Maximum Cable Length

Formula

Description

This formula calculates the maximum one-way cable run before the round-trip voltage drop exceeds the specified limit. It is the inverse of the cable voltage drop formula and is essential for planning DC power distribution, LED lighting installations, security camera wiring, and any system where power must be delivered over distance. The result tells you the farthest you can place a load from its power source while maintaining adequate voltage. If the required distance exceeds this limit, options include using thicker wire, raising the source voltage, or placing power supplies closer to loads.

Variables

• L — Maximum one-way cable length (m)
• Vdrop_max — Maximum acceptable voltage drop (V)
• I — Expected load current (A)
• R_per_m — Wire resistance per meter (Ω/m)

Practical Notes

For a 12V system allowing 3% drop (0.36V) at 5A using 16 AWG wire (0.0132 Ω/m): max length = 0.36 / (2 × 5 × 0.0132) = 2.73m. This shows why low-voltage DC systems are limited to short cable runs. PoE (Power over Ethernet) works up to 100m by using 48V and low currents. For longer distances, consider 24V or 48V systems with local DC-DC converters.