Voltage Drop Calculator: How to Use It (and When It Lies)

The formula

For single-phase, two-wire circuits:

Vd = (2 × K × I × L) / Cmils

For three-phase circuits, replace the 2 with √3 (1.732):

Vd = (1.732 × K × I × L) / Cmils

The inputs

• K: resistivity constant. Copper = 12.9. Aluminum = 21.2. Approximations; actual K varies slightly with temperature.
• I: current in amperes. Use the actual operating current, not the OCPD rating.
• L: length of the run in feet, one way. The 2 (or 1.732) handles the round trip.
• Cmils: circular mils of the conductor. From Chapter 9 Table 8. 14 AWG = 4,110 cmils. 12 AWG = 6,530. 10 AWG = 10,380. 8 AWG = 16,510. 6 AWG = 26,240. 4 AWG = 41,740. Memorize the common ones.

What the NEC actually says about voltage drop

The NEC has informational notes (not enforceable) suggesting:

• Branch circuits: voltage drop should not exceed 3% (NEC 210.19 informational note 4).
• Combined branch + feeder: should not exceed 5% (NEC 215.2 informational note 2).
• Sensitive electronic equipment: tighter limits, vendor-specific.

Informational notes are not code. The exam will still ask you to apply them. On the job, voltage drop becomes a code issue when it’s linked to ampacity correction or when local amendments make it enforceable.

A worked example

Single-phase 120V circuit, 12 AWG copper, 100 ft, 16 A continuous load.

• K = 12.9 (copper)
• I = 16 A
• L = 100 ft
• Cmils = 6,530 (12 AWG)
• Vd = (2 × 12.9 × 16 × 100) / 6530 = 41,280 / 6530 ≈ 6.32 V
• % drop = 6.32 / 120 = 5.3%

That’s above the 3% target. Remedies: bigger conductor (10 AWG drops it to 3.3%), shorter run, or accept the drop if the equipment tolerates it.

Three traps that catch candidates

Trap 1: Confusing OCPD rating with operating current

The voltage drop formula uses the actual current, not the breaker size. A 20A breaker on a circuit pulling 12 A draws based on the 12 A. The exam may mention both, so read carefully.

Trap 2: Forgetting the round trip

Single-phase voltage drop counts both legs (the 2 in the formula). Some candidates calculate one-way and report half the actual drop. Verify that your formula accounts for the round trip.

Trap 3: Single-phase vs three-phase coefficient

Use 2 for single-phase, 1.732 (√3) for three-phase. Don’t use 2 for three-phase or vice versa. The coefficient embeds the phase relationship in the math.

When the calculator is right but the answer is wrong

A calculator gives you a number. It doesn’t tell you whether the number is acceptable. Two scenarios where the math passes but the install fails:

• The 3% target is informational. If your job specs require 2%, the calculator’s 'OK' isn’t.
• Voltage drop on the start-up surge of a motor can be much higher than at running current. Sizing for running current alone undersizes for inrush.

Try the JourneymanIQ voltage drop calculator →