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Voltage-Drop Calculation for Long Branch Circuits

Reference Compliance Updated Jul 24, 2026 ยท Electrical

Why this matters

A branch circuit sized strictly to the ampacity tables can deliver under-voltage at the load if the run is long. The lights flicker, the motor draws excessive current trying to hold torque at low voltage, the electronics drop out, and on a really long run the breaker nuisance-trips because the load is pulling more amps to compensate for the voltage drop. The NEC voltage-drop guidance is in an informational note (not a mandatory rule for most branch circuits), but ignoring it gets a system that operates poorly even though it passes inspection. This article walks the calculation, the NEC reference, and the practical sizing rule that experienced shops apply on long runs.

The NEC reference

NEC 210.19 Informational Note 4 recommends that conductors for branch circuits be sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, with the maximum total voltage drop on both feeders and branch circuits not to exceed 5 percent.

NEC 215.2 Informational Note 2 carries the parallel guidance for feeders: 3 percent on feeders, 5 percent combined.

The notes are recommendations, not requirements, for general-use circuits. They become mandatory for specific applications under other articles:

  • NEC 647.4(D): sensitive electronic equipment, 1.5 percent
  • NEC 695.7: fire pump motors, voltage limits at the motor terminals
  • NEC 625.41: EV charging equipment, manufacturer requirements often tighter than the 3 percent note
  • NEC 690.45 (older edition) and 691: PV system DC circuits, specific limits

For the typical residential or light-commercial branch circuit, the 3 percent rule is a strong recommendation that the inspector may or may not enforce but the load will absolutely enforce.

The basic single-phase calculation

For a single-phase branch circuit:

Voltage drop = 2 x K x I x L / CM

Where:

  • K = resistivity constant for the conductor metal (12.9 ohms-cmil per foot for copper at 75 C, 21.2 for aluminum at 75 C)
  • I = current in amps
  • L = one-way length of the run in feet
  • CM = circular-mil area of the conductor

The "2" accounts for the round-trip (out and back) on a 2-wire single-phase circuit.

To solve for the conductor size, rearrange:

CM required = 2 x K x I x L / (allowable voltage drop)

Allowable voltage drop in volts = source voltage x 0.03 for the 3 percent target.

Worked example

A 20A, 120V branch circuit feeding a 16A continuous load 150 feet from the panel:

  • I = 16A (the load, not the breaker)
  • L = 150 ft
  • K = 12.9 (copper)
  • Allowable VD = 120 x 0.03 = 3.6 V

CM required = 2 x 12.9 x 16 x 150 / 3.6 = 17,200 circular mils

Looking at the conductor table:

  • 12 AWG copper = 6,530 cmil (well below required)
  • 10 AWG copper = 10,380 cmil (below required)
  • 8 AWG copper = 16,510 cmil (just below required, 96 percent of target)
  • 6 AWG copper = 26,240 cmil (well above required)

A code-minimum 12 AWG for a 20A breaker would have 7.2 V drop at this load, or 6 percent. Sizing up to 8 AWG brings the drop into spec at 3.1 percent. Sizing to 6 AWG drops it to 2.0 percent.

The breaker stays at 20A; only the conductor goes up. This is what experienced shops do without thinking on any branch circuit over about 100 feet.

Three-phase variant

For a three-phase circuit (line-to-line or line-to-neutral):

Line-to-line: VD = sqrt(3) x K x I x L / CM Line-to-neutral on a 4-wire wye: same as single-phase formula, treating phase-to-neutral voltage

The square-root-of-3 substitution for three-phase line-to-line accounts for the geometry of the three-phase current return path through the system, not through a dedicated neutral.

When to use the formula vs the chart

Voltage drop tables in code books and design references are fine for quick checks. They assume a fixed power factor (usually 1.0 or 0.85) and a fixed conductor temperature. For unusual cases (high inductive loads with PF below 0.85, conductors run hot near ampacity, very long runs where reactance starts to matter on AC), the full formula with the actual K value and actual load is more accurate.

For inductive loads, the simplified formula above understates the drop. The full formula accounts for both resistance and reactance:

VD = I x (R x cos(theta) + X x sin(theta)) x L

Where R and X are the per-foot resistance and reactance of the conductor and theta is the power-factor angle. This matters on long motor branch circuits.

EV charger circuits as a specific case

Level 2 EV chargers typically draw 32 to 48 A continuous for hours during a charge cycle. NEC 625.42 requires the branch circuit to be sized for 125 percent of the continuous load per 210.19(A) and 215.2(A). Long runs to a detached garage or to an outdoor pedestal often need upsizing for voltage drop on top of the 125 percent continuous-load adder.

A 48A EV charger on a 60A breaker 200 feet from the panel:

  • Code-minimum conductor size for ampacity: 6 AWG copper at 75 C ampacity of 65A (verify against the latest table)
  • Voltage drop at 48A through 6 AWG copper: 2 x 12.9 x 48 x 200 / 26,240 = 9.4 V on a 240V circuit, or 3.9 percent. Already over the 3 percent recommendation.
  • Upsizing to 4 AWG copper (41,740 cmil): VD = 2 x 12.9 x 48 x 200 / 41,740 = 5.9 V or 2.5 percent. Within spec.

The breaker remains at 60A; the conductor goes from 6 AWG to 4 AWG.

Rule of thumb for long runs

Many shops apply a sizing bump:

  • Run length over 100 feet: bump conductor one size from code minimum
  • Run length over 200 feet: bump conductor two sizes from code minimum

This rule of thumb covers most installations without a calculator. The calculation should still be done for any sensitive load (EV chargers, motors, electronic equipment), but the bump rule keeps the bid accurate at the estimation stage.

Documentation

For any branch circuit where voltage drop drove a conductor size larger than the code minimum, the design notes should record:

  • Load amperage used in the calculation
  • One-way length
  • Source voltage
  • Calculated voltage drop percentage
  • Final conductor size and the NEC informational note citation

This protects the installer when an inspector asks why the conductor is upsized.

References

  • NEC 2023, Article 210.19 Informational Note 4, Voltage Drop Recommendations
  • NEC 2023, Article 215.2 Informational Note 2
  • NEC 2023, Article 625, Electric Vehicle Power Transfer System
  • NEC 2023, Article 647.4, Voltage Drop for Sensitive Electronic Equipment
  • NEC 2023, Article 690 and 691, Solar Photovoltaic Systems
  • IEEE Std 141 (Red Book), Recommended Practice for Electric Power Distribution for Industrial Plants
  • ASHRAE/IES Standard 90.1 Energy Standard for Sites and Buildings (efficiency requirements that can drive tighter VD targets)

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