Voltage Drop Calculator

Calculate voltage drop in electrical circuits instantly. Supports DC, single-phase AC, and three-phase AC systems. Uses NEC-compliant formulas with American Wire Gauge (AWG) specifications.

Imperial Metric
Electrical Parameters
V
5V 600V
A
0.5A 400A
ft
1ft 2000ft
0.7 (Motors) 1.0 (Resistive)
Wire Selection

Calculation Results

Voltage Drop
0.00 V
(0.00%)
Within NEC 3% guideline
0-3%
3-5%
>5%
0%Ideal 3%NEC Limit 5%Maximum 10%Critical

Key Results Summary

Metric Your Value NEC Reference
Voltage Drop 0.00 V Lower is better
Voltage Drop % 0.00% ≤3% recommended, ≤5% max
End Voltage at Load 0.00 V Should be close to source
Power Loss (Heat) 0.0 W Lower = more efficient
Total Wire Resistance 0.0000 Ω Round-trip resistance

Your Input Summary

Circuit Type
DC
Source Voltage
120 V
Load Current
15 A
Wire Length
100 ft
Wire Gauge
12 AWG
Wire Material
Copper

What This Means

Excellent - Your Circuit is Well Designed

Your voltage drop of 0.00% is well within the NEC-recommended 3% maximum for branch circuits. The equipment at the end of this wire run will receive adequate voltage for proper operation.

Your selected wire gauge is appropriate for this circuit. No changes needed.

Recommendation

12 AWG copper is adequate for this circuit.

Practical Tips for Your Installation

Use Quality Connections
Poor connections add resistance. Use proper terminals, torque to spec, and consider anti-oxidant compound for aluminum.
Account for Temperature
Hot environments increase resistance. Consider upsizing wire for outdoor or high-ambient installations.
Verify Ampacity Too
Voltage drop is separate from ampacity. Ensure your wire also meets NEC Table 310.16 current limits.
Consider Future Loads
Plan for 80% of wire capacity. Future equipment additions may increase current draw.
Voltage Drop Formula
VD = (2 × L × I × R) ÷ 1000
VD = Voltage Drop (V)
L = One-way length (ft)
I = Current (A)
R = Resistance (Ω/1000ft from NEC Table 8)

Circuit Visualization

Source
120V
Load
120V
0% Voltage Drop 10%

Last updated: January 2026 Formula: NEC Chapter 9, Tables 8 & 9

Key Takeaways

  • 3% maximum voltage drop is recommended for branch circuits per NEC guidelines
  • 5% total maximum for feeders plus branch circuits combined
  • Low-voltage DC systems (12V, 24V) are most sensitive to voltage drop
  • Longer wire runs require larger gauge wire to maintain acceptable voltage
  • Copper wire has 40% lower resistance than aluminum of the same gauge
  • All calculations happen in your browser—your data is never stored

How to Use This Calculator

This calculator determines voltage drop for DC, single-phase AC, and three-phase AC electrical circuits. Follow these step-by-step instructions to get accurate results for your specific application.

Step 1: Select Your Circuit Type

Choose the appropriate circuit type from the dropdown menu at the top of the calculator:

  • DC (Direct Current) — Use for battery systems, solar panel installations, automotive wiring, RV/marine systems, and LED lighting. DC calculations consider pure resistive losses.
  • Single-Phase AC — Use for standard residential circuits (120V/240V), light commercial applications, and typical household branch circuits. This mode includes power factor in calculations.
  • Three-Phase AC — Use for industrial motors, commercial building power distribution, and heavy machinery. Three-phase calculations use the √3 multiplier instead of 2.

Step 2: Choose Your Unit System

Toggle between Imperial (feet) and Metric (meters) using the switch at the top right. The calculator automatically converts your length values when you switch systems. Wire resistance tables are based on imperial units internally.

