Wire Sizing by Ampacity vs Voltage Drop — Which Governs?

Every conductor size must satisfy two independent checks — carrying the current without overheating, and delivering it without excessive voltage loss. Which one forces the bigger wire depends almost entirely on run length.

Two independent checks

• Ampacity depends on the conductor, its insulation temperature rating, ambient temperature, and how many conductors share the raceway. It does not depend on length.
• Voltage drop is directly proportional to length and current. A conductor that is fine at 10 m may violate the 3% limit at 40 m carrying the same current.

The crossover length

Set the voltage-drop equation equal to the limit and solve for length:

where k = 2 for single-phase and √3 for three-phase. Below L_max, the ampacity-selected size is adequate; above it, you must upsize for voltage drop.

Worked comparison

40 A load, 230 V single-phase, 8.0 mm² copper (ampacity 50 A at 75 °C, R ≈ 0.00215 Ω/m), 3% limit:

At 25 m the 8.0 mm² conductor is governed by ampacity and passes. At 60 m it fails voltage drop and you move to 14 mm².

Planning rule of thumb

For 230 V single-phase copper loaded near its ampacity, expect voltage drop to govern beyond roughly 25–35 m. For 400 V three-phase, the higher voltage roughly doubles the allowable length. Long feeders and site-perimeter circuits are nearly always voltage-drop governed.