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NEC Article 210.19(A) Informational Note 4Worked ExampleElectrical

Voltage Drop Calculation for 100ft 12 AWG at 20A 120V

Calculate single-phase voltage drop and percentage loss over 100 feet using 12 AWG copper under NEC recommended 3% limits.

Direct Answer
7.90 V Drop (6.58% Loss - Upsize conductor to 10 AWG to stay below 3%)

Calculated strictly in accordance with NEC Article 210.19(A) Informational Note 4.

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Given Scenario Parameters

distance100
wire Gauge12 AWG
current20
voltage120
phase1

Interactive Calculator (Pre-filled)

Adjust inputs to evaluate variations of this NEC code scenario.

Circuit Parameters & Electrical Load

Calculate wire gauge, voltage drop, and NEC 310.16 ampacity for single-phase, 3-phase, and DC circuits.

Circuit System:
AC Presets:
Standard Breaker Sizes:
Verified NEC Code Proof Available (NEC Article 210.19(A) Note 4)Configuration matches: 100ft 12 AWG at 20A 120V Circuit Proof
Inspect Code Proof

Recommended Conductor & Voltage Drop Sizing

120V • 20A Load • 100 ft run
Recommended Minimum Conductor Size (COPPER):
8 AWGCOPPER (16,510 CM)

Delivers 116.87V at load terminal with 2.61% voltage drop (-3.13V drop).

Actual Voltage Drop3.13V (2.61%)
Voltage at Load116.87V delivered
NEC Base Ampacity50A (50A derated)
Max 3% Distance115.2 ft limit
Copper vs. Aluminum Conductor ComparisonAt 100 ft run
Copper (Cu) Recommendation8 AWG
2.61% DropHigher conductivity
Aluminum (Al) Recommendation6 AWG
2.69% DropLower material weight/cost
Educational and reference calculator only. Conductor sizing must comply with the National Electrical Code (NEC Table 310.16), local AHJ amendments, equipment terminal temperature ratings (60°C/75°C), and short-circuit withstand ratings. Always consult a licensed electrical contractor.
Target ≤ 3% • K = 12.9 Ω·cmil/ft

Circuit Voltage Drop & Conductor Schematic

PASS (< 3%)Single-Phase AC
SOURCE120VMain Panel20ALOAD116.87VEquipment / SubDelivered100 ft One-Way Run (200 ft Loop)8 AWG COPPERΔV = -3.13V (2.61% drop)
Conductor Size8 AWG (copper)
Voltage at Load116.87 Volts (-3.13V)
Voltage Drop %2.61% (Target ≤ 3%)
Max Distance (3%)115.2 ft limit

Conductor Candidate Sizing & Voltage Drop Comparison Table

COPPER Conductors
Conductor SizeCircular MilsBase / Derated AmpacityVoltage DropVoltage at LoadMax 3% DistanceStatus
14 AWG4,110 CM20A / 20A12.55V (10.46%)107.45V28.7 ftFAIL
12 AWG6,530 CM25A / 25A7.9V (6.58%)112.1V45.6 ftFAIL
10 AWG10,380 CM35A / 35A4.97V (4.14%)115.03V72.4 ftWARNING
8 AWGRecommended16,510 CM50A / 50A3.13V (2.61%)116.87V115.2 ftRECOMMENDED
6 AWG26,240 CM65A / 65A1.97V (1.64%)118.03V183.1 ftPASS
4 AWG41,740 CM85A / 85A1.24V (1.03%)118.76V291.2 ftPASS
3 AWG52,620 CM100A / 100A0.98V (0.82%)119.02V367.1 ftPASS
2 AWG66,360 CM115A / 115A0.78V (0.65%)119.22V463 ftPASS
1 AWG83,690 CM130A / 130A0.62V (0.52%)119.38V583.9 ftPASS
1/0 AWG105,600 CM150A / 150A0.49V (0.41%)119.51V736.7 ftPASS
2/0 AWG133,100 CM175A / 175A0.39V (0.33%)119.61V928.6 ftPASS
3/0 AWG167,800 CM200A / 200A0.31V (0.26%)119.69V1170.7 ftPASS
4/0 AWG211,600 CM230A / 230A0.24V (0.2%)119.76V1476.3 ftPASS
250 kcmil250,000 CM255A / 255A0.21V (0.18%)119.79V1744.2 ftPASS
300 kcmil300,000 CM285A / 285A0.17V (0.14%)119.83V2093 ftPASS
350 kcmil350,000 CM310A / 310A0.15V (0.13%)119.85V2441.9 ftPASS
400 kcmil400,000 CM335A / 335A0.13V (0.11%)119.87V2790.7 ftPASS
500 kcmil500,000 CM380A / 380A0.1V (0.08%)119.9V3488.4 ftPASS
600 kcmil600,000 CM420A / 420A0.09V (0.08%)119.91V4186 ftPASS
750 kcmil750,000 CM475A / 475A0.07V (0.06%)119.93V5232.6 ftPASS
1000 kcmil1,000,000 CM545A / 545A0.05V (0.04%)119.95V6976.7 ftPASS
NEC Electrical Code & Safety Disclaimer: This calculation is for design and reference estimation only. Final conductor sizing must comply with the National Electrical Code (NEC Article 310, Table 310.16), local electrical amendments, terminal temperature ratings (typically 75°C), and equipment manufacturer specifications. High-voltage and feeder installations should be verified by a licensed electrical contractor.

Calculation Methodology & Mathematical Steps

1.Calculate Voltage Drop for Recommended 8 AWG (COPPER): (2.0 × 12.9 × 20A × 100 ft) ÷ 16510 CM
= 3.13V (2.61% drop, 116.87V at load)
2.Determine Maximum Distance for 3% Voltage Drop: (0.03 × 120V × 16510) ÷ (2.0 × 12.9 × 20A)
= 115.2 ft maximum one-way length

Step-by-Step Solution

Formal engineering derivation citing applicable National Electrical Code articles.

Governing Code Equation
VD = (2 * K * L * I) / CM
Citing NEC Article 210.19(A) Informational Note 4
1

Identify circular mil area for 12 AWG copper (CM = 6,530) and resistivity constant (K = 12.9 ohms-cmil/ft).

2

Compute voltage drop: (2 * 12.9 * 100 * 20) / 6,530 = 7.90 Volts.

3

Determine percentage drop: (7.90 / 120) * 100 = 6.58% (Exceeds the 3% NEC branch circuit recommendation).

4

Compare with NEC recommendation: 6.58% exceeds the 3% limit. Upsize conductor to 10 AWG to maintain voltage stability.

Conclusion: 7.90 V Drop (6.58% Loss - Upsize conductor to 10 AWG to stay below 3%)
NEC Article 210.19(A) Informational Note 4