Step 3: Enter Electrical Parameters

Configure the left column with your circuit specifications:

  • Source Voltage — The voltage at your power source. Common values: 12V/24V for DC systems, 120V/240V for residential, 208V/480V for commercial/industrial. Range: 5V to 600V.
  • Load Current — The actual current draw of your equipment in amperes. Use the nameplate rating or measured current, not the circuit breaker size. Range: 0.5A to 400A.
  • Wire Length — The one-way distance from the power source to the load. The calculator automatically accounts for the return path. Range: 1 to 2000 feet (or 1 to 600 meters).
  • Power Factor (AC only) — Represents how efficiently your load uses power. Use 1.0 for resistive loads (heaters, incandescent lights), 0.80-0.90 for motors and inductive loads. Default: 0.85.

Step 4: Select Wire Configuration

Configure the right column with your conductor specifications:

  • Wire Gauge (AWG) — Select from 14 AWG (smallest, 2.08 mm²) through 4/0 AWG (largest, 107.2 mm²). The dropdown shows each gauge's cross-sectional area and maximum ampacity for reference.
  • Wire Material — Choose copper (lower resistance, industry standard) or aluminum (higher resistance, lighter weight, lower cost). Copper has approximately 40% lower resistance than aluminum.
  • Conduit Type (AC only) — Select PVC/non-metallic, steel EMT, or aluminum conduit. This affects AC impedance calculations for more accurate results.

Step 5: Review Your Results

Click "Calculate Voltage Drop" to see comprehensive results:

  • Voltage Drop (V) — The actual voltage lost across the wire in volts
  • Voltage Drop (%) — The percentage of source voltage lost (color-coded: green ≤3%, yellow 3-5%, red >5%)
  • End Voltage — The voltage that arrives at your load (source minus drop)
  • Power Loss — Energy wasted as heat in the wire (in watts)
  • Wire Resistance — Total circuit resistance in ohms
  • Recommendation — If voltage drop exceeds 3%, the calculator suggests a larger wire size

Pro Tip: Common Mistakes to Avoid

  • Using breaker size instead of actual load — A 20A breaker doesn't mean 20A load. Use the actual equipment current draw.
  • Entering round-trip distance — Enter only the one-way distance. The formula automatically doubles it for the return path.
  • Forgetting power factor for motors — Motors and transformers have power factors around 0.80-0.90, not 1.0.
  • Ignoring temperature effects — Wire resistance values are for 75°C. Hot environments increase resistance.
  • Only checking ampacity — Always verify both ampacity AND voltage drop. Long runs often need larger wire for voltage drop than ampacity alone would require.

When to Use This Calculator

Voltage drop calculations are essential whenever you're running wire over any significant distance or working with low-voltage systems. Here are specific situations where this calculator provides the most value:

Residential Applications

  • Detached structures — Wiring garages, sheds, workshops, or pool houses that are 50+ feet from the main panel
  • EV charger installation — Sizing wire for 40-50A Level 2 chargers, especially with long garage runs
  • Kitchen and bathroom remodels — Verifying existing wire is adequate for upgraded appliances
  • Landscape lighting — 12V lighting systems are extremely sensitive to voltage drop
  • Hot tub or pool pump circuits — High-current loads at distance from the panel

Commercial and Industrial

  • Motor circuits — Motors can stall or overheat if voltage drops below specifications
  • Large building feeders — Planning main distribution runs from service entrance to subpanels
  • Parking lot lighting — Long runs to pole-mounted fixtures
  • Data centers — Sensitive equipment requires tight voltage tolerances

Specialty Applications

  • Solar PV systems — Optimizing wire size between panels, charge controllers, batteries, and inverters
  • RV and boat electrical — 12V/24V DC systems where every volt matters
  • Automotive audio and lighting — High-power aftermarket installations
  • Off-grid systems — Battery bank wiring where efficiency is critical

Who Benefits Most

This calculator serves electricians planning circuit runs, engineers designing electrical systems, solar installers sizing DC conductors, homeowners planning DIY projects, marine and RV technicians, automotive specialists, and students learning electrical principles. Whether you need a quick field calculation or detailed documentation for a project, this tool provides the accuracy you need.

