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Voltage Drop Calculator

Calculate voltage drop in electrical circuits. Check NEC 3% and 5% compliance with wire size recommendations and power loss estimates.

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Circuit Information

ft
amps
❌Fails NEC Requirements
5.29%
Voltage Drop
6.35V
Drop
113.6V
End Voltage
127.0W
Power Loss
💡Recommendation

Use 8 AWG copper wire to achieve less than 3% voltage drop.

Detailed Results

0.3176 Ω
Wire Resistance
5.29%
Power Loss
$60
Est. Wire Cost

NEC Voltage Drop Guidelines

RuleMax DropApplicationStatus
NEC 210.19(A)(1)3%Branch circuits✗ Fail
NEC 215.2(A)(3)5%Feeder + branch combined✗ Fail

Wire Size Reference (Copper)

AWGAmpacityΩ/1000ft$/ft
14 AWG15A2.525$0.20
12 AWG20A1.588$0.30
10 AWG30A0.999$0.50
8 AWG40A0.628$0.85
6 AWG55A0.395$1.30
4 AWG70A0.249$2.10
2 AWG95A0.156$3.30
1/0 AWG125A0.098$5.30
Voltage Drop Tips
  • Keep branch circuit voltage drop under 3% per NEC recommendation
  • Total drop (feeder + branch) should not exceed 5%
  • Motors are sensitive to voltage drop - size wire carefully
  • Aluminum wire requires one size larger than copper for same ampacity
  • Consider future load growth when sizing wire

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About This Calculator

Excessive voltage drop wastes energy, damages equipment, and can prevent motors from starting. Our comprehensive Voltage Drop Calculator determines if your wire size is adequate for a given circuit length, load, and voltage—helping electricians, contractors, and DIYers ensure both code compliance and optimal electrical system performance.

The National Electrical Code (NEC) recommends limiting voltage drop to 3% on branch circuits and 5% total from service entrance to final outlet. While these are recommendations rather than requirements, most inspectors and Authorities Having Jurisdiction (AHJ) enforce them. For a 120V circuit, 3% means no more than 3.6V drop; for 240V, no more than 7.2V.

In 2026, copper wire prices have increased significantly due to commodity markets, with 12 AWG Romex costing $0.45-0.90 per foot and larger conductors rising proportionally. Understanding voltage drop calculations helps you size wire correctly the first time—avoiding the costly mistake of undersized wire that fails inspection or the unnecessary expense of oversized conductors.

Trusted Sources

NFPA 70 (NEC)(National Electrical Code - the primary reference for electrical installations)IAEI(International Association of Electrical Inspectors)Copper Development Association(Technical resources for copper wire and cable)Aluminum Association(Aluminum building wire resources and installation guides)

How to Use the Voltage Drop Calculator

  1. 1Enter the one-way wire length from panel to load (not round-trip distance).
  2. 2Enter the current (amperage) of the load or circuit breaker rating.
  3. 3Select the wire gauge (AWG) you plan to use or are evaluating.
  4. 4Choose the system voltage (120V, 240V single-phase, or 208V/480V three-phase).
  5. 5Toggle Advanced Mode for copper vs. aluminum selection and temperature derating.
  6. 6Review voltage drop in volts and percentage.
  7. 7Check NEC compliance status (green for ≤3%, yellow for 3-5%, red for >5%).
  8. 8If drop exceeds recommendations, increase wire size and recalculate.
  9. 9Use the wire size recommendation feature for automatic optimal sizing.

Formula

VD = (2 × K × I × L) ÷ CM

Voltage drop (VD) equals 2 times the resistivity constant (K = 12.9 for copper, 21.2 for aluminum), times current (I in amps), times one-way length (L in feet), divided by circular mils (CM) of the wire. The factor of 2 accounts for round-trip current flow (hot and neutral). For three-phase, replace 2 with 1.732 (√3). Voltage drop percentage = (VD ÷ Source Voltage) × 100.

