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CCTV Voltage Drop Calculator: Why That Camera Only Goes Offline After Sunset
Free CCTV voltage drop calculator. Find the right wire gauge for your camera run before long cable pulls cause night time reboot loops.
This one took me embarrassingly long to figure out the first time it happened.
Outdoor camera, decent install, ran fine all day. Every evening, without fail, it would drop offline. Sometimes for a few seconds. Sometimes it would cycle a few times before settling. Customer was convinced the camera was faulty. So was I, honestly. We swapped it. Same behavior on the replacement. Tested the cable for continuity and it passed. Checked the connector, re-terminated it. No change.
What finally cracked it was checking voltage at the camera end instead of the source end, and doing it specifically while the IR LEDs were active. At the switch or power supply, everything read a clean 12V. At the camera, during the day, it was 12V too. The second the IR array kicked on at dusk and pulled more current than daytime operation, the voltage at the camera end sagged to somewhere around 9V. That run was pushing over 200 feet on wire that was fine for a short pull but way too thin once current demand went up.
Why this only shows up at night
Voltage drop isn't a fixed number. It scales with current. A camera doing daytime video draws relatively little. The same camera at night, with IR illuminators and sometimes a heater engaged, can draw noticeably more. Over a long, thin cable run, that extra current means extra drop, and if the voltage at the camera end sags below what it needs to operate cleanly, you get exactly what we saw: fine by day, unstable or stuck in a reboot loop after dark. It's one of the most misdiagnosed CCTV issues on site because the symptom points everywhere except the actual cause. People blame the camera, the DVR, the network, anything but the wire.
The wire gauge matters more than most installers budget for
Use this table exactly:
| Gauge | Typical long run performance |
|---|---|
| 24 AWG | Poor beyond roughly 50 to 75 ft on any real load |
| 22 AWG | Fair for shorter runs, weak past roughly 100 ft |
| 20 AWG | Better, but still marginal on long high draw runs |
| 18 AWG | The safe default once you're past 100 to 150 ft |
If you're reusing old low voltage wiring, old alarm cable, or an existing run that was already in the wall, check the gauge before you trust it for a new camera. A lot of legacy wiring was sized for much lighter loads than a modern IR or PTZ camera actually pulls.
One trick that's saved me more than once on a run I couldn't easily repull: if you're running over Cat5 or Cat6 and only using two pairs for power, paralleling the spare pairs for power delivery cuts your effective resistance roughly in half. It's not a permanent fix for a badly undersized run, but it's genuinely useful as a stopgap, or on a marginal run where a full repull isn't worth it for the distance involved.
What I check now before I leave site
Any run past 100 feet, I don't assume the wire is fine just because it "worked" on the daytime walkthrough. I check voltage at the camera end specifically with IR active, not just on a quick daytime test. If it's sagging, I either upgrade the gauge, shorten the effective run, or use the parallel pair trick as a stopgap until a proper repull.
We built a free CCTV Voltage Drop Calculator for exactly this, out of the same problem we kept running into on site. Put in your run length, wire gauge, and the camera's current draw, and it tells you what voltage is actually reaching the camera before you find out the hard way at 6pm on install day.
Worth checking your camera's rated current draw against an Amp Hour Calculator too if the same run is also feeding a battery backup, and logging the run length against your NVR Storage Calculator notes so the whole install is documented in one place.
If your cameras are PoE rather than a separate 12V run, wire gauge is only half the story. Worth reading how switch power budgets cause the same "fine by day, dead by night" symptom in our PoE Budget Calculator article.
FAQ
Why does my CCTV camera keep going offline only at night?
Night mode IR illuminators and heaters draw more current than daytime operation. On a long or thin cable run, that extra current causes more voltage drop, and if voltage at the camera drops below its operating minimum, the camera can reboot or disconnect intermittently.
What wire gauge should I use for a long CCTV camera run?
For runs under roughly 100 feet, 20 or 22 AWG is usually fine on a 12V system. Past 100 to 150 feet, 18 AWG is the safer default, especially for cameras with IR illuminators or heaters that spike current draw at night.
Can I fix voltage drop without repulling the cable?
On a Cat5/6 run using only two pairs for power, paralleling the unused pairs for power delivery can cut effective resistance roughly in half. It's a useful stopgap on a marginal run, not a substitute for correct gauge on a new install.