When you run low‑voltage cable, the copper resists current and some of the original voltage is lost before it reaches the fixtures. That loss is called voltage drop. Too much drop leaves LEDs dim, off or flashing, it can alter color temperature, and stresses the drivers. In most landscape systems you want the voltage at each lamp to be at the specified voltage by the lamp manufacturer.  Something to note is are your bulbs constant voltage or variable voltage, meaning can they operate within in a voltage range or have to be at a specific current.  This will affect wire run design to compensate for voltage drop, variable voltage makes for an easier install.

Understanding the cause  

Voltage drop happens because wire has resistance that turns part of the electrical energy into heat. The longer the wire and the smaller its gauge, the higher the resistance. More current also increases the loss. If you double the run length or double the load, the drop doubles as well. Twelve‑gauge copper offers roughly 1.6 ohms of resistance per thousand feet, fourteen‑gauge about 2.5 ohms, and sixteen‑gauge about 4.0 ohms. Those numbers give you a feel for why thicker wire is worth the cost on long pulls.

Acceptable limits  

With a twelve‑volt transformer most LED manufacturers promise full brightness when the lamp sees at least 10.8 volts. That corresponds to a ten percent drop. Anything lower shows visible dimming/ flashing and or color shift. If you want every fixture to look identical from day one to year ten, design for a five percent loss, which keeps the farthest lamp above 11.4 volts.  Double check the spec sheets on what your bulbs need to be powered at.

Checking voltage on site  

Use a true-RMS digital multimeter with pointed probes. With the transformer energized and all lamps on, set the meter to AC volts, check the common and 15V (or 12V/13V) leads at the transformer lugs and note the reading. Move to the farthest fixture on that run and measure across the socket contacts while the lamp is still in place. Subtract the fixture voltage from the transformer voltage, divide by the transformer voltage, and multiply by 100. That gives you percent voltage drop. Anything over ~10 % means you should shorten the run, step up the tap, or upgrade to a heavier cable.

Estimating Drop  

When you size a run, always think in terms of the round-trip distance, the “loop” the current follows out on one conductor and back on the other. Double the one-way length, convert that figure to hundreds of feet, and you’ve got the piece of the equation most people forget. Plug in the rule of thumb resistance factors: 12 AWG drops about 2.5 percent per amp per 100-foot loop, 14 AWG about 4 percent, and 16 AWG roughly 6.5 to 7 percent.

The math itself is nothing more than multiplication: percent drop equals the cable factor multiplied by the loop length (in hundreds) multiplied by the load in amps. Keep the answer under ten percent for circuits that finish on a 12-volt tap,  fifteen percent is the outer edge if you start on a 15-volt tap, so the lamps stay bright and live to their expected expiration. If the number comes in high, the fix is mechanical, not mathematical: shorten the run, split the feed, step up to a heavier gauge, or bump the transformer to a higher-voltage tap.

Troubleshooting common problems  

If a lamp looks dull, test its voltage first. Anything below 10.8 on a twelve‑volt system confirms excessive drop. Split the load onto another tap, shorten the cable, or upgrade the wire gauge. If readings vary a lot between fixtures on the same run, check every connection for corrosion or loose set screws because poor joints add resistance.

Final checks before burial  

Always measure voltage at the end of each run while the wire is still on the surface. Adjust the design now rather than return later to dig up wire. Keep a note of the final readings in the job file; they become a baseline for future service calls. By understanding what voltage drop is and confirming it with a simple meter you ensure consistent brightness, longer lamp life, and fewer callbacks.  Remember when in doubt, just ad an extra wire run, it doesn't cost much extra and allows for expansion in the future.