What is Voltage Drop?

Voltage drop is the reduction in electrical potential (voltage) as current flows through a conductor. Every wire has electrical resistance, and according to Ohm's Law (V = IR), this resistance causes a portion of voltage to be "lost" as heat along the wire length.

The National Electrical Code (NEC) recommends keeping voltage drop below 3% for branch circuits and 5% total for feeders plus branch circuits combined. Excessive voltage drop can cause equipment to malfunction, motors to overheat, and lights to dim.

This calculator uses the standard voltage drop formulas from NEC Chapter 9 with wire resistance values from Table 8. It automatically accounts for the round-trip wire distance (to the load and back) in all calculations.

Deep Dive Voltage Drop Formula Explained: DC, AC & 3-Phase Guide

Voltage Drop Formulas

DC Circuits

VD = (2 × L × I × R) ÷ 1000

Single-Phase AC Circuits

VD = (2 × L × I × R × PF) ÷ 1000

Three-Phase AC Circuits

VD = (√3 × L × I × R × PF) ÷ 1000
VD
Voltage drop in volts
L
One-way wire length in feet
I
Load current in amperes
R
Wire resistance in ohms per 1000 feet (from NEC Table 8)
PF
Power factor (typically 0.85 for AC motors, 1.0 for resistive loads)

The factor of 2 in DC and single-phase formulas accounts for the round-trip distance—current must flow to the load AND return through the neutral or ground wire. In three-phase balanced systems, √3 (approximately 1.732) is used because the return current flows through the other phases.

Source: NFPA 70 National Electrical Code, Chapter 9, Tables 8 and 9

Wire Resistance Table (Ohms per 1000 ft at 75°C)

The following table shows DC resistance values from NEC Chapter 9, Table 8. These values are used for voltage drop calculations.

AWG Area (mm²) Copper (Ω) Aluminum (Ω) Copper Ampacity
14 2.08 3.14 5.17 15 A
12 3.31 1.98 3.25 20 A
10 5.26 1.24 2.04 30 A
8 8.37 0.778 1.28 40 A
6 13.3 0.491 0.808 55 A
4 21.2 0.308 0.508 70 A
2 33.6 0.194 0.319 95 A
1/0 53.5 0.122 0.201 125 A
2/0 67.4 0.0967 0.159 145 A
4/0 107.2 0.0608 0.100 195 A

Note: Ampacity values are for copper conductors in raceway at 30°C ambient (NEC Table 310.16). Actual ampacity depends on installation conditions and must be derated for temperature, conduit fill, and other factors.

Voltage Drop by Distance: 120V, 20A, 12 AWG Copper

This chart shows how voltage drop increases with wire distance for a typical 120V, 20A residential circuit using 12 AWG copper wire. Notice how quickly voltage drop exceeds NEC limits at longer distances.

Voltage Drop % vs. One-Way Wire Distance

25 ft
1.2%
50 ft
2.5%
75 ft
3.7%
100 ft
5.0%
125 ft
6.2%
150 ft
7.4%
Within NEC 3% 3-5% Caution Exceeds 5%

For this 20A circuit, 12 AWG wire is only suitable up to about 45 feet to stay within the NEC 3% recommendation. Beyond that distance, upgrading to 10 AWG or larger wire is recommended. See our guide on how to reduce voltage drop for more solutions.

Copper vs. Aluminum: Resistance Comparison

Copper wire has significantly lower resistance than aluminum at every gauge, resulting in less voltage drop for the same wire size. This chart compares the two materials at common wire gauges. For a detailed analysis, see our copper vs. aluminum comparison guide.

14 AWG Cu
3.14 Ω
14 AWG Al
5.17 Ω
10 AWG Cu
1.24 Ω
10 AWG Al
2.04 Ω
6 AWG Cu
0.491 Ω
6 AWG Al
0.808 Ω
2 AWG Cu
0.194 Ω
2 AWG Al
0.319 Ω

Key insight: Aluminum has about 64% higher resistance than copper at the same gauge. To get equivalent voltage drop performance with aluminum, you typically need to go up two wire sizes. For example, 6 AWG aluminum (0.808 Ω) is roughly equivalent to 8 AWG copper (0.778 Ω).