NEC Voltage Drop Requirements and Recommendations

The National Electrical Code provides voltage drop guidance in informational notes:

NEC Voltage Drop Recommendations:

ApplicationMax RecommendedNEC ReferenceNotes
Branch circuit3%210.19(A)(1) FPNFrom panel to outlet
Feeder3%215.2(A)(3) FPNFrom service to sub-panel
Total circuit5%215.2(A)(3) FPNService to final outlet
Sensitive equipment2%OptionalComputers, medical equipment

Important Distinction: These are Fine Print Notes (FPN), meaning recommendations for efficiency, not mandatory requirements. However:

  • Most AHJs (Authorities Having Jurisdiction) enforce them
  • Many specifications require compliance
  • Exceeding 5% may cause equipment problems
  • Insurance and liability considerations apply

What Voltage Drop Percentages Mean:

Voltage DropStatusEffect
≤3%ExcellentMeets NEC recommendation for branch circuits
3-5%AcceptableMeets NEC total recommendation but at limits
5-8%MarginalMay cause problems, some equipment affected
8-10%PoorMotors may not start, lights noticeably dim
>10%UnacceptableEquipment damage likely, fire hazard

Actual Voltage at Load (120V system):

Drop %Voltage at LoadStatus
3%116.4VRecommended
5%114VMaximum
10%108VProblematic

Wire Gauge Reference and Ampacity

American Wire Gauge (AWG) uses a counterintuitive system: smaller numbers = thicker wire = lower resistance = less voltage drop.

Copper Wire Properties (75°C Rating, THWN/THHN):

AWGDiameter (mils)Circular MilsΩ/1000ft @ 75°CAmpacity
1464.14,1103.1415A
1280.86,5301.9820A
10101.910,3801.2430A
8128.516,5100.77840A
6162.026,2400.49155A
4204.341,7400.30870A
3229.452,6200.24585A
2257.666,3600.19495A
1289.383,6900.154110A
1/0324.9105,6000.122125A
2/0364.8133,1000.0967145A
3/0409.6167,8000.0766165A
4/0460.0211,6000.0608195A

Aluminum Wire (61% Conductivity of Copper):

AWGΩ/1000ftAmpacityCopper Equivalent
123.25-Not recommended
102.04-Not recommended
81.2830A10 AWG copper
60.80840A8 AWG copper
40.50855A6 AWG copper
20.31975A4 AWG copper
1/00.201100A2 AWG copper
2/00.159115A1 AWG copper
4/00.100150A1/0 AWG copper

Key Rule: For aluminum, use wire two sizes larger than copper for equivalent performance.

2026 Wire Pricing and Cost Considerations

Copper prices have reached five-year highs, significantly impacting electrical wire costs:

2026 Romex (NM-B) Pricing:

SizePrice per Foot250ft RollTypical Use
14/2 NM-B$0.35-0.55$90-14015A lighting circuits
12/2 NM-B$0.45-0.70$115-17520A general circuits
10/2 NM-B$0.75-1.10$190-27530A dryer, A/C
10/3 NM-B$1.10-1.50$275-37530A dryer with neutral
8/3 NM-B$2.00-2.75$500-69040A range
6/3 NM-B$3.25-4.25$815-1,06050A range, sub-panel

THHN/THWN (Conduit Wire) Pricing:

SizeCopper/ftAluminum/ftTypical Use
12 AWG$0.25-0.40-Branch circuits
10 AWG$0.40-0.60-30A circuits
8 AWG$0.65-0.95$0.35-0.50Sub-feeders
6 AWG$1.00-1.50$0.50-0.7550-60A feeders
4 AWG$1.60-2.30$0.80-1.1570A feeders
2 AWG$2.50-3.50$1.25-1.75100A feeders
1/0 AWG$4.00-5.50$2.00-2.75125A service
4/0 AWG$8.00-11.00$4.00-5.50200A service

Cost of Upsizing for Voltage Drop:

UpgradeAdditional Cost/100ftWhen Worth It
12 → 10 AWG$30-50Runs >55 ft
10 → 8 AWG$50-85Runs >88 ft
8 → 6 AWG$60-100Runs >140 ft
6 → 4 AWG$90-130Runs >220 ft

Key Insight: The cost of upsizing wire is often minimal compared to reinstallation costs if undersized wire fails inspection.

