---
title: "Xingguang | How do I calculate voltage drop on a long LED run?"
description: "Voltage drop follows circuit current, conductor cross-section, actual routed length and material, and lighting circuits are usually held under three percent, so it is calculated before the cable size is chosen."
url: "https://www.xingguanglightingcn.com/en/blog/led-voltage-drop"
lang: "en"
updated: "2026-10-28"
markdown: "https://www.xingguanglightingcn.com/en/blog/led-voltage-drop.md"
---

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# How do I calculate voltage drop on a long LED run?

[Voltage drop](https://www.xingguanglightingcn.com/en/glossary#voltage-drop) follows circuit current, conductor cross-section, actual routed length and material, and lighting circuits are usually held under three percent, so it is calculated before the cable size is chosen.

## 1. The formula and the two variables

For a single-phase circuit, estimate the drop as two times length times current times resistance per metre, and use the square root of three for three-phase, with the actual routed length rather than the straight-line distance.

Copper resistance per metre falls as cross-section grows, so what decides the drop is the ratio of current to conductor size rather than the luminaire wattage on its own.

## 2. Where the limit sits

A common engineering criterion is no more than three percent at the far end of a lighting circuit and five percent on power circuits, though the project code governs.

When it is exceeded, the visible symptoms are hard starting and low output at the far end, and in bad cases a driver that restarts repeatedly.

Luminaires with 100–277 V wide input tolerate a low supply better, but that eases the symptom rather than the line loss.

## 3. Avoiding it at specification stage

Calculate the drop at the furthest luminaire before choosing the conductor size, which is cheaper than upsizing afterwards.

On very long runs, splitting the supply into [zones](https://www.xingguanglightingcn.com/en/glossary#hazardous-zone) is usually more economical than simply going up a cable size, because line losses fall with the square of current.

Send us the routed length and the luminaire wattage and Xingguang will suggest an input voltage range and a zoning approach for your supply arrangement.

Send the distance to the furthest point and the luminaire wattage to WhatsApp +8613809877696 and we will return a voltage-drop estimate.

WhatsApp: [+8613809877696](https://wa.me/8613809877696)
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Email: [aaronlamp@hotmail.com](mailto:aaronlamp@hotmail.com?subject=Website%20inquiry%3A%20industrial%20LED%20lighting)

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