What is the NEC formula for voltage drop?

The NEC voltage drop formula for single-phase circuits is VD = (2 × K × I × L) / CM. For three-phase circuits, replace the 2 with 1.732 (√3). K is the resistivity constant (12.9 for copper, 21.2 for aluminum at 75°C), I is the load current in amps, L is the one-way conductor length in feet, and CM is the circular mil area of the conductor from NEC Chapter 9 Table 8.

What is the maximum allowable voltage drop per NEC?

NEC 210.19(A) Informational Note No. 4 recommends no more than 3% voltage drop on branch circuits and no more than 5% total voltage drop from the service entrance to the furthest outlet. While these are recommendations rather than mandatory Code requirements, most authorities having jurisdiction (AHJs) and engineers treat them as standards.

What K value should I use for voltage drop?

Use K=12.9 for copper conductors and K=21.2 for aluminum conductors. These values are at 75°C, which is the standard temperature rating for most building wire (THHN/THWN-2). Some references use K=10.4/17.0 (at 25°C) or K=12.5/20.6 (at 60°C) — always match the K value to the conductor temperature rating.

Is copper or aluminum better for long runs?

Copper has lower resistivity (K=12.9 vs 21.2) so it produces less voltage drop per foot in the same wire size. However, aluminum is significantly cheaper per ampere of capacity. For long feeders and service entrances, aluminum is commonly used because you can upsize the conductor at lower total cost. For branch circuits, copper is standard because the smaller conduit and easier terminations offset the material cost difference.

Voltage Drop Calculator

Calculate voltage drop per NEC for single-phase and three-phase circuits. Copper and aluminum, AWG 14 through 2000 kcmil, with pass/fail status and the full formula showing your work.

Source Voltage

Phase

Conductor Material

Conductor & Circuit

Conductor Size

One-Way Length (ft)

Load Current (amps)

Results

NEC 2026

Enter one-way length and load current to see voltage drop results.

Single-phase: VD = (2 x K x I x L) / CM

Three-phase: VD = (1.732 x K x I x L) / CM

Understanding NEC Voltage Drop

The 3% and 5% Rules

NEC 210.19(A) Informational Note No. 4 recommends no more than 3% voltage drop on branch circuits and 5% total (feeder + branch) from the service entrance to the furthest outlet. These are recommendations, not Code requirements — but most AHJs and engineers treat them as de facto standards.

Copper vs. Aluminum

Copper (K=12.9) has lower resistivity than aluminum (K=21.2), meaning less voltage drop for the same wire size. Aluminum requires approximately two sizes larger than copper for equivalent performance. Aluminum is common for feeders and service entrances where the cost savings outweigh the larger conduit.

When Voltage Drop Matters Most

Long runs to outbuildings, well pumps, and EV chargers are the most common voltage drop problems. A 200-foot run to a 50A EV charger on 6 AWG copper at 240V drops 9.85V (4.1%) — marginal. Upgrading to 4 AWG brings it to 6.19V (2.6%). Motors are especially sensitive because low voltage increases current draw and heat.

NEC 2026 Changes

NEC 2026 restructured Article 220 into the new Article 120, changed general lighting from 3 VA/sq ft to 2 VA/sq ft, and introduced new EV charging provisions. The voltage drop formula and conductor resistance values in Chapter 9 Table 8 remain unchanged — this calculator is current through NEC 2026.

This tool provides calculations for informational purposes only and does not constitute professional engineering, construction, or safety advice. Verify all results independently before use in design, construction, or compliance decisions. See our Terms of Service.

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