PoE budget math: the camera works on the bench, dies on the switch
Why your PoE camera works when you bench-test it but won't power up when plugged into the live switch. Class limits, total switch budget, cable length, and the four show commands that tell you exactly what's wrong.
The bench test lies
You bench-tested the camera with a PoE injector or a small desktop switch. It powered up immediately, you can see it in the VMS, all good. You take it to site, plug it into the live access switch, and the switch says "denied" or the camera simply won't come up.
Four things cause this, in order of how often we see them:
1. Total switch budget is exhausted. 24-port switches ship with 370W or 740W budgets. Add up the camera wattages, leave 20% headroom, and the budget is gone by camera 14. 2. Port is configured for a lower class. The port is set to Class 3 (15.4W) but the camera is Class 4 (25W) — common with PTZ or IR-bullet cameras. 3. Cable is too long. Beyond 100m the voltage drop means the camera doesn't get what it negotiated. 4. Cable quality. CCA (copper-clad aluminium) cable looks like Cat6 but the resistance is 80% higher. PoE fails first; data follows.
The 4 show commands
On a Cisco switch:
show power inline
Look at "AdminCum/Watts" vs "Used/Watts". If they're within 20% of each other, you're close to the wall. If they're equal, the next device you plug in will be denied.
show power inline <int>
Shows the negotiated class and actual draw for a single port. If the camera says "Class 4" but the port is set to "Class 3", that's your problem.
show interface <int> status
``Look for "err-disabled" or "admin down". PoE-related err-disable happens when the camera keeps redrawing too much current, often from a bad cable or a failing camera.
show log | include POWER
The actual error message — almost always "denied" or "over-current" — tells you which of the four causes is in play.
The math, worked out
48-port switch, 740W total budget (typical for an HPE 1950 or Aruba 2930F class). Cameras at 12W each, IR-bullet with heater.
- 740W × 0.8 headroom = 592W usable - 592 / 12 = 49 cameras
Sounds fine for 48 ports, but two of the 48 ports are uplinks, and you have one AP at 25W, one door controller at 15W. Now you're at 47 cameras × 12 + 25 + 15 = 564 + 40 = 604W. Over budget.
Solutions in order of preference:
1. Replace the switch with a higher-budget model (e.g. 1100W budget for 48 ports) — simplest fix 2. Add a second PoE switch and split the load — costs less, but two switches to manage 3. Use a PoE+ midspan injector on the higher-draw devices, freeing the switch budget 4. Reduce the heater load by enabling PoE scheduling (turn the heater off between midnight and 6 a.m.)
Cable length — the silent killer
IEEE 802.3 says 100m. The reality:
| Cable | Effective PoE range |
|---|---|
| Cat5e solid copper, 23 AWG | ~95m at 25W |
| Cat6 solid copper, 23 AWG | ~100m at 25W |
| Cat6A solid copper, 23 AWG | ~100m at 25W |
| CCA "Cat6" | ~70m at 25W |
| Stranded patch (not for runs) | Don't |
If the run is 80–100m, you're probably fine with solid copper. If it's 110m, plan for a PoE extender or a switch in the middle. If it's 150m, run fibre to a media converter with PoE on the far end.
Field checklist before you buy
1. Add up the cameras' wattage, including heaters and IR 2. Add 25% headroom 3. Check the switch's rated PoE budget, not the per-port limit 4. Check the cable spec — solid copper, 23 AWG minimum, no CCA 5. Check the run lengths — anything over 90m needs a plan
Get all five right and the "works on the bench, dies on site" problem disappears.
When to call us
PoE issues at a live site where the cameras are already on the wall and the customer is asking why the switch says "denied" — we get this call weekly. 30-minute remote fix if you can give us the show power inline output and the camera model. Toll-free 1800-5GHZ-NET.
Frequently asked
How much power does a PoE camera need?
Class 2 (IEEE 802.3af) devices draw up to 7W, Class 3 (802.3at) up to 25W, Class 4 (PoE+ / 802.3at Type 2) up to 25W but with LLDP negotiation. Most PTZ cameras and IR-bullet cameras with heaters need 25W. Static indoor domes are usually 7W.
My camera is rated 15W but the switch says 'denied' — why?
The switch port is configured for Class 3 (15.4W) but the camera is Class 4 (25W), or the total switch PoE budget is exhausted. Run 'show power inline' on the switch — the 'AdminCum/Watts' column is your total budget.
What's the maximum cable run for PoE?
100 metres (328 feet) per the IEEE spec. Beyond that, voltage drop on the cable means the camera won't get the power it negotiated. For longer runs, use a PoE extender or a midspan injector at 100m intervals.
Still need a hand?
If the article didn't fix it, raise a ticket and we'll get on it. Most P1 calls land within an hour.