Voltage Drop Explained. Why the Far End Runs Dim and How to Stop It

Every low voltage lighting installer meets voltage drop sooner or later. Usually it shows up at dusk on the first night, when the homeowner asks why the last ten feet look different from the first ten. It is easy to prevent on paper and expensive to fix after the ladders are back on the truck.

What is voltage drop?

Wire is a conductor, and every conductor has some resistance. Push current through that resistance and a little voltage gets used up in the wire itself. It turns into heat along the run and never reaches the lights. Ohm’s law covers it. The voltage lost equals the current flowing times the resistance of the wire.

Two details catch people out. First, current travels out on one conductor and back on the other, so the wire that counts is the full round trip. A run that measures fifty feet on the house is a hundred feet of conductor. Second, the loss adds up along the way. The first light sees almost full voltage. Each light after it sees a little less. The last light on the run sees the least.

Why does the far end go dim or change color?

An LED light is built to work inside a voltage window. Feed it less and it puts out less light. On a single color run that shows up as a slow fade from the power end to the far end.

Color changing lights add a second symptom. They mix red, green and blue emitters to make every other color, and those emitters do not all need the same voltage to light. Red runs happily on less. Green and blue need more. When the voltage sags, blue and green fall off first while red hangs on. Set the run to white and the far end drifts warm, yellow or pink while the near end still looks clean.

Lights with onboard regulation behave a little differently. They hold their brightness until the voltage falls below what the electronics can work with, and then they flicker, lose color or drop out all at once. Either way the cause is the same. Too little voltage is arriving at the end of the wire.

The four factors

Layout matters as much as any of them. In a daisy chain, the first stretch of wire carries the current for every light downstream, so the loss stacks up quickly. When each light or small group gets its own feed from a central point, each wire carries only its own share and the lights see much closer to the same voltage.

How do you plan a run to avoid it?

Do the thinking on paper before the first fastener goes in. A good plan covers these points.

• Measure the real wire path. Count the distance from the power source to the first light, including the trip up the wall and around obstacles. That stretch carries the whole load.
• Add up the load on each run at full brightness. Use the manufacturer’s stated wattage, and size for white at full output if the lights are color changing. White with every emitter on is normally the heaviest draw.
• Stay inside the manufacturer’s limits. Every system has a maximum run length and a maximum load per power source. Those limits are set for that product. Treat them as hard stops.
• Put power where the lights are. A central mounting spot with two shorter runs beats an end mounted spot with one long run.
• Feed long runs from the middle where the product supports it. Each leg is then half the length and carries half the current.
• Split big jobs into several runs. Voltage drop is figured run by run. Total load on the power source is figured for the whole system. Check both, because passing one tells you nothing about the other.

Then run the numbers. There is a voltage drop calculator on the Brighter Nights Lighting Technologies site for planning. Confirm what it gives you against the product instructions. It sits with the rest of the BNLT calculators and tools.

How do you check it on a job?

You need a multimeter and a few minutes. Make this the last step on every install.

• Turn the whole run on at full brightness. On color changing lights, set it to white. If the product has a cool white setting, test that as well.
• Measure the voltage at the output of the power source with the load running. That is your starting number.
• Measure again at the last light on the run, also under load. That is your worst case.
• Compare the far end reading to the minimum operating voltage the manufacturer publishes for that light. If you are above it with room to spare, the run is good.
• Stand back and look. Walk to the street and compare the first light to the last on white. Your eye is the final test, because the homeowner will judge it the same way.
• Write the two readings on the job file. If a problem shows up later, you will know whether the run changed or was always borderline.

What if a finished run comes up short?

Start with the connections. A loose, corroded or half seated connector adds resistance at one spot and produces the same symptoms as a run that is too long. Measure on each side of a suspect joint under load. A clear step down across one connector tells you where the trouble is.

If the connections are sound, the design is the problem and the fix is one of the four factors. Shorten the wire by moving the power source or feeding the run from the middle. Reduce the load by splitting the run in two. Go to heavier wire if the system allows it. Where the manufacturer allows it, split off the far section and give it its own power source, so that no section is ever fed by two sources at once. On a traditional landscape lighting job with a multi-tap power source, moving that run to a higher tap sends more voltage out to make up for what the wire loses. That covers the loss without removing it, so check the nearest light afterward to be sure it is still inside its window.