Electrical Circuits•AC & DC Conductor Sizing•Updated October 22, 2026

Voltage Drop Calculator

Calculate voltage drop for DC, single-phase, and balanced three-phase circuits. Determine voltage loss, percentage drop, receiving voltage, and evaluate adjacent wire sizes.

Input Parameters

Circuit Type

Source / System Voltage (Volts)

Load Current (Amps) Amps

Operating continuous or peak load current carried through the circuit conductors.

One-Way Circuit Distance (Feet) Feet

Enter one-way distance. The calculator accounts for the return path automatically.

Distance Notice: Enter one-way distance. The calculator accounts for the return path automatically. Do not manually double your measurement.

Conductor Material

Conductor Wire Size (AWG / kcmil)

Standard AWG and kcmil conductor sizes with 75°C stranded direct-current resistance from NEC Chapter 9 Table 8.

Selected Comparison Threshold

Comparison Threshold Target (%) %

Selected comparison threshold used for visual evaluation. Commonly 3% for branch circuits and 5% for combined runs. This is an engineering comparison target, not a universal code mandate.

View Calculated Result & Matched Gear↓

Calculated Voltage Drop

4.75 V

3.96% potential drop of 120V source over 75 ft one-way

Voltage at Load

115.25 V

Available terminal voltage

Voltage Drop %

3.96%

Target: ≤ 3%

Total Loop Resistance

0.2970 Ω

2-wire loop @ 75°C

Conductor Resistance

1.98 Ω

Per 1,000 ft @ 75°C stranded

Exceeds selected comparison threshold (3%)

Calculated drop of 3.96% exceeds your selected 3% comparison target. Review the adjacent wire sizes below to compare larger conductors.

Calculation Baseline & Parameters

Resistance Basis: 75°C stranded (NEC Ch. 9 Tbl 8)

Distance Mode: One-way + automatic return path

Conductor R: 1.98 Ω / 1k ft

Multiplier: 2 (complete loop)

Conductor Comparison Table

Target: ≤ 3%

Wire Size	R (Ω/1k ft)	Voltage Drop	Drop %	Load Voltage	Comparison
14 AWG	3.14	7.54 V	6.28%	112.46 V	> 3%
12 AWG(Selected)	1.98	4.75 V	3.96%	115.25 V	> 3%
10 AWG	1.24	2.98 V	2.48%	117.02 V	≤ 3%
8 AWG	0.778	1.87 V	1.56%	118.13 V	≤ 3%
6 AWG	0.491	1.18 V	0.98%	118.82 V	≤ 3%
4 AWG	0.308	0.74 V	0.62%	119.26 V	≤ 3%
3 AWG	0.245	0.59 V	0.49%	119.41 V	≤ 3%
2 AWG	0.194	0.47 V	0.39%	119.53 V	≤ 3%
1 AWG	0.154	0.37 V	0.31%	119.63 V	≤ 3%
1/0 AWG	0.122	0.29 V	0.24%	119.71 V	≤ 3%
2/0 AWG	0.0967	0.23 V	0.19%	119.77 V	≤ 3%
3/0 AWG	0.0766	0.18 V	0.15%	119.82 V	≤ 3%
4/0 AWG	0.0608	0.15 V	0.12%	119.85 V	≤ 3%
250 kcmil	0.0515	0.12 V	0.10%	119.88 V	≤ 3%
300 kcmil	0.0429	0.10 V	0.09%	119.90 V	≤ 3%
350 kcmil	0.0367	0.09 V	0.07%	119.91 V	≤ 3%
500 kcmil	0.0258	0.06 V	0.05%	119.94 V	≤ 3%

At the selected inputs, wire sizes marked with green calculate at or below the selected voltage-drop comparison threshold.

Critical Engineering Boundary:Voltage-drop analysis does NOT replace conductor ampacity verification, overcurrent protection sizing, insulation temperature ratings (60°C, 75°C, 90°C), ambient temperature derating, conduit fill adjustments, terminal lug compatibility, or local electrical code compliance. Always verify conductor ampacity under NEC Section 310.16 prior to physical installation.

Need to determine circuit amperage or reduce voltage drop?

Convert appliance wattage to circuit current with our Watts to Amps Calculator. For solar arrays, learn how series stringing increases voltage to minimize wire resistance losses in our Solar Panels Series vs Parallel Guide.

Commercial electrical panel with heavy AWG copper conductors connected to terminal lugs and circuit breakers with digital multimeter measuring line voltage.

Figure 1: Conductor Routing & Voltage Drop Architecture: Heavy-gauge electrical conductors routed through distribution panels. As current flows over extended circuit lengths, conductor internal resistance reduces voltage available at downstream electrical loads.

Circuit Voltage Drop & Return Path Architecture

Visualizing electrical potential drop from the supply panel through 75 ft of 12 AWG Copper conductors to the load terminals.