Maximum One-Way Wire Distance for 3% Drop

Use this table to quickly determine the maximum wire length that keeps voltage drop within the NEC 3% recommendation for common circuit configurations. All values assume copper conductors.

Circuit 14 AWG 12 AWG 10 AWG 8 AWG 6 AWG 4 AWG
120V / 15A 46 ft 72 ft 116 ft 184 ft 293 ft 468 ft
120V / 20A 34 ft 55 ft 87 ft 138 ft 220 ft 351 ft
240V / 20A 69 ft 109 ft 174 ft 277 ft 440 ft 701 ft
240V / 30A — 73 ft 116 ft 184 ft 293 ft 468 ft
240V / 50A — — 70 ft 111 ft 176 ft 281 ft
12V / 10A 6 ft 9 ft 15 ft 23 ft 37 ft 58 ft
24V / 10A 11 ft 18 ft 29 ft 46 ft 73 ft 117 ft
48V / 20A — 11 ft 17 ft 28 ft 44 ft 70 ft

How to read this table: Find your circuit voltage and current in the left column, then look across to see the maximum one-way wire length for each gauge. The "—" entries indicate the wire gauge cannot safely handle that current (ampacity exceeded). For 12V and 24V systems, distances are dramatically shorter—highlighting why low-voltage systems need heavier wire.

Real-World Voltage Drop Examples

The following examples illustrate common scenarios where voltage drop calculations are essential. Each example shows realistic inputs, calculated results, and practical recommendations.

Example 1: Homeowner Adding a Backyard Workshop

Sarah, a woodworking hobbyist, is wiring a new 20A circuit to her detached workshop 150 feet from the main panel. She initially planned to use standard 12 AWG wire.

Voltage: 120V
Current: 20A
Length: 150 ft
Wire: 12 AWG Copper
Calculation: VD = (2 × 150 × 20 × 1.98) ÷ 1000 = 11.88V
✗ Voltage Drop: 11.88V (9.9%) — Exceeds NEC 5% maximum

The problem: At 9.9% voltage drop, Sarah's power tools would only receive 108V instead of 120V. Motors would run hot, struggle to start, and potentially trip thermal overloads.

Solution: Using 6 AWG copper wire: VD = (2 × 150 × 20 × 0.491) ÷ 1000 = 2.95V (2.46%). This meets the NEC 3% guideline, ensuring full performance from her table saw and dust collector.

Example 2: EV Charger Installation

Marcus is installing a 48A Level 2 EV charger in his garage, which is 80 feet from the electrical panel. His electrician suggested 6 AWG wire for the 60A circuit.

Voltage: 240V
Current: 48A (continuous)
Length: 80 ft
Wire: 6 AWG Copper
Calculation: VD = (2 × 80 × 48 × 0.491 × 0.85) ÷ 1000 = 3.21V
✓ Voltage Drop: 3.21V (1.34%) — Excellent, well within guidelines

Result: The charger receives 236.8V, providing optimal charging speed. The 6 AWG wire handles both the ampacity requirement (55A rated) and keeps voltage drop minimal. No upsizing needed.

Example 3: Solar Panel Array to Charge Controller

Jennifer is installing a 48V off-grid solar system. She needs to run wire from her roof-mounted panels to the charge controller in her utility room, a distance of 45 feet.

Voltage: 48V DC
Current: 35A (peak)
Length: 45 ft
Wire: 8 AWG Copper
Calculation: VD = (2 × 45 × 35 × 0.778) ÷ 1000 = 2.45V
⚠ Voltage Drop: 2.45V (5.1%) — Slightly exceeds 5% guideline

Analysis: In solar applications, voltage drop directly reduces harvested energy. A 5.1% drop means Jennifer loses 5.1% of her solar production to heat in the wires.

