Voltage Drop Explained: Causes, Calculation, and Prevention

Understand why voltage drops in electrical circuits and how to calculate it. Covers wire resistance, distance effects, and practical solutions.

📅 June 9, 2026 ✍️ CoreCalx Team
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What Causes Voltage Drop?

Every conductor has resistance. As current flows through resistance, voltage is lost according to Ohm's Law:

V_drop = I × R_wire

The longer the wire and the more current flowing, the greater the voltage drop. This is not a minor concern — in many installations, voltage drop is the primary factor that determines cable size, not current-carrying capacity.

The Voltage Drop Formula

For a single-phase circuit:

V_drop = 2 × I × (R/km) × L / 1000

Where I = current (A), R/km = resistance per km of cable (Ω/km), L = one-way cable length (m). The factor of 2 accounts for supply + return path.

For three-phase: replace 2 with √3.

Percentage drop:

Drop% = (V_drop / V_supply) × 100

Real-World Example: Outdoor Lighting

A 10A outdoor lighting circuit at 230V, running 40m on 2.5mm² copper cable (7.41 Ω/km):

V_drop = 2 × 10 × 7.41 × 40 / 1000 = 5.93V (2.6%)

Within the 5% limit, but for a 5V LED system over the same distance, this would be catastrophic.

5V Systems Are Extremely Sensitive: A 0.25V drop on a 5V rail is 5% — enough to cause visible dimming in LED displays. Use our Cable Cross-Section Calculator to verify your installations.

Factors That Increase Voltage Drop

FactorEffect
Longer cable runLinear increase in drop
Higher currentLinear increase in drop
Smaller wire gaugeExponential increase in resistance
Higher temperature~0.4%/°C more resistance for copper
Poor connectionsAdds significant resistance at junctions

How to Minimize Voltage Drop

  • Use larger cable: Doubling the cross-sectional area roughly halves the resistance.
  • Reduce cable length: Place power sources closer to loads.
  • Increase supply voltage: Higher voltage means lower current for the same power.
  • Use three-phase: The √3 factor is better than the ×2 factor of single-phase.
  • Use parallel conductors: Two cables in parallel double the effective cross-section.

NEC and IEC Requirements

  • NEC 210.19: 3% max for branch circuits
  • NEC 215.2: 3% max for feeders
  • IEC 60364: 4-5% recommended for final circuits
  • BS 7671: 3% for lighting, 5% for other

Use our Cable Cross-Section Calculator to check voltage drop for your specific installation.