You're running low-voltage lighting: 24VDC, Class 2, clean and simple. Then the architect hands you a plan with a load 200 feet from the nearest transformer. You spec 18-gauge wire, do the voltage drop math, and realize the numbers don't work. So, you upsize the wire. And upsize again. Eventually someone on the crew asks: "At what point does it make more sense to just move the power supply closer?"
That's exactly the right question. And the answer isn't just about convenience, there's real science behind it, a 19th-century physics principle called Kelvin's Law, and a hard code boundary at 100 watts that should be guiding your design decisions from the start.
This article breaks it down in plain terms: what voltage drop actually does to a circuit, why low-voltage wire has limitations, where the crossover point is, and why a 24VDC, 100-watt Class 2 power supply is the sweet spot for most installations.
Voltage Drop 101: Why Every Foot Counts in Low-Voltage Wiring
Voltage drop is simple: when current flows through a wire, it loses voltage to resistance. The longer the wire, the more resistance, the more voltage lost before it reaches the load. The formula is:
Voltage Drop = Current × Resistance
Here's the key point: delivering the same amount of power at a lower voltage requires more current. A 100-watt load operating on 24VDC draws about 4.2 amps, while the same 100-watt load on 120VAC draws less than 1 amp. Since voltage drop is directly related to current, low-voltage circuits experience much greater voltage drop over the same wire run.
That 4:1 difference in current means the 24VDC wire has to work four times as hard. To carry the same effective power the same distance, you need a significantly heavier conductor. This is why wire gauge matters so much more in low-voltage systems, and why installers hit the wall faster than they expect.
A Real-World Example
Say you have a 150-watt, 24VDC fixture 200 feet from the power supply. At that wattage and distance, 18-gauge wire won't come close to meeting a 3% voltage drop limit: you're looking at something in the 10–12 gauge range, depending on your tolerance. That's commercial-grade copper being run for a low-voltage circuit.
Meanwhile, your Class 1 supply feeding the power supply? That's probably 14 gauge at 120VAC, a much lighter conductor carrying the same wattage hundreds of feet with no problem. The low-voltage side is doing all the heavy lifting.
Kelvin's Law: The Science Behind the Crossover
In 1881, Lord Kelvin (yes, the same scientist who created the Kelvin color temperature scale) worked out a principle that still holds up today: there is an economically optimal wire size for any circuit, and it's the point where the cost of the conductor equals the cost of the energy lost to resistance in that conductor over time.
In modern electrical design, we apply a practical version of this principle: when the cost of upgrading the low-voltage wire exceeds the cost of moving the power source closer, you've hit the crossover. Don't buy more copper, buy a shorter run.
Here's a way to think about it that most electricians find intuitive:
- Your Class 1 supply wire (typically 14-guage at 120VAC) can carry a given load a very long distance with minimal voltage drop.
- Your low-voltage wire starts losing the battle fast as load or distance increases.
- The moment the math requires your low voltage wire to be heavier than your Class 1 supply feed, you've crossed Kelvin's line. You're spending more on copper than it would cost to relocate the transformer.
The crossover isn't just theoretical. At a 150-watt load and 200-foot run on 24VDC, the required wire gauge to stay within a 3% drop limit is heavier than the 14-guage line voltage feed that's delivering power to the transformer in the first place. That's the signal. Stop upsizing. Move the source.
The 100-Watt Hardline: Why Class 2 Is Your Best Friend
NEC Article 725 defines a Class 2 circuit as one that's inherently limited in power, specifically to 100 watts at 24VDC or less. Below that threshold, you get significant installation advantages:
- No conduit required in most jurisdictions
- Simplified wiring methods
- Reduced fire and shock hazard classification
- Lower inspection friction and faster rough-in
The moment you go over 100 watts, you're in Class 1 territory. The NEC treats it differently. The AHJ inspects it differently. Your liability exposure changes. And here's the thing: the voltage drop math also gets much harder, because more wattage means more current, which means shorter practical run lengths on any given wire gauge.
The industry standard recommendation is this: design to Class 2 limits. Keep loads at or under 100 watts per circuit. Anything above that should be treated as Class 1 from the start: planned, permitted, and wired accordingly.
Trying to squeeze a 150-watt load into a Class 2 installation is a technical violation and a practical headache. Don't do it. Design the system right or reclassify it.
The Sweet Spot: 24VDC, 100-Watt Class 2
If you're designing a low-voltage lighting or controls system and you have flexibility in the spec, a 24VDC, 100-watt Class 2 power supply is the best starting point for most applications. Here's why:
24VDC vs. 12VDC
12VDC is everywhere in landscape lighting, but it’s challenging on long runs. Double the voltage (to 24V) and you cut current in half for the same wattage. Cut the current in half, and your voltage drop over distance drops by half as well. Practical run lengths roughly double. For any application where the load is more than 50 feet from the supply, 24VDC wins.
100 Watts vs. More
Staying at or under 100 watts keeps you firmly in Class 2. You get the simplified installation methods, you avoid the Kelvin crossover zone at typical architectural run lengths, and 18-guage wire handles the job adequately at reasonable distances. At 24VDC and 100 watts, 18-guage gives you a usable run of roughly 60–70 feet at 3% drop — enough for most room-level or zone-level installations.
When 70 feet isn't enough, the right answer is usually: add another supply point. Not a bigger wire.
The Decision Framework: Wire Up or Source Closer?
Use this logic on your next job when you hit a voltage drop problem:
- Is the load under 100 watts? If yes, you're in Class 2. Optimize wire gauge and run length within that classification.
- If you're upsizing low voltage wire past 14-gauge, stop and do the math. Compare the cost of heavier wire against the cost of a second power supply, a shorter low voltage run, and a longer Class 1 feed.
- If your low voltage wire must be heavier than your Class 1 supply feed, you've crossed Kelvin's line. Move the source.
- If the load exceeds 100 watts, treat the entire circuit as Class 1 from the design phase. Don't retrofit compliance.
Summary: The Rules That Save You Wire, Time, and Money
Voltage drop is a current problem, not a wattage problem. Low-voltage circuits carry more current for the same load, which means every foot of run costs more than the equivalent line-voltage run. The wire gauge you need scales fast, and there's a point, Kelvin's crossover, where buying copper stops making sense and relocating the transformer starts.
The NEC drew a smart line at 100 watts and 24VDC for Class 2 circuits. That line exists for safety reasons, but it also aligns almost perfectly with the practical limits of 18-gauge wire in real-world installations. Respect that line. Design to it. When a load or a run pushes you past it, reclassify the circuit or add a supply point. Don't try to wire your way out of a system design problem.
The sweet spot: 24VDC, 100 watts, Class 2. It allows for longer runs on smaller wire, keeps installation simple and NEC-friendly, and makes it obvious when you've reached the point where adding a power supply is more effective than upsizing the wire.
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