75°C Stranded Model

One-Way User InputYou specify the physical separation (75 ft). The calculation engine handles conductor return paths automatically.

Conductor Properties12 AWG Copper stranded conductor evaluated at a standardized 75°C internal resistance reference basis.

Terminal VoltageEquipment receives 115.25 V under full 16 A load. Actual voltage depends on steady line conditions.

Understanding Voltage Drop in Electrical Circuit Design

Voltage drop is the reduction in electrical potential between a power source and connected equipment as current flows through circuit conductors. While copper and aluminum are effective conductors, every physical wire possesses finite internal resistance.

Under Ohm's Law (V = I × R), current traversing conductor resistance converts electrical energy into heat. This loss reduces the usable terminal voltage delivered to connected equipment.

Why Circuit Distance and Load Current Multiply Losses

Conductor resistance scales directly with physical distance. For example, running a feeder 150 feet produces three times the resistance of a 50-foot run of identical wire.

Voltage drop also scales directly with load current. When long distances combine with high currents, severe voltage loss can degrade motor performance, dim lighting, trigger electronic lockouts, or overheat conductors.

One-Way Physical Distance vs Complete Return Path

Electricians measure the one-way linear distance from the panel to equipment. However, electric current requires a complete closed loop to flow.

In two-wire DC and single-phase AC systems, current flows out along the supply wire and back along the return conductor. The formula applies a multiplier of 2 to cover both circuit legs.

Three-Phase Balanced Circuits and the √3 Multiplier

In balanced three-phase systems, currents in the three phase conductors are 120 degrees out of phase. Because their balanced vector sum at the neutral point equals zero, neutral conductors carry zero net return current under balanced conditions.

Line-to-line voltage drop between phase conductors is calculated by multiplying phase current by the line factor of √3 (approximately 1.73205), rather than the factor of 2 used in single-phase circuits.

Conductor Material: Copper vs Aluminum Resistance

Copper features an electrical resistivity of approximately 1.68 × 10⁻⁸ Ω·m, while aluminum presents approximately 2.82 × 10⁻⁸ Ω·m at standard temperatures. For the same AWG size, aluminum conductor resistance is roughly 1.6 times higher than copper.

Consequently, aluminum circuits typically require stepping up one to two trade wire sizes (for example, using 2 AWG aluminum instead of 4 AWG copper) to achieve equivalent voltage drop and ampacity.

National Electrical Code (NEC) Informational Notes on Voltage Drop

The National Electrical Code (NFPA 70) addresses voltage drop primarily through advisory Informational Notes rather than mandatory blanket rules. These non-mandatory guidelines help engineers balance installation cost against operational efficiency.

NEC Section 210.19(A) Informational Note No. 2 and Section 215.2(A)(1) recommend limiting voltage drop to 3% on the furthest branch circuit or feeder. Total combined drop across feeder and branch circuits should not exceed 5%.

Important Code Distinction: Commonly cited 3% and 5% values reflect NEC informational notes and good engineering design practice, not universal legal mandates. However, mandatory voltage drop limits do exist in specific code sections, such as Article 695 for fire pump installations. Always consult the locally adopted code edition and governing authority for binding legal requirements.

Why Voltage Drop Alone Does Not Determine Conductor Ampacity

Conductor sizing requires satisfying two independent criteria: ampacity thermal limits and voltage drop limits. Conductor ampacity (NEC Section 310.16) defines the maximum continuous current a wire carries safely without insulation overheating. Ambient temperature and conduit fill adjustments directly derate this allowable ampacity.

A short 10-foot circuit might calculate a negligible 0.5% voltage drop, yet remain dangerously undersized if current exceeds allowable ampacity. Safe branch circuit design requires verifying both thermal ampacity and voltage drop limits independently.

Voltage Drop Calculation Formulas

The mathematical formulas used to calculate circuit loop resistance, voltage drop, percentage loss, and receiving terminal voltage across DC, single-phase, and balanced three-phase systems.

Formula

DC & 1-Phase: VD = 2 × I × (L ÷ 1,000) × R | 3-Phase: VD = √3 × I × (L ÷ 1,000) × R | VD% = (VD ÷ V_source) × 100

Variables & Constants

Load CurrentI (Amperes (A))

Operating electrical current flowing through the circuit under normal load conditions.

One-Way DistanceL (Feet (ft))

One-way physical linear distance from the upstream supply panel or power source to the connected electrical load.

Conductor ResistanceR (Ohms per 1,000 ft (Ω/kft))

Direct-current conductor resistance at 75°C stranded baseline obtained from NEC Chapter 9 Table 8.

Nominal Source VoltageV_source (Volts (V))

Nominal electrical system supply voltage measured at the power source or distribution panel.

