W | ~0.5 W | 27 W |
| Total | 239.6 W |
You need a switch that can supply at least 240 W of PoE power. A 24-port switch with a 370 W budget would work with comfortable headroom. A 195 W budget switch would not.
Step 4: Add Safety Margin
Add 15–20% margin for:
- Devices that draw more at startup (cameras with motors, APs during peak traffic)
- Cable aging (resistance increases over time in outdoor deployments)
- Future additions
240 W × 1.2 = 288 W minimum recommended switch budget.
Common Mistakes
Forgetting Startup Surge
PTZ cameras, access points, and IP phones often draw significantly more power during boot. A camera that normally draws 12 W might pull 18 W for the first 30 seconds while it initializes motors and optics.
If all your cameras power on simultaneously (like after a power outage), your switch needs to handle the combined surge — or support per-port power-up sequencing.
Mixing Classes Incorrectly
PoE power classes (0–8) define how much power a device needs. The switch uses the class to allocate its budget. If a device declares Class 3 (12.95 W max) but actually draws 15 W peak, the switch may refuse to power it — or cut power when it exceeds the class limit.
Check your device datasheets. Don't assume the class tells the whole story.
Ignoring Ambient Temperature
Copper resistance increases with temperature. If your cables run through a hot attic or industrial environment, you'll lose more power than calculations at 25°C suggest. The resistance temperature coefficient for copper is about 0.393% per °C.
At 60°C ambient, your cable resistance is roughly 14% higher than at 25°C. That means more loss and less power at the device.
Not Accounting for Cable Quality
Not all Cat5e is created equal. Cheap cables with thin conductors (some use 26 AWG instead of 24 AWG) have significantly higher resistance. A CCA (copper-clad aluminum) cable can have 50–60% more resistance than solid copper. Always use solid copper cable for PoE runs.
Switch Selection Tips
When choosing a PoE switch, look beyond port count:
- Total PoE budget. A 48-port switch with 370 W can't power 48 devices at 15.4 W each (that would need 740 W). Budget determines how many ports can actually deliver full power simultaneously.
- Per-port maximum. Make sure each port can deliver the class you need. Some "PoE+" switches only support PoE+ on half the ports.
- Power management. Good switches support priority-based power management — critical devices stay powered when the budget is exceeded, less critical ones get shut down gracefully.
For Industrial and Outdoor Deployments
If you're deploying PoE in harsh environments:
- Use outdoor-rated, UV-resistant cable (UV-resistant PE jacket)
- Consider armored cable for areas with physical stress
- Use surge protectors on every outdoor run — lightning-induced surges will destroy your switch
- Account for temperature derating on both cable loss and switch power capacity
- If running conduit, calculate the cable fill ratio — packed conduits run hotter
The Quick Formula Reference
For anyone who just wants the math:
P_switch = Σ [(P_device + P_cable_loss) × quantity] × safety_factor
Where:
P_cable_loss = I² × R_cable
R_cable = (cable_length_m × 0.188 Ω/m) for Cat5e, 2-pair, round trip
safety_factor = 1.15 to 1.20
Want to skip the spreadsheet? The PoE Power Budget Planner handles cable loss calculations, standard compliance, and per-device budgeting. Enter your devices and cable lengths — get the total budget you need instantly.