Solution: Upgrading to 6 AWG: VD = (2 × 45 × 35 × 0.491) ÷ 1000 = 1.55V (3.2%). This recovers approximately 2% of her solar production—a significant gain over the system's lifetime.

Example 4: Industrial Three-Phase Motor Circuit

Carlos, an industrial electrician, is sizing wire for a new 25 HP air compressor in a manufacturing plant. The motor is located 200 feet from the MCC (Motor Control Center).

Voltage: 480V 3-Phase
Current: 50A
Length: 200 ft
Wire: 6 AWG Copper
Power Factor: 0.85
Calculation: VD = (√3 × 200 × 50 × 0.491 × 0.85) ÷ 1000 = 7.23V
✓ Voltage Drop: 7.23V (1.51%) — Excellent for motor starting

Why this matters: Motors draw 6-8 times their running current during startup. Low voltage drop ensures the motor can accelerate the compressor to full speed without overheating or tripping on overcurrent. The 6 AWG wire provides ample margin.

Example 5: 12V Landscape Lighting — A Cautionary Tale

David is installing low-voltage landscape lights along his 75-foot driveway. He purchased a 150W transformer and standard 14 AWG landscape wire, connecting ten 10W LED fixtures.

Voltage: 12V DC
Current: 8A (total for all fixtures)
Length: 75 ft
Wire: 14 AWG Copper
Calculation: VD = (2 × 75 × 8 × 3.14) ÷ 1000 = 3.77V
✗ Voltage Drop: 3.77V (31.4%) — Severe, lights will be dim or fail

The problem: At 31.4% voltage drop, the lights at the end of the run receive only 8.2V instead of 12V. They'll be noticeably dimmer than fixtures near the transformer, and some may flicker or fail entirely.

Solution: Using 10 AWG wire: VD = (2 × 75 × 8 × 1.24) ÷ 1000 = 1.49V (12.4%). Still high for lighting—consider 8 AWG for 0.93V (7.8%), or split into two runs with a multi-tap transformer.

Example 6: RV Battery to Inverter Connection

Linda upgraded her RV to a 3000W pure sine wave inverter. The batteries are mounted 8 feet from the inverter compartment. At full load, the inverter draws 280A from the 12V battery bank.

Voltage: 12V DC
Current: 280A (peak)
Length: 8 ft
Wire: 4/0 AWG Copper
Calculation: VD = (2 × 8 × 280 × 0.0608) ÷ 1000 = 0.27V
✓ Voltage Drop: 0.27V (2.3%) — Acceptable for high-current DC

Critical consideration: Even with the largest common wire gauge (4/0), the 8-foot run produces measurable voltage drop. Longer runs would require parallel cables or relocating the inverter closer to the batteries. At 280A, even small resistance creates significant heat.

Example 7: Commercial Kitchen Equipment Circuit

A restaurant is installing a new commercial pizza oven rated at 40A on a 208V single-phase circuit. The run from the panel to the kitchen is 120 feet through the drop ceiling.

Voltage: 208V Single-Phase
Current: 40A
Length: 120 ft
Wire: 8 AWG Copper
Power Factor: 1.0 (resistive)
Calculation: VD = (2 × 120 × 40 × 0.778 × 1.0) ÷ 1000 = 7.47V
⚠ Voltage Drop: 7.47V (3.59%) — Slightly above 3% recommendation

Consideration: While 3.59% exceeds the 3% guideline, it's within the 5% total limit. For a resistive heating load, this is acceptable—the oven will simply take slightly longer to reach temperature. However, upgrading to 6 AWG (4.71V, 2.26%) would be better practice.

Step-by-Step Example Calculation

Problem: Calculate the voltage drop for a 120V, 15A residential circuit using 12 AWG copper wire over a 100-foot run.

