Exact DC Voltage Drop Calculator

Pre-calculated engineering gauge table for standard mobile, marine, and off-grid DC circuits. Values reflect 105°C marine-grade tinned copper maintaining a maximum 3.0% voltage drop over round-trip circuit length:

Engineering Derivations

How to Calculate DC Loop Resistance & Voltage Sag

In direct current (DC) electrical circuits, current must flow from the power source through the positive feed wire, pass through the electrical load, and travel back to the power source through the negative return wire. As a result, the total circuit resistance equals the sum of both conductors, commonly referred to as the DC loop resistance ($R_{\text{loop}}$).

Step-by-Step Loop Resistance Calculation Example:

Consider a 12.8V LiFePO4 battery powering a 30A DC refrigerator/compressor load located 15 feet away using 10 AWG uncoated stranded copper wire.

1. Determine Total Conductor Length: The one-way run is 15 feet, so the total round-trip circuit path is $2 \times 15\text{ ft} = 30\text{ feet}$.
2. Lookup Conductor Resistance: Per NEC Chapter 9, Table 8, 10 AWG copper has a direct current resistance of $1.21\ \Omega\text{ per 1,000 feet}$ at 75°C ($0.00121\ \Omega/\text{ft}$).
3. Calculate DC Loop Resistance: $$\begin{aligned} R_{\text{loop}} &= 30\,\text{ft} \times 0.00121\,\frac{\Omega}{\text{ft}} \\[6pt] &= 0.0363\,\Omega \end{aligned}$$
4. Calculate Voltage Drop: $$\begin{aligned} V_{\text{drop}} &= 30\,\text{A} \times 0.0363\,\Omega \\[6pt] &= 1.089\,\text{V} \end{aligned}$$
5. Calculate Percentage Voltage Sag: $$\begin{aligned} \text{Voltage Sag} &= \left(\frac{1.089\,\text{V}}{12.8\,\text{V}}\right) \times 100 \\[6pt] &= 8.51\% \end{aligned}$$
6. Calculate Heat Loss in Wiring: $$\begin{aligned} P_{\text{loss}} &= I^2 \times R_{\text{loop}} \\[6pt] &= (30\,\text{A})^2 \times 0.0363\,\Omega \\[6pt] &= 32.67\,\text{W} \end{aligned}$$

Because 8.51% exceeds the recommended 3% voltage drop threshold for critical equipment, the wire gauge must be upsized to 6 AWG (0.0147 Ω loop resistance), which limits voltage drop to 0.44V (3.4%). To calculate the required wire size automatically for any load or distance, use our DC Wire Size Calculator.

Calculation Assumptions

• Linear DC ohmic resistance per NEC Chapter 9, Table 8.
• Constant circuit current and fully symmetrical positive/negative conductor lengths. For single-wire vehicle installations utilizing metal body grounding, see our Chassis Ground vs. Dedicated Return Guide.

Engineering Limitations

• Assumes steady-state conductor temperature of 75°C; colder wires exhibit lower resistance, hotter wires exhibit higher resistance.
• Terminal lug and contact resistance are omitted from the calculated wire body.

Quick-Reference DC Voltage Drop Table (12V Nominal)

This reference table illustrates total circuit voltage drop and voltage sag percentage across a nominal 12.0V DC battery system for common conductor sizes over standard one-way distances (10 ft and 25 ft). Calculations account for the complete two-wire round-trip loop resistance at 75°C per NEC Chapter 9, Table 8.

Note: Entries marked with (✓) satisfy the strict ABYC E-11 3% critical circuit voltage drop requirement (0.36V max drop on 12.0V nominal).

DC Voltage Drop & Line Loss Formulas