Calculating voltage drop for AV and security cable runs

Low-voltage does not mean low-consequence. Push 12 or 24 volts down a long thin conductor and a meaningful slice of it never arrives. The device at the far end sees less than its rated voltage, and the way that shows up is maddening: a camera that reboots only when its IR illuminators kick in at night, a display that flickers under load, a door strike that clicks but does not throw. None of it looks like a cable problem, which is exactly why it eats so many service hours.

Why the far end starves

Voltage drop is Ohm's law doing its job. Every conductor has resistance, resistance rises with length and falls with cross-section, and current flowing through that resistance burns off voltage as heat along the way. The longer the run and the thinner the wire, the more voltage is gone before it reaches the load. Double the distance and you double the drop. Halve the conductor area and you double it again.

The rule of thumb most integrators use is a ceiling of three percent drop for sensitive gear, five percent where the device tolerates more. The trouble with rules of thumb is that they do not tell you which gauge clears the bar for this run, at this current, over this distance.

Run the number before you pull the wire

That is a calculation, not a guess, and it is worth doing before the cable is on the drum. You need four things: conductor length, gauge, the load current, and the supply voltage. Out comes the drop in volts and as a percentage, which tells you immediately whether to step up a gauge or move the power supply closer. Our voltage drop calculator takes those four inputs and gives you the answer in the browser, no spreadsheet.

The two fixes, and when to use each

When a run fails the drop test you have two levers. Step up the conductor size, which lowers resistance, or shorten the electrical distance by relocating the power supply or adding a local injector midway. On a camera job with a dozen runs from one head end, upsizing every conductor is expensive; a mid-span power point often costs less and fixes several runs at once. On a single long haul, a heavier gauge is simpler. The calculation tells you which conversation to have.

Length is where estimates lie

Voltage drop is only as honest as the length you feed it, and plan-straight-line lengths are always short. The real conductor climbs into the ceiling, runs the containment route, drops down the wall and loops into the enclosure. Estimate off a calibrated floor plan that already accounts for vertical drops and slack and the drop figure means something. Guess the length and you have calculated a precise answer to the wrong question. We wrote about getting those lengths right in estimating true cable lengths, and keeping them tied to the cable schedule keeps the whole job honest.

Design it out, do not troubleshoot it in

Voltage drop is one of the few field faults that is fully predictable at design time. Four numbers per run, checked before anyone pulls cable, and the 2am intermittent that would have cost you a return visit simply never happens. Cheaper to calculate than to chase.