Identify Wire Resistance

From NEC Chapter 9, Table 8, 12 AWG copper wire has a resistance of:

R = 1.98 Ω per 1000 ft

Apply the DC/Single-Phase Formula

Using the standard voltage drop formula:

VD = (2 × L × I × R) ÷ 1000

Substitute Values

VD = (2 × 100 × 15 × 1.98) ÷ 1000 VD = 5,940 ÷ 1000 VD = 5.94 volts

Calculate Percentage

VD% = (5.94 ÷ 120) × 100 = 4.95%

Evaluate Against NEC Guidelines

At 4.95%, this exceeds the NEC 3% recommendation for branch circuits. Options to reduce voltage drop:

  • 10 AWG copper: 3.72V (3.1%) — still marginal
  • 8 AWG copper: 2.33V (1.94%) — acceptable
  • Shorter run: Reduce wire length if possible

In-Depth Guides

Master voltage drop calculations with our comprehensive educational articles covering formulas, applications, and best practices.

View all articles →

Frequently Asked Questions

The National Electrical Code (NEC) recommends a maximum of 3% voltage drop for branch circuits and 5% total for feeders plus branch circuits combined. For sensitive electronics and computer equipment, many engineers recommend keeping voltage drop under 2%. These are recommendations rather than code requirements, but following them ensures reliable equipment operation and energy efficiency.

There are four main ways to reduce voltage drop: (1) Use larger gauge wire—this is the most common solution; (2) Shorten the wire run by relocating the panel or equipment closer; (3) Reduce the load current by using more efficient equipment or splitting the load across multiple circuits; (4) Increase the system voltage if possible (e.g., 240V instead of 120V). Using copper instead of aluminum wire also helps, as copper has lower resistance.

The factor of 2 accounts for the round-trip distance that current must travel. In a simple DC or single-phase circuit, current flows from the source to the load through the "hot" wire, then returns through the neutral (or ground) wire. Both wires have resistance, so the total circuit resistance is doubled. The wire length you enter should be the one-way distance; the formula automatically doubles it.

DC voltage drop calculations only consider the wire's resistance. AC calculations must also account for reactance (inductive and capacitive effects) and power factor. The power factor represents how efficiently the circuit uses power—resistive loads like heaters have a power factor of 1.0, while motors typically have power factors around 0.85. AC calculations generally result in slightly higher voltage drops than DC for the same current and wire size.

Yes, wire resistance increases with temperature. The NEC tables are based on 75°C (167°F) conductor temperature. In hot environments or when wires are carrying heavy loads, the actual temperature may be higher, increasing resistance by 10-15% or more. For critical applications, consider using the next larger wire size to account for temperature effects and provide a safety margin.

Southwire's voltage drop calculator uses the same NEC Chapter 9 formulas and wire resistance values. This calculator provides identical results for the same inputs, with additional features including three-phase support, visual circuit diagrams, wire size recommendations, and the ability to toggle between imperial and metric units. Both are suitable for professional electrical design calculations.

Wire size depends on both ampacity (safe current-carrying capacity) and voltage drop. First, select a wire size that meets the ampacity requirements from NEC Table 310.16. Then, verify that the voltage drop is acceptable using this calculator. For long runs, voltage drop often requires larger wire than ampacity alone would suggest. Always size conductors for both requirements and choose the larger wire. See our complete wire size chart for detailed ampacity and resistance values.

At 12V, even a small voltage drop in volts represents a large percentage. A 1V drop on a 120V circuit is only 0.83%, but that same 1V drop on a 12V circuit is 8.3%. This is why low-voltage lighting requires heavier gauge wire for the same current. A typical 50-foot run with 5A load needs at least 10 AWG wire to stay under 5% drop—much heavier than the 16-18 AWG wire commonly sold with landscape kits. Learn more in our LED landscape lighting guide.

This calculator uses the same NEC Chapter 9, Table 8 resistance values and formulas as professional tools including Southwire's voltage drop calculator. Results are identical for the same inputs. The main differences are in additional features—this calculator adds three-phase support, circuit visualization, automatic wire recommendations, and unit conversion. For a detailed comparison, see our Southwire calculator comparison.

