Does voltage drop differ between AC and DC circuits?

This calculator uses the DC-resistance-based estimate, which is accurate for typical branch-circuit sizes. For large conductors on long AC runs, inductive reactance should also be considered.

Voltage Drop Calculator

Calculate voltage drop in volts and percentage for an existing conductor gauge, verify compliance with the 3% branch limit, and compare adjacent wire sizes.

Unit System:

1. Circuit Specifications NEC Chapter 9 Table 8

Conductor Material

Wire Gauge (AWG)

Current / Load

Supply Voltage

Run Distance

Length Mode

Current Type

Voltage Drop vs. Wire Gauge

The interactive curve below displays voltage drop across available wire sizes. Your chosen gauge is highlighted against the threshold.

Reference values only: DC conductor resistances follow NEC Chapter 9, Table 8. Sizing for high-power long-distance AC feeders should account for conductor skin-effect and conduit inductive reactance.

Advanced Settings (Parallel Conductors & Custom Drop Limit)

Parallel Conductors Per Leg

Maximum Acceptable Drop Target

How Voltage Drop is Calculated

Electrical resistance causes a loss of electrical potential over distance according to Ohm's Law (V = I × R):

V_drop = (2 × Length × Current × Resistance) / 1000

Where Length is one-way distance in feet, Current is in amperes, and Resistance is the conductor resistance in ohms per 1,000 feet. The round-trip circuit doubles the resistance (conducted through hot and return path).

Worked Example

With a 20 A, 120 V circuit running 75 ft of 10 AWG copper (resistance ≈ 1.24 Ω/kft):

V_drop = 2 × 75 × 20 × 1.24 / 1000 = 3.72 V

The percentage drop is (3.72 / 120) × 100 = 3.10%. Because this slightly exceeds the 3% target, stepping up to 8 AWG (0.778 Ω/kft) drops the loss down to 2.33 V (1.94%).

Frequently Asked Questions

What is considered an acceptable voltage drop?

Standard design practice (NEC 210.19(A)) targets 3% or less on branch circuits, and 5% total drop across both feeder and branch circuits combined.

How can I reduce voltage drop?

You can: (1) Increase wire gauge size (lower AWG), (2) Shorten the run distance, (3) Raise circuit voltage (e.g. converting 120 V loads to 240 V halves current for identical power), or (4) Run parallel conductors.