Short Circuit Voltage Drop Calculator
Estimate short circuit voltage drop for your project with our free calculator. Get accurate material quantities, costs, and specifications.
Formula
VD (1-phase) = 2 x I x R x L | VD (3-phase) = 1.732 x I x R x L
Voltage drop equals the current (amps) times the conductor resistance per unit length (ohms/ft) times the one-way conductor length (feet). For single-phase, multiply by 2 for the round-trip path. For three-phase, multiply by 1.732 (square root of 3). Short circuit current at the end of the run is estimated as the source voltage divided by the total circuit resistance.
Worked Examples
Example 1: 120V Branch Circuit
Problem: A 120V, single-phase circuit supplies 16A through 150 feet of 12 AWG copper wire. Calculate voltage drop.
Solution: Resistance: 1.98 ohms/1000ft\nVoltage drop: 2 x 16 x (1.98/1000) x 150 = 9.50V\nPercent: 9.50 / 120 x 100 = 7.92%\nThis exceeds the 3% NEC recommendation.\nUpgrade to 10 AWG: 2 x 16 x (1.24/1000) x 150 = 5.95V (4.96%)
Result: 12 AWG: 9.50V (7.92%) - Too high | 10 AWG: 5.95V (4.96%)
Example 2: 480V Three-Phase Feeder
Problem: A 480V, 3-phase feeder supplies 200A through 300 feet of 3/0 AWG copper.
Solution: Resistance: 0.0766 ohms/1000ft\nVoltage drop: 1.732 x 200 x (0.0766/1000) x 300 = 7.96V\nPercent: 7.96 / 480 x 100 = 1.66%\nWell within 3% limit.
Result: 7.96V drop (1.66%) - Compliant with NEC 3% recommendation
Frequently Asked Questions
What is acceptable voltage drop per NEC?
The NEC recommends (but does not mandate as a hard requirement) a maximum voltage drop of 3% for branch circuits and 5% total for the combination of feeder and branch circuit. NEC Article 210.19(A) Informational Note No. 4 and Article 215.2(A) Informational Note No. 2 provide this guidance. While not a code violation to exceed these values, excessive voltage drop causes motors to overheat, lighting to dim, and electronic equipment to malfunction. Many engineers design for 2% on feeders and 3% on branch circuits to stay well within limits.
How does wire size affect voltage drop?
Larger wire sizes have lower resistance per foot, which directly reduces voltage drop. For example, upgrading from 10 AWG copper (1.24 ohms/1000ft) to 8 AWG copper (0.778 ohms/1000ft) reduces resistance by 37%. The relationship is that doubling the wire cross-sectional area (going up approximately 3 AWG sizes) roughly halves the resistance and voltage drop. For long runs, it is often more economical to upsize the wire by one or two sizes rather than deal with the ongoing energy losses and performance issues from excessive voltage drop.
How do I calculate voltage drop for three-phase circuits?
Three-phase voltage drop uses a different formula than single-phase. For single-phase: VD = 2 x I x R x L, where the factor of 2 accounts for the current traveling through both the hot and neutral conductors. For three-phase: VD = 1.732 x I x R x L, where 1.732 (square root of 3) replaces the factor of 2 because of the 120-degree phase relationship between the three conductors. This means three-phase circuits have approximately 13% less voltage drop than single-phase circuits for the same wire size, current, and distance, making them more efficient for long runs.
Is my data stored or sent to a server?
No. All calculations run entirely in your browser using JavaScript. No data you enter is ever transmitted to any server or stored anywhere. Your inputs remain completely private.
Is Short Circuit Voltage Drop Calculator free to use?
Yes, completely free with no sign-up required. All calculators on NovaCalculator are free to use without registration, subscription, or payment.
How do I interpret the result?
Results are displayed with a label and unit to help you understand the output. Many calculators include a short explanation or classification below the result (for example, a BMI category or risk level). Refer to the worked examples section on this page for real-world context.