Copper is the standard choice for most applications due to its lower resistance (about 40% less than aluminum), smaller required wire size, and easier termination. Aluminum makes economic sense for large feeders (typically 4 AWG and larger) where the cost savings outweigh the need for larger wire and special AL-rated terminations. For branch circuits, especially those requiring flexibility during installation, copper is almost always preferred. See our detailed copper vs aluminum comparison for more guidance.

For solar installations, you'll typically calculate voltage drop for three segments: (1) from panels to combiner box, (2) from combiner to charge controller or inverter, and (3) from batteries to inverter. Use DC mode for all segments. Solar professionals often target 2% maximum drop per segment to maximize energy harvest—every percentage point of voltage drop represents lost generation capacity. Our solar voltage drop guide covers this in detail.

This calculator is a planning and verification tool, not a substitute for professional expertise. Consult a licensed electrician for: (1) any work requiring permits, (2) service entrance and main panel work, (3) situations where safety is critical (wet locations, hazardous areas), (4) when results seem unusual or you're unsure, and (5) all actual installation work if you're not qualified. This calculator helps you understand requirements and verify calculations, but electrical work is regulated and potentially dangerous.

NEC Voltage Drop Guidelines

Circuit Type Max Recommended Code Reference
Branch Circuits 3% NEC 210.19(A) Informational Note No. 4
Feeders 3% NEC 215.2(A) Informational Note No. 2
Total (Feeder + Branch) 5% NEC 210.19(A) Informational Note No. 4
Sensitive Equipment 2% (recommended) IEEE 141 (Red Book)

Important: These are NEC recommendations, not requirements. The NEC informational notes state that conductors "should be sized" to meet these guidelines for "reasonable efficiency of operation." Local authorities having jurisdiction (AHJ) may have additional requirements. Always verify with local codes before installation.

Full Guide NEC Voltage Drop Guidelines: The 3% and 5% Rules Explained

Understanding Your Results

After calculating voltage drop, use these guidelines to interpret what the numbers mean for your specific application and what actions to take.

Voltage Drop Percentage Ranges

Drop % Status What It Means Action Required
0-2% Excellent Optimal efficiency; exceeds NEC recommendations No action needed. Wire sizing is conservative.
2-3% Good Within NEC 3% guideline for branch circuits Acceptable for most applications. Proceed with installation.
3-5% Marginal Exceeds branch circuit recommendation but within total limit Consider larger wire, especially for motors or sensitive loads.
5-10% Poor Exceeds NEC maximum; efficiency loss and potential issues Upgrade wire size. Motors may overheat, lights will dim.
>10% Critical Severe loss; equipment may malfunction or fail Must upgrade wire. Consider shorter run or higher voltage.

Effects of Voltage Drop by Load Type

Load Type Voltage Drop Tolerance Symptoms of Excessive Drop
Incandescent Lights Somewhat tolerant (up to 5%) Dimmer output, warmer color temperature, reduced lifespan
LED Lights Sensitive (keep under 3%) Flickering, uneven brightness, driver failure in extreme cases
Motors Critical (keep under 3%) Overheating, difficulty starting, reduced torque, tripped overloads
Electronics/Computers Very sensitive (keep under 2%) Erratic behavior, data errors, premature component failure
Resistive Heaters Tolerant (up to 5%) Longer time to reach temperature, reduced heat output
Welders Critical (keep under 2%) Poor arc stability, weak welds, overheating

Power Loss Considerations

The power loss value shows how much energy is wasted as heat in your conductors. This has two important implications:

  • Energy cost: A circuit with 50W of power loss running 8 hours/day wastes 146 kWh/year—about $15-25 annually at typical rates. For high-current circuits, this can add up significantly.
  • Heat generation: That wasted power becomes heat in your wires. In conduit runs with multiple circuits, excessive heat can require derating or cause insulation damage.

When to Be More Conservative

Consider targeting lower voltage drop (under 2%) in these situations:

  • Motor circuits where startup inrush is a concern
  • Circuits feeding sensitive electronic equipment
  • Solar PV systems where every percentage point affects energy harvest
  • Long feeders that also supply branch circuits (to leave room for branch circuit drop)
  • Circuits that may have additional loads added in the future

Wire Size Quick Reference by Application

Use this table as a starting point for common residential and commercial applications. Always verify voltage drop for your specific circuit length.

