Voltage Drop Calculator

Formula

Vd = (2 × L × I × R) / 1000

Vd is the total voltage drop in volts (V); 2 accounts for both the hot and neutral conductors in a single-phase circuit; L is the one-way conductor length in feet; I is the circuit load current in amperes (A); R is the conductor direct-current resistance in ohms per 1,000 feet (sourced from National Electrical Code Chapter 9, Table 8); and dividing by 1,000 scales the length to the per-thousand-foot resistance value.

Worked example

Conductor

cu

Wire size

12

One-way length

100

Load current

15

Source voltage

120

• Voltage drop: 5.79 V (4.83%)
• Voltage at load: 114.21 V
• Within 3% guideline?: No, use thicker wire

Understanding Voltage Drop in Electrical Circuits

When electrical current moves through a conductor, it encounters resistance inherent to the metal. This resistance causes a drop in electrical potential between the supply panel and the destination device. While short wire runs inside a home exhibit negligible loss, long circuits—such as those feeding detached garages, outbuildings, deep-well pumps, or outdoor lighting—can experience significant loss.

The Voltage Drop Calculator provides electricians, contractors, and DIY planners with a reliable way to evaluate conductor performance. By factoring in conductor material, American Wire Gauge (AWG) size, circuit length, and load current, the tool checks whether your installation operates safely within recognized performance standards.

The Mathematical Formula

For single-phase alternating current circuits, the calculator uses the well-established DC resistance relationship outlined in National Electrical Code (NEC) Chapter 9, Table 8:

$$\text{Voltage Drop } (V_d) = \frac{2 \times L \times I \times R}{1000}$$

$$\text{Percentage Drop } (%) = \left(\frac{V_d}{V_s}\right) \times 100$$

• $L$ (Length): The one-way physical distance from the panel to the load in feet.
• $2$ (Circuit Multiplier): Accounts for the round-trip distance traveled by current across both the hot and neutral conductors.
• $I$ (Current): The design current drawn by the load in amperes.
• $R$ (Resistance): The conductor resistance expressed in ohms per 1,000 feet at typical operating temperature.
• $V_s$ (Source Voltage): The nominal electrical potential supplied at the breaker panel (e.g., 120 V or 240 V).

Step-by-Step Worked Example

Consider a common residential scenario: powering a 15-ampere power tool in an outbuilding located 100 feet from the main 120-volt breaker panel using standard 12 AWG copper wire.

1. Identify the Conductor Resistance: According to standard NEC tables, uncoated solid copper wire of size 12 AWG has a resistance ($R$) of approximately $1.93\text{ }\Omega$ per 1,000 feet.
2. Apply Circuit Parameters:
   • Conductor material: Copper (cu)
   • Wire size: 12 AWG
   • One-way length ($L$): $100\text{ ft}$
   • Load current ($I$): $15\text{ A}$
   • Source voltage ($V_s$): $120\text{ V}$
3. Compute the Voltage Drop: $$V_d = \frac{2 \times 100 \times 15 \times 1.93}{1000} = \frac{5790}{1000} = 5.79\text{ V}$$
4. Calculate Percentage Loss: $$%\text{ Drop} = \left(\frac{5.79}{120}\right) \times 100 = 4.83%$$
5. Determine Voltage at the Load: $$V_{\text{load}} = 120\text{ V} - 5.79\text{ V} = 114.21\text{ V}$$

Because $4.83%$ exceeds the recommended $3%$ threshold for branch circuits, the calculator alerts you with the recommendation: "No, use thicker wire." Stepping up to 10 AWG copper ($1.21\text{ }\Omega / 1000\text{ ft}$) on the same 100-foot run drops the loss to $3.63\text{ V}$ ($3.03%$) or 8 AWG ($0.764\text{ }\Omega / 1000\text{ ft}$) down to $2.29\text{ V}$ ($1.91%$).

How to Interpret the Results

The 3% and 5% Benchmarks

The National Electrical Code includes Informational Notes (such as NEC 210.19(A) Informational Note No. 4) recommending:

• Maximum 3% drop on any individual branch circuit or feeder.
• Maximum 5% total drop across the combined feeder and branch circuit system from the service entrance to the furthest outlet.

Staying within these boundaries ensures optimal motor torque, prevents incandescent and LED lighting flicker, and protects sensitive electronics from low-voltage brownout damage.

Voltage at the Load

Compare the resulting load voltage against your equipment nameplate. Most North American appliances rated for 115 V operate safely down to 108 V or 110 V (within $\pm 10%$ of nominal utility voltage). If your calculated drop brings the terminal voltage below device specifications, premature motor burnout or intermittent electronic reset can occur.

Practical Wire Sizing Recommendations

• Route Planning: Always add 10% to 15% to straight-line tape measurements to account for panel make-up loops, junction boxes, and vertical transitions.
• Continuous vs. Intermittent Loads: If a load operates continuously for three hours or more, sizing guidelines mandate rating the circuit at 125% of the continuous draw for ampacity, though voltage drop calculations should use actual steady-state current.
• Terminal Ratings: When upsizing wire to combat voltage drop, verify that the breaker and device lugs accept the larger wire gauge. A 10 AWG or 8 AWG wire may require pigtailing down to 12 AWG at the receptacle if the screw terminals are not rated for larger conductors.

Limitations and When to Use Specialized Engineering

This calculator utilizes direct-current resistance values, which provide excellent accuracy for residential and light commercial circuits operating at power factors near 1.0 (resistive loads) with smaller conductors (up to 1/0 AWG).

However, for heavy industrial systems utilizing large cables (250 kcmil and above) running inside magnetic steel conduit, alternating current inductive reactance ($X_L$) and circuit power factor significantly impact total impedance. In those specialized three-phase commercial installations, use the NEC Chapter 9 Table 9 effective impedance method rather than basic DC resistance formulas.

Comparison

Scenario	Input	Result
Lower input	One-way length: 80	Voltage drop: 4.63 V (3.86%)
Base example	One-way length: 100	Voltage drop: 5.79 V (4.83%)
Higher input	One-way length: 120	Voltage drop: 6.95 V (5.79%)

Common mistakes

• Doubling the one-way distance in the input field when the single-phase formula already accounts for the return path.
• Assuming circuit breaker rating is the actual operating current rather than calculating the connected design load.
• Using copper resistance values when running aluminum conductors, which causes substantial underestimation of voltage drop.
• Measuring physical room dimensions instead of accounting for vertical rises, conduit bends, and panel offsets.
• Overlooking ambient temperature increases in attics or exterior conduit runs that elevate conductor resistance.

Checklist

• Measure the one-way circuit distance along the actual wire path rather than straight-line distance.
• Verify the continuous or non-continuous nature of the electrical load to apply appropriate amperage figures.
• Identify conductor material correctly, noting that aluminum exhibits roughly 1.6 times higher resistance than copper.
• Check source voltage at the main service panel under operating conditions for optimal accuracy.
• Aim to keep branch circuit drop under 3% and total feeder-plus-branch drop under 5% per NEC recommendations.
• Consult local electrical codes and equipment nameplates for minimum operating voltage tolerances.

Calculators Percentage Editorial Team

Calculator guides from the Calculators Percentage team. These introductory examples are educational, not professional advice.

Last reviewed: November 12, 2026

This tool is designed for preliminary planning and educational guidance based on standard National Electrical Code direct-current resistance tables. Always verify conductor sizing with a licensed electrician and consult your local jurisdiction's adopted electrical codes before performing electrical installations.