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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.
Illustrative worked example. The scenario below is a hypothetical, not a record of a specific job. Its numbers can be reproduced in the Voltage Drop Calculator.
Take an outdoor camera on a separate 12V supply that runs fine all day but drops offline every evening, sometimes for a few seconds and sometimes cycling a few times before settling. Swapping the camera doesn't help, the cable passes a continuity test, and re-terminating the connector changes nothing. That combination points away from the camera and the connector and toward the voltage that actually reaches the camera end under load.
Assumptions: 12V DC supply, 22 AWG solid copper at 20°C, a run of 200 ft one way (400 ft of conductor in the loop, about 6.46 Ω), and a camera drawing 0.15A by day and 0.5A at night with the IR LEDs on.
| Condition | Current | Voltage drop | Voltage at camera |
|---|---|---|---|
| Daytime | 0.15A | 0.97V (8.1%) | about 11.0V |
| Night, IR on | 0.5A | 3.23V (26.9%) | about 8.8V |
The supply still reads a clean 12V at its own terminals in both cases. If the camera needs roughly 10.8V (12V minus 10%) to run cleanly — an assumption for this example, so check the real camera's datasheet — it would be fine by day and reset after dusk. Measuring the voltage at the camera end with the IR active is how you would see it.
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 the pattern in the example: 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
The right gauge depends on how much current the camera pulls, not on distance alone. As one example, here is how far each gauge gets on a 12V supply with a 0.5A camera load (IR on) and a 10% planning drop, using solid copper at 20°C. Double the current and these distances roughly halve.
| Gauge | Approx. max one-way run at 12V, 0.5A, 10% drop |
|---|---|
| 24 AWG | About 46 ft |
| 22 AWG | About 74 ft |
| 20 AWG | About 118 ft |
| 18 AWG | About 187 ft |
These are example figures, not a rule for every camera. Use your camera's actual maximum current in the Voltage Drop Calculator to check your own run.
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 option on a run that can't easily be repulled: 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 to check before leaving site
On any long or thin run (as a rough rule of thumb, past 100 feet), don't assume the wire is fine just because it "worked" on the daytime walkthrough. Check the voltage at the camera end specifically with IR active, not just on a quick daytime test. If it's sagging, upgrade the gauge, shorten the effective run, or use the parallel pair trick as a stopgap until a proper repull.
The free Voltage Drop Calculator is built for exactly this. 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 the camera is installed.
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?
It depends on the camera's maximum current, including IR and heater load, and on the run length, so there is no single gauge that fits every job. As an example only, at 12V, 0.5A, copper and a 10% planning drop, 22 AWG reaches about 74 ft one way, 20 AWG about 118 ft and 18 AWG about 187 ft. At double the current those distances roughly halve. Enter your camera's actual maximum current in the calculator to check your run.
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.