Exact DC Voltage Drop Calculator
Diagnostic tool for existing wiring: enter your known wire gauge, circuit run length, and current load to calculate exact voltage sag, terminal voltage, and heat loss. To size an unknown wire gauge for a new circuit, use our DC Wire Size Calculator.
Enter the values above and click Calculate to view recommendations and engineering specifications.
Engineering Derivations
Calculation Assumptions
- Linear DC ohmic resistance per NEC Chapter 9, Table 8.
- Constant circuit current and fully symmetrical positive/negative conductor lengths.
Engineering Limitations
- Assumes steady-state conductor temperature of 75°C; colder wires exhibit lower resistance, hotter wires exhibit higher resistance.
- Terminal lug and contact contact resistance are omitted from the calculated wire body.
Evaluating a 24V 25A auxiliary light bar run through 20 feet of 10 AWG copper reveals a loop resistance of 0.0484 Ω, producing a 1.21V drop (5.04%) and 30.3W of dissipated heat in the harness.
Voltage drop in direct current circuits is a direct physical consequence of Ohm's law ($V = I \times R$). Because DC electricity must complete a continuous loop from the power source through the load and back to the source, the total resistance includes both the positive feed and the negative return conductor.
Frequently Asked Questions
What is an acceptable voltage drop for 12V DC circuits?
Under ABYC E-11 standards, 3% maximum voltage drop (0.36V on a 12.0V nominal system) is required for critical circuits such as bilge pumps, navigation lights, electronic controls, and battery inverter cables. Non-critical branch circuits (general cabin interior lighting, utility sockets) permit up to 10% voltage drop (1.20V loss).
How do you calculate DC voltage drop manually?
DC voltage drop is calculated using Ohm's Law: Voltage Drop (V) = (2 * Length in feet * Resistance per 1,000 ft * Current in Amps) / 1,000. For metric units: V = (2 * Length in meters * Conductor Resistivity * Current) / Cross-Sectional Area (mm²). Conductor resistance values are obtained from NEC Chapter 9 Table 8.
What happens if voltage drop is too high in a 12V system?
Excessive voltage drop causes appliances to operate below their minimum operating threshold, resulting in low-voltage inverter cutoffs, flickering LED fixtures, sluggish pump motors, and battery undercharging. Furthermore, the lost energy is converted into heat along the conductor, increasing thermal stress on insulation.
Does conductor temperature affect voltage drop?
Yes. Copper resistivity increases by approximately 0.393% per degree Celsius rise above 20°C (68°F). In hot environments such as engine bays (60°C / 140°F) or rooftop solar conduit, wire resistance increases significantly, creating higher voltage drop and requiring thicker wire gauge.