Your PoE Switch Says 30W. Your Camera Gets 24.

Your PoE+ switch says 30W. Your PTZ camera gets 24. And it reboots itself at 3 AM.

The formula is V_drop = I × R_loop. With Cat5e 24 AWG at 90 meters, that’s 0.51A × 15Ω = 7.5V lost. Result: your camera starts at 42.5V (the 802.3at minimum). On paper it works. In practice, any temperature swing pushes it over the edge.

( copper resistance rises ~0.4% per degree Celsius. A 40°C ceiling costs you 8% extra drop )

Every deployment I review for tv4agent tells the same story:

→ Camera that “randomly reboots” → cheap CCA cable (50% higher resistance) → Camera that “won’t power on” → 110m cable run, exceeding IEEE spec’s 100m → Camera that works in winter but fails in summer → no thermal headroom

The 802.3at PoE+ switch delivers 30W per port. But the camera is guaranteed only 25.5W at 100m of certified pure copper Cat5e. Every extra meter, every degree, every grade of cheap copper costs margin you don’t have.

What nobody tells you: the IEEE 802.3bt standard (PoE++ Type 3 and 4, 60-90W) splits current across all 4 pairs of the cable. That gives you half the drop of PoE+ on 2 pairs. If your camera is an outdoor PTZ with heater, you need bt, not at.

P.D. How many meters of cable is your farthest camera from the switch? Pure copper 23-24 AWG or CCA? If the answer is “I don’t know” or “whatever was in stock”, today’s a good day to run the math before the next outage.

📰 Source: IEEE 802.3at/bt standard + V_drop = I × R_loop with R_cable 24 AWG = 0.0842 Ω/m at 20°C.