How to Calculate Voltage Drop per NEC

The NEC recommends a maximum of 3% voltage drop on branch circuits and 5% total from service entrance to the farthest outlet (NEC 210.19(A) Informational Note No. 4). While this is technically a recommendation rather than a hard requirement, most Authorities Having Jurisdiction (AHJs) enforce it as a practical standard. Excessive voltage drop causes motors to overheat, lights to dim, and sensitive electronic equipment to malfunction.

For long runs — warehouse lighting feeds, parking lot circuits, agricultural buildings, EV charger circuits in detached garages — voltage drop is often the controlling factor in wire sizing, not ampacity. An electrician who sizes wire only by ampacity may pass the Table 310.16 check but fail the voltage drop check, requiring an expensive re-pull.

The standard voltage drop formula from NEC Chapter 9 informational notes:

Single-phase: VD = (2 × K × I × L) / CM Three-phase: VD = (1.732 × K × I × L) / CM

Where:

• VD = voltage drop in volts
• K = resistivity constant: 12.9 for copper, 21.2 for aluminum (at 75°C)
• I = load current in amperes
• L = one-way circuit length in feet
• CM = circular mil area of the conductor (from NEC Chapter 9, Table 8)

The factor of 2 accounts for the round-trip current path (out and back). For three-phase circuits, the factor changes to 1.732 (√3) because the return path uses phase relationships rather than a neutral conductor.

To express voltage drop as a percentage: VD% = (VD / V_source) × 100

Scenario: A 20A, 120V branch circuit feeding a receptacle outlet 150 feet from the panel. 12 AWG THHN copper in EMT conduit.

Step 1: Look up the circular mil area of 12 AWG from NEC Chapter 9, Table 8: 6,530 CM

Step 2: Apply the formula: VD = (2 × 12.9 × 20 × 150) / 6,530 VD = 77,400 / 6,530 VD = 11.85 volts

Step 3: Calculate the percentage: VD% = (11.85 / 120) × 100 = 9.88%

Result: 9.88% exceeds the 3% branch circuit recommendation. This circuit needs a larger conductor.

Step 4: Find the minimum wire size. Working backward from the 3% limit: Required CM = (2 × 12.9 × 20 × 150) / (120 × 0.03) Required CM = 77,400 / 3.6 Required CM = 21,500

From Table 8: 6 AWG = 26,240 CM — this is the first size that meets the voltage drop limit. However, a 20A circuit on 6 AWG copper is expensive. Consider splitting the circuit or relocating the panel.

Scenario: A 200A, 480V three-phase feeder running 300 feet from the switchgear to a panel. 3/0 AWG THHN copper.

Step 1: 3/0 AWG = 167,800 CM (NEC Chapter 9, Table 8)

Step 2: Apply the three-phase formula: VD = (1.732 × 12.9 × 200 × 300) / 167,800 VD = 1,339,416 / 167,800 VD = 7.98 volts

Step 3: Calculate the percentage: VD% = (7.98 / 480) × 100 = 1.66%

Result: 1.66% is well within the 3% recommendation for feeders. This conductor size is adequate for voltage drop. Verify that the total (feeder + branch) remains under 5%.

Using the wrong K value. K = 12.9 for copper and 21.2 for aluminum at 75°C. Some references use 10.4/17.0 (at 25°C) or other temperature-dependent values. The NEC calculation method uses the 75°C values because conductors operate at elevated temperatures under load.

Forgetting the factor of 2. The formula includes a factor of 2 (single-phase) or 1.732 (three-phase) to account for the return path. Omitting this factor gives half the actual voltage drop.

Measuring total circuit length instead of one-way. L in the formula is the one-way distance from source to load, not the total wire length. The factor of 2 already accounts for the return path.

Ignoring voltage drop on long home runs. An electrician who sizes wire only by Table 310.16 ampacity may choose 12 AWG for a 20A circuit. At 200 feet, that produces 13.2% voltage drop — far exceeding 3%. Always check voltage drop on runs over 100 feet.

For most branch circuits under 100 feet, Table 310.16 ampacity is the controlling factor. The wire sized for ampacity will also meet the 3% voltage drop recommendation.

But for longer runs, voltage drop often requires a larger conductor than ampacity alone. Rules of thumb:

• 120V circuits: Voltage drop typically controls above 75–100 feet
• 240V circuits: Voltage drop typically controls above 150–200 feet
• 480V circuits: Voltage drop typically controls above 300–400 feet

Higher voltage systems tolerate longer runs because the same voltage drop in volts represents a smaller percentage of the source voltage.

Always check both ampacity (Table 310.16 with derating) and voltage drop. The required wire size is the larger of the two.