Maximum Wire Run Distances

These tables show maximum one-way distances for 3% voltage drop at full rated load:

120V Single-Phase Circuits:

Load14 AWG12 AWG10 AWG8 AWG6 AWG
10A45 ft70 ft115 ft180 ft290 ft
15A30 ft50 ft75 ft120 ft190 ft
20A-35 ft55 ft90 ft145 ft
30A--40 ft60 ft95 ft
40A---45 ft70 ft

240V Single-Phase Circuits:

Load14 AWG12 AWG10 AWG8 AWG6 AWG4 AWG
15A60 ft95 ft155 ft245 ft390 ft620 ft
20A45 ft70 ft115 ft185 ft290 ft465 ft
30A-50 ft75 ft120 ft195 ft310 ft
40A--55 ft90 ft145 ft230 ft
50A---70 ft115 ft185 ft

208V/240V Three-Phase Circuits:

Load10 AWG8 AWG6 AWG4 AWG2 AWG
30A90 ft145 ft230 ft370 ft580 ft
50A55 ft85 ft140 ft220 ft350 ft
75A-60 ft95 ft150 ft235 ft
100A--70 ft110 ft175 ft

Note: These distances assume copper wire at 75°C. Reduce by 20-40% for aluminum.

Common Long-Run Applications

Certain installations commonly require voltage drop calculations:

Outbuildings (Garage, Shop, Barn):

Distance100A ServiceRecommended Wire
50 ft3% drop2 AWG copper or 1/0 aluminum
100 ft3% drop1/0 AWG copper or 3/0 aluminum
150 ft3% drop2/0 AWG copper or 4/0 aluminum
200 ft3% drop3/0 AWG copper or 250 MCM aluminum

Well Pumps and Water Systems:

HPRunning Amps (240V)Max Distance @ 10 AWG@ 8 AWG
1/2 HP5A230 ft365 ft
3/4 HP7A165 ft260 ft
1 HP9A125 ft200 ft
1.5 HP12A95 ft150 ft
2 HP15A75 ft120 ft

EV Charger Installations (240V):

Level 2 RatingCurrentMax Distance @ 6 AWG@ 4 AWG
7.2 kW30A95 ft155 ft
9.6 kW40A70 ft115 ft
12 kW50A55 ft90 ft

Pool/Spa Equipment:

EquipmentTypical LoadConsiderations
Pool pump10-20A @ 240VOften 100+ ft from panel
Spa heater30-50A @ 240VHigh continuous load
Pool heater50-100A @ 240VSize for nameplate + 25%

Solutions for Long Runs:

  1. Upsize wire - Most common, adds material cost
  2. Install sub-panel - Reduces long circuit runs
  3. Run higher voltage - 240V drops half as much as 120V
  4. Parallel conductors - For very large loads

Effects of Voltage Drop on Equipment

Voltage drop affects different equipment types in specific ways:

Motor Loads (Most Sensitive):

Voltage DropEffect on Motors
5%Starting torque reduced 10%, may struggle under load
10%Starting torque reduced 19%, may not start
15%Starting torque reduced 28%, definite no-start

Motor current increases as voltage drops, potentially:

  • Tripping overload protection
  • Overheating windings
  • Reducing motor lifespan by 50%+
  • Causing nuisance tripping of breakers

Lighting Loads:

Type5% Drop10% Drop
Incandescent16% dimmer, 50% shorter life30% dimmer, 75% shorter life
LEDMinor dimmingPossible flickering, driver stress
FluorescentMay not start reliablyFrequent starting failures
HIDColor shift, may not restrikeStarting failures

Heating Elements (Resistive Loads): Power output follows voltage squared:

Voltage DropPower Reduction
3%6% reduction
5%10% reduction
10%19% reduction
15%28% reduction

A 5,000W heater at 10% voltage drop delivers only 4,050W.

Electronic Equipment:

EquipmentMinimum VoltageEffect of Low Voltage
Computers104-108VRandom crashes, data loss
Refrigerators105-110VCompressor damage
Sensitive electronics108-114VMalfunction, damage
Battery chargersVariesIncomplete charging

Copper vs. Aluminum Wire

Choosing between copper and aluminum involves cost, performance, and application considerations:

Material Properties:

PropertyCopperAluminum
Conductivity100% (reference)61%
WeightHeavier70% lighter
Ω/1000ft (12 AWG)1.983.25
Expansion rate0.0000094/°F0.0000128/°F
Cost (relative)100%40-50%

When to Use Aluminum:

  • Service entrance conductors (4/0, 3/0, 2/0)
  • Sub-panel feeders (4 AWG and larger)
  • Long runs where weight matters
  • Budget-constrained large feeders
  • Underground service laterals

When to Use Copper:

  • Branch circuits (always)
  • Connections to standard outlets/switches
  • Where space is limited
  • Exposed or flexible applications
  • Motor connections (less expansion)

Aluminum Installation Requirements:

