Voltage Drop Calculator

About the Voltage Drop Calculator

This voltage drop calculator determines voltage drop in copper conductor circuits per NEC voltage-drop guidelines. Use it to verify cable sizing meets the NEC 210.19 informational note (3% on branch circuits, 5% combined with feeders), troubleshoot low-voltage symptoms, or evaluate whether a long run requires upsized conductors.

Built using NEC Chapter 9 Table 8 conductor resistance values for uncoated copper. Adjust for aluminum (multiply result by ~1.6) or for elevated conductor temperatures (>75°C).

How voltage drop is calculated

Single-phase: VD = 2 × L × I × R ÷ 1000
Three-phase: VD = √3 × L × I × R ÷ 1000

L is one-way length in feet, I is current in amps, R is conductor resistance in ohms per 1000 feet. The factor of 2 in single-phase accounts for both the hot and neutral conductor; √3 in three-phase reflects the line-to-line voltage relationship.

Worked example

A 30A motor circuit, 240V single-phase, 100 ft from panel, using #10 AWG copper THHN (R = 1.21 Ω/1000 ft): VD = (2 × 100 × 30 × 1.21) ÷ 1000 = 7.26 V, which is 3.0% drop. At the 3% NEC informational guideline limit. A 200 ft run would require upsizing to #8 AWG to stay within 3%.

When to use this calculator

• Verifying conductor size meets NEC 3% voltage-drop guidance on branch circuits
• Sizing feeders for long runs (warehouses, agricultural buildings, outdoor lighting)
• Troubleshooting motors that won’t start or run hot (often caused by voltage drop)
• Specifying conductors for utility services with long meter-to-building runs
• Comparing copper vs aluminum cost trade-offs for large feeders
• Validating EV charger installations (which often require dedicated 60+ amp circuits)

Frequently Asked Questions

What is acceptable voltage drop?

NEC 210.19 informational note recommends ≤3% on branch circuits and ≤5% combined for feeder + branch. These are guidelines, not enforceable requirements, but most jurisdictions and good practice follow them. Motors should not see more than 10% drop during starting.

How do you calculate voltage drop?

For single-phase: VD = (2 × L × I × R) ÷ 1000. For three-phase: VD = (√3 × L × I × R) ÷ 1000. L is one-way length in feet, I is amps, R is conductor resistance in Ω per 1000 ft.

Why does voltage drop matter for motors?

Motors operating at reduced voltage draw more current, generate more heat, and produce less torque. Sustained voltage drop > 5% shortens motor life dramatically. Starting torque drops with the square of voltage — a 10% voltage drop reduces starting torque by 19%.

Does aluminum wire have different voltage drop?

Yes — aluminum has about 60% the conductivity of copper, so it has higher resistance for the same gauge. To match copper voltage drop, aluminum needs to be one or two AWG sizes larger.

How do I reduce voltage drop on a long run?

Three options: (1) upsize the conductor (most common), (2) increase voltage at the source (use 480V instead of 240V), or (3) shorten the run by relocating equipment. Upsizing is usually cheapest unless the run is very long.

Does this account for conductor temperature?

Conductor resistance is given at 75°C. At higher temperatures, resistance increases by about 0.4% per °C. For circuits operating at high ambient temperature or with thermal insulation, multiply VD by 1.05–1.10 for a more conservative result.

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