Circuit MultiplierMultiplier (Unitless)

Factor of 2 for two-wire DC and single-phase AC circuits; factor of √3 (approximately 1.732) for balanced three-phase AC circuits.

Resistance values reflect stranded conductors at 75°C (167°F) based on NEC Chapter 9 Table 8.
Distances are entered as one-way measurements. The formula accounts for the return conductor path automatically.
Three-phase calculations assume balanced loads across all three phases.
This resistance-based model does not account for AC inductive reactance or load power factors.

Step-by-Step Worked Example: 120V Single-Phase AC Branch Circuit (16A, 75 ft, 12 AWG Copper)

Example Scenario: A dedicated 120V single-phase 20-amp branch circuit supplies a continuous 16-amp workshop power tool located 75 feet away from the main service panel using 12 AWG stranded copper conductors.

Identify Circuit Multiplier and Conductor Resistance

Multiplier = 2 (Two-wire AC circuit) | R = 1.98 Ω per 1,000 ft (12 AWG Cu at 75°C)

Single-phase circuits require current to flow through both the ungrounded conductor and the return neutral. From NEC Chapter 9 Table 8, 12 AWG stranded copper has an internal direct-current resistance of 1.98 Ohms per 1,000 feet at 75°C.

Calculate Total Circuit Loop Resistance

R_loop = 2 × (75 ft ÷ 1,000) × 1.98 Ω = 0.2970 Ohms

Multiplying one-way distance in thousands of feet by the circuit multiplier and conductor resistance gives the combined loop resistance for the complete electrical circuit.

Calculate Voltage Drop in Volts

VD = 16 Amps × 0.2970 Ohms = 4.752 Volts

By Ohm's Law, current flowing through circuit loop resistance causes a potential drop of 4.752 Volts between the breaker panel and the load.

Calculate Percentage Voltage Drop

VD% = (4.752 V ÷ 120 V) × 100 = 3.96%

Dividing voltage loss by the nominal supply voltage shows a 3.96% drop, which exceeds the commonly referenced 3% branch circuit threshold.

Calculate Net Receiving Voltage at Connected Equipment

V_load = 120 V - 4.752 V = 115.248 Volts (approx 115.25 V)

The connected equipment receives 115.25 Volts at its terminals under steady 16-Amp operating current.

Result: At 16 Amps over a 75-foot one-way distance, 12 AWG copper produces a 4.752V drop (3.96%), which exceeds the selected 3% comparison threshold. Stepping up to 10 AWG copper (R = 1.24 Ω/kft) reduces voltage drop to 2.976V (2.48%), placing the circuit at or below the 3% target.

Voltage Drop Sizing Assumptions & Variables

Core technical assumptions, conductor properties, and mathematical baselines used in the voltage drop calculation engine.

Conductor Resistance Baseline

Default: NEC Chapter 9 Table 8 (75°C Stranded)

Typical Range: Standard trade AWG / kcmil sizes

Practical Impact: Calculations use stranded copper and aluminum resistance values from NEC Chapter 9 Table 8 at a 75°C (167°F) reference temperature. Operating conductors at lower temperatures (such as 25°C or 60°C) exhibits slightly lower resistance, while elevated ambient temperatures increase resistance.

Circuit Multiplier & Return Path

Default: 2 for DC & 1-Phase AC | √3 (1.732) for 3-Phase AC

Typical Range: Two-wire complete loop or balanced 3-phase line-to-line

Practical Impact: DC and single-phase calculations automatically account for outgoing and return conductors using a factor of 2. Balanced three-phase calculations use the line-to-line multiplier of √3 under the assumption of balanced phase currents.

Mathematical Model Scope

Default: Resistance-Based Direct-Current & AC Model

Typical Range: Unity power factor / direct-current resistance

Practical Impact: This version implements a pure resistance-based model. AC inductive reactance (NEC Chapter 9 Table 9) and load power factor can cause additional voltage drop in large conductors (such as 1/0 and larger) and long commercial conduits.

Comparison Threshold

Default: 3% (User Configurable)

Typical Range: 1% to 5% typical planning ranges

Practical Impact: The 3% and 5% values reflect common engineering guidelines and NEC informational notes for reasonable efficiency. They do not constitute universal mandatory code limits.