Application Typical Voltage Typical Current Minimum Wire Recommended for Long Runs
General lighting circuit 120V 15A 14 AWG 12 AWG (>50 ft)
Kitchen/bath outlets 120V 20A 12 AWG 10 AWG (>75 ft)
Electric dryer 240V 30A 10 AWG 8 AWG (>75 ft)
Electric range 240V 50A 6 AWG 4 AWG (>50 ft)
EV charger (Level 2) 240V 40-50A 6 AWG 4 AWG (>75 ft)
Central A/C (3-ton) 240V 20-25A 10 AWG 8 AWG (>100 ft)
Subpanel feeder (60A) 240V 60A 6 AWG 4 AWG (>100 ft)
Subpanel feeder (100A) 240V 100A 3 AWG 2 AWG (>75 ft)
12V landscape lighting 12V DC 5-10A 12 AWG 10-8 AWG (always verify!)
Solar panel strings 48V DC 10-40A 10 AWG 6-4 AWG (roof to inverter)

Note: "Minimum Wire" refers to NEC ampacity requirements. "Recommended for Long Runs" accounts for voltage drop. For circuits over the lengths indicated, calculate your specific voltage drop to determine the correct wire size.

Copper vs. Aluminum: Complete Comparison

Choosing between copper and aluminum conductors involves trade-offs in resistance, cost, weight, and installation requirements.

Factor Copper Aluminum
Resistance (relative) 1.0× (baseline) 1.6× higher (requires larger wire)
Weight (same ampacity) 1.0× (baseline) 0.5× lighter (half the weight)
Material cost Higher (≈$3-5/lb) Lower (≈$1-2/lb)
Installed cost (same ampacity) Higher overall 20-40% savings on large feeders
Thermal expansion Lower Higher (requires special terminations)
Oxide formation Conductive oxide Insulating oxide (requires antioxidant)
Typical applications Branch circuits, all sizes Large feeders, service entrance (4 AWG+)
NEC requirements Standard terminations AL-rated terminations required

When to Use Each Material

  • Use copper for branch circuits, motor connections, residential wiring, and any application where space is limited or voltage drop is critical.
  • Consider aluminum for large service entrance conductors (100A+), long feeder runs where cost savings justify the larger wire size, and overhead utility connections.

For voltage drop calculations, remember that aluminum requires approximately 1.5-2 wire sizes larger than copper to achieve the same voltage drop. For example, if copper 6 AWG meets your requirements, aluminum 4 AWG would be the equivalent.

Maximum One-Way Wire Length for 3% Voltage Drop

This reference table shows the maximum one-way wire length (in feet) to stay within the NEC 3% voltage drop guideline for common circuit configurations using copper wire.

Circuit 14 AWG 12 AWG 10 AWG 8 AWG 6 AWG 4 AWG
120V / 15A 57 ft 91 ft 145 ft 231 ft 366 ft 584 ft
120V / 20A — 68 ft 109 ft 173 ft 275 ft 438 ft
240V / 20A — 136 ft 218 ft 347 ft 550 ft 877 ft
240V / 30A — — 145 ft 231 ft 366 ft 584 ft
240V / 50A — — — 139 ft 220 ft 351 ft
12V DC / 10A 11 ft 18 ft 29 ft 46 ft 73 ft 117 ft
12V DC / 20A 6 ft 9 ft 15 ft 23 ft 37 ft 58 ft
48V DC / 20A 23 ft 36 ft 58 ft 92 ft 147 ft 234 ft

How to use: Find your circuit voltage and current in the left column. Read across to find the maximum one-way distance for each wire size. If your actual distance exceeds the table value, use the next larger wire size or calculate your exact voltage drop using this calculator.

Note: Values assume copper conductors at 75°C and power factor of 1.0. For aluminum, multiply distances by approximately 0.6.