Voltage Drop Considerations for Long Runs of Low-Wattage LED Fixtures

[emmamama]

Fellow electricians and engineers,

I’m designing a pathway lighting system for a residential estate using low-wattage (8W) 120V LED bollard fixtures, spaced ~20 ft apart along a 300-foot one-way run from the panel. Total load: 12 fixtures × 8W = 96W (~0.8A at 120V).

The fixtures are UL-listed, constant-voltage type (not Class 2), and specify an operating range of 105–130V.

My question centers on voltage drop:

Using 12 AWG THWN in PVC, the calculated voltage drop is ~2.1% (about 2.5V) at full load—well under the common 3% informal guideline.
However, I’ve heard conflicting opinions: “Since LEDs draw so little current, voltage drop isn’t a concern,” vs. “LED drivers can behave unpredictably below minimum voltage—even if power loss seems negligible.”
Does NEC provide any specific guidance for voltage drop with electronic LED loads?

I know NEC doesn’t mandate a max VD except in fine print notes (e.g., 210.19 FP Note 4, 215.2 FP Note 2), but from a performance and warranty standpoint, should we still enforce stricter limits for sensitive electronics?

Also: if the first fixture sees 119V and the last sees 116V, is that truly acceptable—even if within the manufacturer’s spec? Could it cause flicker, reduced lifespan, or color shift over time?

 

[ptonsparky]

Welcome to the Forum.

How do you know all those issues aren't a sign of normal aging? Moving the fixtures every few weeks to minimize the affects may help.

You may see flickering no matter what you do.

The NEC is minimum. You could use 4/0 if that will make you feel better. Better design of the fixtures would be preferred vs stricter limits.

 

[infinity]

The fixtures are UL-listed, constant-voltage type (not Class 2), and specify an operating range of 105–130V.

Also: if the first fixture sees 119V and the last sees 116V, is that truly acceptable—even if within the manufacturer’s spec? Could it cause flicker, reduced lifespan, or color shift over time?

I don't see an issue with the numbers that you've posted. Even in the worst case scenario the lowest voltage at the last fixture is 11 volts higher than the minimum required by the operating range so why would there be any problems?

 

[winnie]

Voltage drop is always a consideration, even with low current loads. Only after you do the calculations can you really know if there is an issue.

With the numbers posted, I see no problem. The end of run voltage is greater than the minimum fixture operating voltage. Of course the manufacture specs can be optimistic, and some LEDs are sensitive to flicker even with full voltage available. Flicker in LEDs is totally a question of the drive circuit design, and can be anything from negligible to horrible.

I would be curious about one point not mentioned in the specifications: what is the inrush current to the power supply when it gets energized, and how well does it tolerate voltage drop during startup.

If there is any chance that your customer will want to add load (holiday lighting, brighter lights, etc) I would consider that _now_ before burying anything.

-Jonathan

 

[ptonsparky]

OP.
Check your math. I don't think you come anywhere close to 2.5 volt drop. At worst would be if the entire load is at the end of the run. It is not. It is at the start and drops at each fixture.

 

[wwhitney]

I’m designing a pathway lighting system for a residential estate using low-wattage (8W) 120V LED bollard fixtures, spaced ~20 ft apart along a 300-foot one-way run from the panel. Total load: 12 fixtures × 8W = 96W (~0.8A at 120V).

A couple comments:

An 8W LED fixture may draw more than 8W/120V in current, as the power factor may not be 1.0. Best to check the fixture's spec for current draw.

If you have 12 identical loads on a circuit, which is all one conductor size, and you want to calculate the voltage drop at the last fixture, you can use the normal voltage drop formula with the total load at the average distance from the panel of the fixtures. So if your 300 ft run has the farthest fixture at 300 ft away, and the closest at 80 ft away (based on 12 fixtures, 20 foot spacing), and the spacing is even, then the average distance is (300+80)/2 = 190 ft.

Cheers, Wayne

 

[ramsy]

Does NEC provide any specific guidance for voltage drop with electronic LED loads?

220.18(B) prohibits the use of Watts for LED lighting.

Therefore any VD calc must use nameplate Amps.

 

[LarryFine]

Because the lights have power supplies that have an input-voltage range and regulate the output voltage, the lights will all behave the same, and there won't be any difference in brightness.