Voltage Drop Calculator — Wire Length & Gauge | WireStandard

Enter current (A), one-way length, wire gauge, system voltage, and phase to compute voltage drop in volts and percent. The calculator compares your result against the NEC 215.2(A)(1)(b) informational thresholds: 3% for branch circuits and 5% combined. Undersized conductors cause motor stalls, lighting flicker, and code violations on feeder circuits — catching the problem before installation saves re-pulling wire.

Voltage Drop Formula

Single-phase: VD = 2 × L × I × ρ / A. Three-phase: VD = √3 × L × I × ρ / A. L = one-way length (m), I = current (A), ρ = 0.0172 Ω·mm²/m (copper, 20 °C IACS), A = conductor cross-section (mm²). The √3 ≈ 1.732 factor replaces the factor-of-2 return-path multiplier for three-phase circuits. At 75 °C operating temperature, ρ rises roughly 1–3 % (NEC Chapter 9 Table 8); apply a temperature derating for precision.

Worked Example

Load: 30 A continuous, 240 V single-phase, one-way run 200 ft (60.96 m), AWG 6 (13.3 mm²). VD = (2 × 60.96 × 30 × 0.0172) / 13.3 = 4.73 V = 1.97 % — passes the NEC 3 % guideline. Downsize to AWG 8 (8.37 mm²): VD = (2 × 60.96 × 30 × 0.0172) / 8.37 = 7.52 V = 3.13 % — fails. Upsizing one AWG gauge reduced voltage drop by a third; the wire cost difference is trivial compared to a callback.

Frequently Asked Questions

What is the difference between single-phase and three-phase voltage drop?

Single-phase circuits use a factor of 2 (current travels out on one conductor and returns on another). Three-phase circuits replace that with √3 ≈ 1.732 because the three phases partially cancel each other. For the same wire and load, three-phase VD is about 13 % lower than single-phase.

Is the 3 % limit a hard NEC requirement?

No. NEC 215.2(A)(1)(b) is an Informational Note, not an enforceable rule. However, NEC 695.7 mandates a maximum 15 % VD for fire pump feeders, and many utilities and AHJs adopt 3/5 % as a code requirement. For sensitive electronics and motor feeders, treating the guideline as mandatory protects equipment and avoids nuisance trips.

Should I use 60, 75, or 90 °C resistivity?

75 °C is the standard default for new construction under NEC — most terminations are rated 75 °C so that column governs ampacity. For life-safety feeders (fire pumps, emergency circuits), use 90 °C conductor resistivity in your VD calculation to account for worst-case heating. Raising temperature from 20 °C to 75 °C increases copper ρ by approximately 21 %, so precision calculations should apply the correction.