A hybrid fiber-power cable carries both optical fiber and copper DC power conductors in a single jacket. It simplifies FTTA and small-cell installations — but the copper conductors have a finite distance limit due to voltage drop. Beyond that distance, the remote radio unit (RRU) doesn’t receive enough voltage to operate. This guide explains how to calculate the voltage drop and size the conductors correctly.
The Voltage Drop Formula
For a DC circuit, voltage drop per conductor:
V_drop = I × R = I × (ρ × L / A)
Where: I = current (A), ρ = copper resistivity (0.0172 Ω·mm²/m at 20°C), L = one-way cable length (m), A = conductor cross-section (mm²). For the round-trip (two conductors), multiply V_drop by 2.
Maximum Distance by Conductor Size (48V DC, 300W RRU, 6.25A)
| Conductor (mm²) | Max Distance (m) | Voltage at RRU |
|---|---|---|
| 1.5 | 80 | 39.2V (below spec) |
| 2.5 | 130 | 40.1V |
| 4.0 | 210 | 41.5V |
| 6.0 | 310 | 42.8V |
For FTTA system context, see FTTA solutions. Beyond 300m, consider separate power cable or higher supply voltage. For hybrid cable product options, see hybrid fiber optic cable guide and hybrid cable structure and applications.
Key Takeaways
- Voltage drop limits hybrid cable distance. At 48V DC with 2.5mm² conductors, maximum practical distance is ~130m for a 300W RRU.
- Larger conductors extend reach but increase cable diameter and cost. 6mm² conductors double the reach vs 2.5mm² but add significant cable weight.
- Beyond 300m, separate power and fiber cables are usually more cost-effective.
Need a Hybrid Cable Voltage Drop Analysis?
Tell us your RRU power, distance, and supply voltage — we’ll recommend the optimal conductor size.