  1. Use AL/CU rated devices and breakers
  2. Apply anti-oxidant compound
  3. Torque connections to specification
  4. Allow for thermal expansion
  5. Use compression connectors, not set screws
  6. Size two gauges larger than copper equivalent

2026 Cost Comparison (200A Service, 100ft):

MaterialWire SizeCostInstallation
Copper3/0 AWG$1,100-1,500Standard
Aluminum4/0 AWG$450-650Requires AL-rated equipment
Savings-$600-900Worth it for long feeders

Three-Phase Voltage Drop

Three-phase systems have different voltage drop characteristics:

Three-Phase Formula:

VD = (√3 × K × I × L) ÷ CM
   = (1.732 × K × I × L) ÷ CM

Why √3? In three-phase systems, current flows through only one wire at a time (not round-trip like single-phase), but the phase relationship creates a 1.732 multiplier.

Common Three-Phase Voltages:

VoltageApplicationPhase-to-Neutral
208VCommercial, small industrial120V
240VIndustrial deltaN/A
277VCommercial lightingN/A
480VIndustrial motors277V
600VHeavy industrial347V

Three-Phase Advantages:

  • More efficient power transmission
  • Smaller wire for same power
  • Less voltage drop per kW delivered
  • Motors run smoother

208V vs. 240V Three-Phase:

Factor208V Wye240V Delta
SourceUtility transformerDelta transformer
NeutralAvailable (120V)Not typically available
Motor compatibility208V rated230V rated
Voltage dropSlightly higherSlightly lower

Example Calculation (480V, 100A, 200ft, 1/0 copper):

VD = (1.732 × 12.9 × 100 × 200) ÷ 105,600
VD = 446,760 ÷ 105,600
VD = 4.23V (0.88%)

Three-phase at 480V experiences much less percentage drop than single-phase at 120V.

Temperature and Derating Factors

Wire resistance increases with temperature, affecting voltage drop:

Temperature Correction Factors:

Ambient Temp60°C Wire75°C Wire90°C Wire
70°F (21°C)1.081.041.00
86°F (30°C)1.001.001.00
104°F (40°C)0.820.880.91
122°F (50°C)0.580.750.82
140°F (60°C)-0.580.71

Resistance Change with Temperature: Copper resistance increases approximately 0.4% per °C above 20°C.

Bundled Conductors (NEC 310.15(B)(3)):

ConductorsAdjustment Factor
4-60.80 (80%)
7-90.70 (70%)
10-200.50 (50%)
21-300.45 (45%)

Conduit Fill Derating: More wires = more heat = higher resistance = more voltage drop.

Ambient Temperature Locations:

LocationTypical TempConsideration
Indoor conditioned77°FStandard
Attic120-150°FSignificant derating
Roof conduit130-160°FSevere derating
Underground68-77°FMinimal adjustment
Direct sunlight+30°FAdd to ambient

Practical Impact: A circuit calculated at 2.8% voltage drop at 75°F may exceed 3% at 120°F attic temperatures.

Pro Tips

  • 💡When in doubt, go up one wire size—the extra cost is typically 15-30% and avoids failed inspections.
  • 💡For motors, always size wire for 125% of nameplate full-load amps per NEC 430.22.
  • 💡Use 240V instead of 120V for long runs—voltage drop percentage is cut in half.
  • 💡Calculate at the full expected load, not the breaker rating, for more accurate results.
  • 💡Consider future load growth—upsizing now is cheaper than rewiring later.
  • 💡For aluminum wire, always use anti-oxidant compound and AL-rated devices.
  • 💡Temperature derating in hot locations (attics, rooftops) can push a marginal circuit over limits.
  • 💡Sub-panels are often more cost-effective than oversized wire for distant loads.
  • 💡Document your voltage drop calculations for permit inspections.
  • 💡For EV chargers, size for full rated capacity even if you start with lower charging rates.
  • 💡Conduit fill derating affects both ampacity AND increases resistance due to heat.
  • 💡Three-phase 208V is NOT the same as single-phase 240V—motors must be rated correctly.

Frequently Asked Questions

No, NEC 3% and 5% limits are recommendations in Fine Print Notes, not enforceable requirements. However, most local Authorities Having Jurisdiction (AHJ) and inspectors enforce them. Additionally, exceeding 5% can cause equipment problems and liability issues. Always check with your local building department.

Nina Bao
Written byNina Bao• Content Writer
Updated January 5, 2026

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