---
title: "Xingguang | Voltage drop on long runs: it is the cable"
description: "There is one formula for single phase: ΔU ≈ 2 × I × L × ρ / A, with ρ = 0.0175 Ω·mm²/m for copper. Twenty 150 W fixtures at the end of a 30 m run draw about 13 A, and on 2.5 mm² that is roughly 5.5 V, or 2.4% of 230 V. Stretch the same load to 40 m and it becomes 7.3 V, over the line."
url: "https://www.xingguanglightingcn.com/en/spec/voltage-drop-long-cable-run"
lang: "en"
updated: "2026-10-28"
markdown: "https://www.xingguanglightingcn.com/en/spec/voltage-drop-long-cable-run.md"
---

[Home](https://www.xingguanglightingcn.com/en/) / [Selection by specification](https://www.xingguanglightingcn.com/en/spec/) / Voltage drop on long runs: it is the cable

# Voltage drop on long runs: it is the cable

There is one formula for single phase: ΔU ≈ 2 × I × L × ρ / A, with ρ = 0.0175 Ω·mm²/m for copper. Twenty 150 W fixtures at the end of a 30 m run draw about 13 A, and on 2.5 mm² that is roughly 5.5 V, or 2.4% of 230 V. Stretch the same load to 40 m and it becomes 7.3 V, over the line. This is a wiring parameter, not a fixture parameter, but what the customer sees is that the lights are dim.

[Get a Quote](https://wa.me/8613809877696)

## Key parameters

| Power | 100–200 W |
| --- | --- |
| Luminous flux | 12,000–25,000 lm |
| IP rating | IP66 |
| Input voltage | 220–240 V |
| Frequency | 50 / 60 Hz |
| Allowed voltage drop | ≤ 3% end to end, AC side |
| Suggested cable / run | 2.5 mm² over 30 m, about 20 × 150 W |

[Product family: Floodlights](https://www.xingguanglightingcn.com/en/product-categories/floodlight)

## Voltage is the item most often skipped

This page is 220–240 V at 50 / 60 Hz. [Wide-range input](https://www.xingguanglightingcn.com/en/glossary#wide-voltage) is not about "works anywhere" — it is about not derating or tripping when the supply sits low or high. In regions with unstable grids, narrow-range drivers fail noticeably more often.

The other thing that gets skipped is inrush: when high-power fixtures start together the inrush adds up and trips the breaker. Where twenty units switch on at once, size the breaker on the row, not on one fixture — undersize it and the symptom is "it trips the moment you switch on", which few people trace back to the lights.

## Voltage drop: right voltage, still dim

Allowed drop on this page is ≤ 3% end to end, AC side. It shows up worst on low-voltage and DC systems: a 24 V run of 30 m on 2.5 mm² can lose more than 10% by the far end, the fixtures visibly dimmer and the furthest ones dimmest. On site that looks like a faulty fixture; it is an undersized cable.

On the AC side the working figure is to keep total drop under 3%. That is a wiring parameter, not a fixture parameter — but it decides what the customer sees, so size the cable during the quote, not during commissioning when the lights get blamed.

## Four things to verify before ordering

① Whether flux and [efficacy](https://www.xingguanglightingcn.com/en/glossary#efficacy) are quoted for the complete fixture or the light source — a 10–15% difference that changes the fixture count. ② Whether [colour temperature](https://www.xingguanglightingcn.com/en/glossary#cct) and CRI are measured or nominal values; mixed CCT within one batch is the most common arrival dispute. ③ Driver brand and model, plus its derating curve at high temperature — it is the only part that really sets life. ④ The mounting kit list: chain, bracket and anchors in the box, or the site stops for a day.

## Concepts on this page

[Voltage drop](https://www.xingguanglightingcn.com/en/glossary#voltage-drop)[Flood light](https://www.xingguanglightingcn.com/en/glossary#flood-light)[Luminous efficacy](https://www.xingguanglightingcn.com/en/glossary#efficacy)

![Floodlight FL-160](https://www.xingguanglightingcn.com/assets/img/prod-fl160.webp)

### [Floodlight FL-160](https://www.xingguanglightingcn.com/en/products/floodlight-160)

Flood

## Need a Lighting Plan?

Tell us your scenario, Jie Zhai will help you choose.

[Get a Quote](https://wa.me/8613809877696)
[All specification pages](https://www.xingguanglightingcn.com/en/spec/)