Voltage Drop Calculator

How to use this voltage drop calculator

What this calculator does

This calculator estimates the resistive voltage lost along a conductor run, the percentage of the source voltage that is lost, the voltage available at the load, the equivalent circuit resistance, and the electrical power dissipated in the conductors. It uses conductor resistivity at approximately 20 °C and the standard geometric path factor for DC, single-phase AC, or balanced three-phase AC. It is a planning and comparison tool, not an ampacity, fault-current, protection, insulation, thermal, or code-compliance determination. The NIST explanation of amperes, volts, and ohms provides the SI relationships behind the displayed units.

When to use it

Use the calculator when comparing wire sizes for a long feeder, checking whether a low-voltage load may receive adequate voltage, estimating the effect of parallel conductors, or reviewing how a higher current or longer run changes losses. It is also useful for preliminary DC battery wiring, lighting circuits, equipment feeds, and balanced three-phase runs where a resistance-only estimate is appropriate.

How to calculate

1. The calculator opens with a complete demonstration: DC, one copper conductor, 8 AWG, 300 ft one-way length, 220 V, and 1.2 A. Results and a validated example workbook are immediately available.
2. Choose Current type and phase, then enter the Number of conductors operating in parallel.
3. Select the Wire material and Wire size in AWG units. A standard AWG choice fills Cross-sectional area; editing the area selects Custom area.
4. Enter Wire length and choose ft or m. Changing the unit converts the current value rather than relabeling it.
5. Enter Initial voltage in volts and Load current in amperes. Read the live result cards and the nearby-AWG table.
6. Select Download Excel to export the current validated model. Reset clears the demonstration and calculated state; export remains unavailable until a new complete valid state is entered.

Input guide

Current type and phase is required and accepts DC, Single-phase AC, or Three-phase AC. DC and single-phase use a factor of 2 for the outward and return path; balanced three-phase uses √3. Choosing the wrong phase changes the modeled circuit resistance and voltage drop. Number of conductors is a required whole number from 1 to 20; an example is 1. More equal parallel conductors reduce equivalent resistance, but the calculation assumes even current sharing.

Wire material is required and sets resistivity for Copper, Aluminum, Silver, or Gold near 20 °C. Copper is the startup example. Higher resistivity raises the result. Real conductor resistance varies with alloy, temperature, stranding, and manufacturing tolerance; the Copper Development Association conductivity guide explains why material and temperature matter.

Wire size in AWG units is required unless Custom area is used. The example is 8 AWG. A lower AWG number generally means a larger area and lower voltage drop. Cross-sectional area is a required positive decimal in mm²; the example is 8.367. Use a period as the decimal separator, no thousands separators, and no scientific notation. Do not confuse conductor diameter with cross-sectional area.

Wire length is the required positive one-way distance, entered as a plain decimal with ft or m selected; the example is 300 ft. Doubling length doubles resistance and drop. Do not enter round-trip length because the phase factor already handles the path. Initial voltage is required, greater than zero, and entered in V; the example is 220. A higher source voltage does not change drop in volts under this model, but it reduces the percentage drop. Load current is required, entered in A, and may be zero or positive; the example is 1.2. Higher current raises voltage drop linearly and power loss quadratically through I²R.

Output guide

Voltage drop is the estimated loss in volts and is the primary result. Voltage drop as a percentage divides that loss by Initial voltage; zero means no modeled load current, while a larger percentage signals less voltage reaching the load. Voltage at the end is source voltage minus drop. A negative value means the requested current, length, and conductor combination is physically unsuitable within this simple fixed-current model.

Equivalent circuit resistance combines material, cross-sectional area, length, phase factor, and parallel-conductor count in ohms. Power loss in conductors is current multiplied by voltage drop, equivalent to I²R, in watts. 3% drop threshold is exactly 3% of source voltage, and Margin to 3% is that threshold minus the estimated drop; a negative margin means the estimate exceeds the comparison level. One-way conductor length reports the normalized metric length used in the formula.

The summary pills repeat phase summary, wire summary, distance summary, and guideline summary from the same current model. The Nearby AWG comparison table lists Wire size, Area, Voltage drop, Drop percentage, Voltage at end, and 3% check for adjacent standard sizes. It is a sensitivity table, not an automatic wire recommendation or ampacity approval.

Worked example

For the opening values, 300 ft converts to 91.44 m. Copper resistivity is modeled as 0.017241 Ω·mm²/m, 8 AWG has an area of 8.367 mm², the DC factor is 2, and there is one parallel conductor. Equivalent resistance is 2 × 91.44 × 0.017241 ÷ 8.367 = 0.376842 Ω. At 1.2 A, the voltage drop is 1.2 × 0.376842 = 0.452 V. That is 0.206% of 220 V, leaving 219.548 V at the load and dissipating about 0.543 W in the conductors.