Voltage Drop Calculator

The standard formula for voltage drop in a complete DC wire run (or single-phase AC) is: V_drop = 2 × L × I × r, where L is the one-way wire length in feet, I is the current in amperes, and r is the wire's resistance per foot in Ω/ft. The factor of 2 accounts for the round-trip path of current — it travels from the source to the load through one conductor and returns through the other. The voltage at the load end is V_end = V_source − V_drop, and the percentage drop is (V_drop ÷ V_source) × 100. Power lost in the wire as heat is P_loss = V_drop × I (equivalently I² × R_total, where R_total = 2 × L × r).

The voltage drop across a resistor is calculated using Ohm's Law: V = I × R, where V is the voltage drop in volts, I is the current flowing through the resistor in amperes, and R is the resistance in ohms. For example, a 10Ω resistor with 2A flowing through it produces a voltage drop of 2 × 10 = 20V. The power dissipated as heat in the resistor is P = I² × R = V²/R = V × I. For a 10Ω resistor at 2A: P = 2² × 10 = 40 watts. This formula applies to any purely resistive component — a wire, a heater element, a current-sense resistor, or a voltage divider.

The National Electrical Code (NEC) recommends in Article 210.19(A) that the voltage drop on branch circuits not exceed 3% of the source voltage, and in Article 215.2(A) that feeders not exceed 3% either — making the total combined drop from the service entrance to the furthest outlet no more than 5%. These are recommendations (using the word 'should'), not hard requirements like the mandatory rules. However, most engineers and inspectors treat them as design targets. For a 120V residential circuit, 3% means a maximum drop of 3.6V; for a 12V DC system, 3% is only 0.36V — which makes wire sizing much more critical in low-voltage installations.

Voltage drop in a circuit is caused by the electrical resistance of the conductors carrying current. Every wire has resistance determined by its material (copper, aluminium), cross-sectional area (gauge), and length. When current flows through a resistive conductor, Ohm's Law dictates that a voltage differential develops across it (V = I × R). Other contributors include: poor or corroded connections (which add contact resistance at terminals, splices, and connectors), undersized wire for the load, very long wire runs that accumulate resistance over distance, and high current draws that amplify the drop even on low-resistance wire. Temperature also affects resistance — copper's resistance increases by approximately 0.393% per degree Celsius above 20°C.

Wire gauge (AWG — American Wire Gauge) has a direct and significant effect on voltage drop. AWG uses an inverse scale: a lower AWG number means a thicker wire with a larger cross-sectional area, which has lower resistance per foot and therefore less voltage drop. For example, 14 AWG copper wire has a resistance of about 0.00258 Ω/ft, while the thicker 10 AWG wire has only 0.00102 Ω/ft — 2.5× less resistance. Upgrading from 14 AWG to 12 AWG (0.00162 Ω/ft) reduces the resistance per foot by 37%, proportionally reducing voltage drop on the same run at the same current. For long wire runs or high-current loads, stepping up one or two gauge sizes is often the most cost-effective way to bring voltage drop within NEC limits.

Voltage drop and voltage loss are often used interchangeably, but there is a nuanced distinction. Voltage drop specifically refers to the reduction in voltage across a particular component or wire segment due to resistance — it is the difference in potential between the two ends of that element (V = I × R). Voltage loss is a more general term that can describe the cumulative effect of multiple drops across an entire circuit. In engineering practice, 'voltage drop' usually refers to the drop across a single conductor or device, while 'voltage loss' may describe the total reduction from source to load including all connections. The energy is not destroyed — it is converted to heat in the conductor — but it is 'lost' from the perspective of the useful voltage available to the load.

There are four main strategies to reduce voltage drop: (1) Use a larger wire gauge — thicker wire (lower AWG number) has less resistance per foot and therefore less drop for the same current and length. (2) Shorten the wire run — halving the one-way distance halves the drop. (3) Reduce the current — using a higher voltage system (e.g. 240V instead of 120V for the same wattage) cuts the current in half and reduces the drop by half, since P = V × I and I decreases for the same P. (4) Use higher-conductivity materials — copper has significantly lower resistance than aluminium; silver is even better but cost-prohibitive for most uses. For 12V DC systems (solar, RV, marine), oversizing the wire is especially important because the voltage is already low and even a fraction of a volt drop is a significant percentage of the supply.

For DC circuits and single-phase AC circuits, the voltage drop formula is the same: V_drop = 2 × L × I × r, using twice the one-way length to account for both the hot and neutral conductors. For three-phase AC circuits, the formula changes to V_drop = √3 × L × I × r (approximately 1.732 × L × I × r), because the three conductors share the return path and the geometry reduces the effective round-trip length. The wire resistance values (Ω/ft) are the same physical property regardless of AC or DC — though AC circuits have an additional component called inductive reactance (X_L) that adds to impedance at high frequencies or with large inductive loads. For typical residential and commercial 60 Hz wiring, the reactance is small compared to resistance and the DC formula gives a very close approximation.

To calculate voltage drop for a 12V DC system (solar panels, RV, marine, battery storage), select the 'Wire Run → Voltage Drop' mode and enter: Source Voltage = 12, Current = your load's current draw in amps, One-Way Wire Length in feet from battery to load, and Wire Resistance per foot for your chosen AWG wire. For example, a 10A LED light bar 20 feet from a 12V battery using 10 AWG wire (0.00102 Ω/ft) gives V_drop = 2 × 20 × 10 × 0.00102 = 0.408V, leaving 11.592V at the load — a 3.4% drop. To find the maximum wire length that keeps drop within 3%, use the 'Max Wire Length' mode: with 12V source, 10A current, 3% max drop, and 10 AWG wire, the maximum one-way run is 0.36 ÷ (2 × 10 × 0.00102) ≈ 17.6 feet.