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Voltage Drop Calculator

Calculate voltage drop and verify NEC compliance for wire runs

Voltage Drop

7.92 V

Drop %

6.6%

At Load

112.1 V

NEC Status

Violation

Voltage Drop

7.92 V

Drop %

6.6%

At Load

112.1 V

Power Loss

158.4 W

Exceeds 5% Total Limit

Voltage drop of 6.6% exceeds the NEC 5% total recommendation. Use a larger wire gauge or shorter run.

Voltage Analysis

Source Voltage120.0 V
Voltage at Load112.1 V
Voltage Drop7.9 V

Wire Details

Wire Gauge12 AWG
Materialcopper
Resistance1.980 Ω/1000 ft
One-Way Length100 ft
Total Wire Length200 ft (round-trip)

Example Calculations

1100 ft run, 20A at 120V, 12 AWG copper

Inputs

System Voltage120V
PhaseSingle-Phase
Wire Length100 ft
Load Current20 A
Wire Gauge12 AWG
MaterialCopper

Result

Voltage Drop7.92 V
Voltage Drop %6.60%
Voltage at Load112.1 V
Power Loss158.4 W
NEC StatusViolation (>5%)

Vdrop = 2 x 100 x 20 x 1.98 / 1000 = 7.92V. R for 12 AWG copper = 1.98 ohms/1000ft. Vdrop% = 7.92/120 = 6.6%. Exceeds the NEC 5% total limit. Consider upsizing to 10 AWG.

250 ft run, 20A at 120V, 12 AWG copper

Inputs

System Voltage120V
PhaseSingle-Phase
Wire Length50 ft
Load Current20 A
Wire Gauge12 AWG
MaterialCopper

Result

Voltage Drop3.96 V
Voltage Drop %3.30%
Voltage at Load116.0 V
Power Loss79.2 W
NEC StatusWarning (>3%)

Vdrop = 2 x 50 x 20 x 1.98 / 1000 = 3.96V. Vdrop% = 3.96/120 = 3.30%. Just above the 3% branch circuit recommendation.

Frequently Asked Questions

Q

What is acceptable voltage drop per NEC?

NEC recommends no more than 3% voltage drop for branch circuits and 5% total for feeder plus branch circuits combined. For a 120V circuit, 3% = 3.6V maximum drop and 5% = 6V maximum.

  • NEC 210.19(A) Informational Note: 3% max for branch circuits
  • NEC 215.2(A) Informational Note: 5% max total (feeder + branch)
  • These are recommendations, not code requirements – but inspectors enforce them
  • Sensitive electronics (computers, medical) may need ≤2% drop
  • Motors running below rated voltage draw more current and overheat
System Voltage3% Max Drop5% Max DropVoltage at Load (5%)
120V3.6V6.0V114.0V
208V6.2V10.4V197.6V
240V7.2V12.0V228.0V
277V8.3V13.9V263.2V
480V14.4V24.0V456.0V
Q

How do I calculate voltage drop?

For single-phase: Vdrop = 2 x Length x Current x Resistance / 1000. For three-phase: Vdrop = 1.732 x Length x Current x Resistance / 1000. Length is one-way distance in feet, resistance is per 1000 ft.

  • The "2" in single-phase accounts for hot and neutral conductors
  • The "1.732" in three-phase is the square root of 3 (√3)
  • Length is ONE-WAY distance – formula already doubles it
  • Resistance values come from NEC Chapter 9, Table 8
  • For AC circuits, use AC resistance (slightly higher than DC due to skin effect)
Q

Does aluminum wire have more voltage drop than copper?

Yes. Aluminum has about 1.6 times the resistance of copper for the same gauge. To get equivalent voltage drop, you typically need to go up 2 wire sizes with aluminum (e.g., use 10 AWG aluminum instead of 12 AWG copper).

  • Aluminum resistivity: ~1.6× higher than copper at same gauge
  • Rule of thumb: Go up 2 AWG sizes for aluminum equivalent
  • Aluminum is lighter and cheaper – common for feeders and service entrance
  • Aluminum requires anti-oxidant compound at all connections
  • Use AL/CU rated breakers and connectors with aluminum wire
Wire GaugeCopper Resistance (Ω/1000ft)Aluminum Resistance (Ω/1000ft)Aluminum Equivalent
14 AWG3.145.1712 AWG Al
12 AWG1.983.2510 AWG Al
10 AWG1.242.048 AWG Al
8 AWG0.7781.286 AWG Al
6 AWG0.4910.8084 AWG Al
Q

How do I reduce voltage drop in long wire runs?

Use a larger wire gauge (lower AWG number), reduce circuit length, increase voltage (240V vs 120V), or reduce the load current. Doubling wire gauge size roughly halves the voltage drop.

  • Upsize wire: Going from 12 AWG to 10 AWG cuts drop by ~37%
  • Increase voltage: 240V circuits have half the drop of 120V at same wattage
  • Relocate panel: Shorter runs = less drop (move sub-panel closer)
  • Reduce load: Split high-draw devices onto separate circuits
  • For runs over 100 ft, always check voltage drop before installing

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Last Updated: Mar 9, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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