Parameter	Model Default	Typical Field Range	Practical Impact
Conductor Resistance Baseline	NEC Chapter 9 Table 8 (75°C Stranded)	Standard trade AWG / kcmil sizes	Calculations use stranded copper and aluminum resistance values from NEC Chapter 9 Table 8 at a 75°C (167°F) reference temperature. Operating conductors at lower temperatures (such as 25°C or 60°C) exhibits slightly lower resistance, while elevated ambient temperatures increase resistance.
Circuit Multiplier & Return Path	2 for DC & 1-Phase AC \| √3 (1.732) for 3-Phase AC	Two-wire complete loop or balanced 3-phase line-to-line	DC and single-phase calculations automatically account for outgoing and return conductors using a factor of 2. Balanced three-phase calculations use the line-to-line multiplier of √3 under the assumption of balanced phase currents.
Mathematical Model Scope	Resistance-Based Direct-Current & AC Model	Unity power factor / direct-current resistance	This version implements a pure resistance-based model. AC inductive reactance (NEC Chapter 9 Table 9) and load power factor can cause additional voltage drop in large conductors (such as 1/0 and larger) and long commercial conduits.
Comparison Threshold	3% (User Configurable)	1% to 5% typical planning ranges	The 3% and 5% values reflect common engineering guidelines and NEC informational notes for reasonable efficiency. They do not constitute universal mandatory code limits.

Voltage Drop Frequently Asked Questions

What is voltage drop?

Voltage drop is the loss of electrical potential that occurs as electric current flows through the internal electrical resistance of circuit conductors from the power source to connected equipment. All physical conductors present a small amount of resistance. According to Ohm's Law (V = I × R), current passing through this resistance creates a voltage reduction along the length of the wire, resulting in a lower receiving voltage at the electrical load.

How do you calculate voltage drop?

To calculate voltage drop using a resistance-based model, multiply the circuit multiplier by the load current in Amps, the one-way conductor length divided by 1,000 feet, and the conductor's resistance in Ohms per 1,000 feet. For DC and single-phase AC circuits, the multiplier is 2 to account for the outgoing and return conductors. For balanced three-phase AC circuits, the multiplier is √3 (approximately 1.732) for line-to-line calculations. Then divide the calculated voltage drop by the source voltage and multiply by 100 to find the percentage drop.

Is voltage drop calculated using one-way or round-trip distance?

Field engineers and electricians measure one-way distance between the power source panel and the connected load. However, the calculation formula itself accounts for the round-trip path by applying a multiplier of 2 for two-wire DC and single-phase AC circuits. On our calculator, enter the one-way physical distance, and the calculation engine automatically accounts for the complete return conductor path.

Why is voltage drop more significant on 12V systems?

A given voltage loss represents a much higher percentage on a low-voltage circuit than on a high-voltage circuit. For example, a 1.2-volt drop on a 120-volt circuit is just 1%, whereas the exact same 1.2-volt drop on a 12-volt system represents 10% of total system voltage. Furthermore, to deliver the same amount of power (Watts = Volts × Amps), a 12-volt system requires 10 times the current of a 120-volt system, multiplying resistive voltage loss.

How does wire gauge affect voltage drop?

Wire gauge directly determines conductor cross-sectional area and internal resistance. Under the American Wire Gauge (AWG) standard, smaller numerical gauge numbers indicate physically thicker conductors with lower electrical resistance. Moving up by three gauge sizes (for example, from 12 AWG to 9 AWG or approximately from 10 AWG to 7 AWG) roughly doubles the cross-sectional area and halves conductor resistance, reducing voltage drop by roughly half under identical current and distance.

When is √3 used in voltage drop calculations?

The square root of 3 (approximately 1.732) is used in balanced three-phase alternating current calculations to determine line-to-line voltage drop. In a balanced three-phase system, current returns across the other two phase conductors with a 120-degree phase shift rather than flowing through a single dedicated return conductor, resulting in an effective line-to-line voltage drop multiplier of √3 rather than 2.

What does the NEC say about voltage drop?

The National Electrical Code (NEC / NFPA 70) mentions voltage drop in informational notes, such as Informational Note No. 2 to Section 210.19(A) for branch circuits and Section 215.2(A)(1) for feeders. These notes recommend that conductors be sized to limit voltage drop to 3% on branch circuits or feeders, and no more than 5% overall for branch circuits and feeders combined, for reasonable efficiency. Informational notes in the NEC are advisory recommendations for electrical performance, not mandatory code requirements, though certain specific sections or local jurisdictions may enforce mandatory limits.

Preliminary Planning & Electrical Engineering Disclaimer

This calculator provides preliminary electrical calculations based on simplified mathematical models and user-supplied parameters. It does not constitute professional engineering advice, electrical inspection approval, or installation sign-off.
Voltage-drop analysis does not replace conductor ampacity verification, overcurrent protection device sizing, insulation temperature ratings (60°C, 75°C, 90°C), ambient temperature derating, conduit fill adjustments, terminal lug temperature limitations, or local electrical code compliance.
Working on electrical wiring, distribution panels, and high-energy circuits involves substantial risk of electrical shock, arc flash, personal injury, and fire. Always consult a qualified licensed electrician or electrical engineer to verify physical conductor sizing, overcurrent protection, and installation compliance.

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