PoE Power Budgets & Cat6 Cable Voltage Drops: The Physics Behind Camera Reboot Loops

By Gary Pearce — Lead Physical Infrastructure Engineer at ADI Communications (Direct: 07860 244 274)

A common mystery in outdoor IP CCTV and wireless access point installations is the "dusk reboot loop": cameras operate perfectly in bright daylight, but the moment the sun sets and high-power EXIR infrared night-vision LEDs ignite, the camera resets, flashes, and falls offline in an infinite reboot cycle.

In this engineering guide, we dissect Ohm's Law along twisted-pair conductors, compare solid copper vs hazardous Copper Clad Aluminium (CCA), and share our interactive PoE Voltage Drop & Cable Resistance Calculator.

1. Ohm's Law & Conductor Resistance

Power over Ethernet (PoE) delivers direct current across dedicated twisted pairs. As distance increases, the resistance of the conductor loop attenuates the supplied voltage:

$$V_{\text{drop}} = I_{\text{load}} \times R_{\text{loop}} = \left( \frac{P_{\text{camera}}}{V_{\text{source}}} \right) \times (L \times r_{\text{conductor}})$$

$$V_{\text{terminal}} = V_{\text{source}} - V_{\text{drop}}$$

Cable Loop Resistance Benchmarks (Round-Trip):

• Solid Bare Copper 23 AWG (Cat6 Benchmark): $0.0734 \ \Omega/\text{m}$ (7.34 $\Omega$ per 100m)
• Solid Bare Copper 24 AWG (Cat5e): $0.0938 \ \Omega/\text{m}$ (9.38 $\Omega$ per 100m)
• Copper Clad Aluminium (CCA - High Hazard): $0.1450 \ \Omega/\text{m}$ (14.50 $\Omega$ per 100m — Nearly double the resistance!)

2. Why Cameras Reboot When Night Falls

Under daylight conditions, a standard 4K IP turret camera draws only 4 to 6 Watts ($I \approx 0.10\text{ A}$). Voltage drop across a 60m run is negligible (< 0.8V), so the camera receives ~47.2V.

At dusk, the ambient light sensor triggers:

1. High-power EXIR 850nm / White-Light LED arrays illuminate.
2. Internal heater elements activate in cold weather.
3. Power demand spikes to 18 to 24 Watts ($I \approx 0.45\text{ A}$ to $0.50\text{ A}$).

On a 60-metre CCA cable, round-trip resistance is $60 \times 0.145 = 8.7\ \Omega$. The voltage drop spikes to:

$$V_{\text{drop}} = 0.48\text{ A} \times 8.7\ \Omega = 4.18\text{ V}$$

If the source switch supplies 48V, terminal voltage drops below 42V (the threshold for IEEE 802.3at PD chips). The internal power converter enters undervoltage lockout, shutting down the camera. With the load removed, voltage springs back up, the camera boots up, turns on IR LEDs, and reboots again indefinitely.

3. Interactive Web Tool & Simulation Engine

We have open-sourced our complete PoE electrical analysis engine:

• Interactive Web App: ADI PoE Voltage Drop Calculator
• GitHub Suite: gazpearce/cctv-security-hub
• GitLab Mirror: gitlab.com/aiagent1510/cctv-security-hub
• GitHub Gist Formula: PoE Resistance & Voltage Drop Model

4. Solid Copper Certification Standards

1. Pure Solid Copper (23 AWG): Always specify CAI-benchmarked solid bare copper cable. Reject any cable marked "CCA" or "Aluminium".
2. RJ45 Pass-Through Shielding: Use shielded external Cat6 connectors with solid internal ground wires to discharge atmospheric static.
3. Downward Drip Loops: Form clean drip loops at masonry penetrations to stop water tracking into PoE ports.

For certified data cabling, PoE power budget verification, and outdoor network drops across Yorkshire, consult Gary Pearce at ADI Communications or call 07860 244 274.