How do you calculate voltage drop?

For single-phase: VD = (2 × K × L × I) ÷ CM. For three-phase: VD = (1.732 × K × L × I) ÷ CM. K is 12.9 for copper or 21.2 for aluminum, L is the one-way length in feet, I is the current in amps, and CM is the conductor's circular mils. Percent drop = VD ÷ source voltage × 100.

What is an acceptable voltage drop?

The NEC includes informational notes recommending no more than 3% voltage drop on a branch circuit and no more than 5% on the combined feeder and branch. These are recommendations, not enforceable requirements, though many inspectors and equipment manufacturers treat them as limits.

Is the 3% voltage drop limit required by code?

No — the 3% and 5% figures appear as informational notes in the NEC, not as mandatory rules. However, some jurisdictions adopt them, and equipment warranties may require them, so always verify with the authority having jurisdiction.

What are K and circular mils?

K is the resistivity constant for the conductor material (about 12.9 for copper, 21.2 for aluminum at 75°C). Circular mils (CM) is the conductor's cross-sectional area — for example, 12 AWG is 6,530 CM and 10 AWG is 10,380 CM. Larger conductors have more circular mils and less voltage drop.

Voltage Drop Calculator

Free voltage drop calculator for single- and three-phase circuits. Enter conductor material, wire gauge, length, and current to get voltage drop and percent drop against NEC guidance.

How the voltage drop calculation works

Voltage drop is the voltage lost to conductor resistance over the length of a run. The standard field formula uses the conductor’s circular mils and a material constant:

Single-phase: VD = (2 × K × L × I) ÷ CM

Three-phase: VD = (1.732 × K × L × I) ÷ CM

K = resistivity constant: 12.9 for copper, 21.2 for aluminum (at ~75°C)
L = one-way circuit length in feet
I = load current in amps
CM = conductor circular mils (looked up from the wire size)

Then % drop = VD ÷ source voltage × 100.

Worked example

30 A down 150 ft of 10 AWG copper (CM = 10,380), single-phase:

VD = (2 × 12.9 × 150 × 30) ÷ 10,380 = 116,100 ÷ 10,380 ≈ 11.2 V

On a 240 V circuit that’s about 4.7% — over the 3% branch recommendation, so you’d consider upsizing to 8 AWG.

Circular mils reference

AWG	Circular mils
14	4,107
12	6,530
10	10,380
8	16,510
6	26,240
4	41,740
2	66,360
1/0	105,600
2/0	133,100
4/0	211,600

A note on the NEC

The 3% (branch) and 5% (total) figures are informational notes in the NEC — recommendations, not enforceable requirements. Many inspectors and equipment manufacturers still treat them as limits, so always verify with the authority having jurisdiction (AHJ). This calculator assumes a steady-state, ~75°C conductor; consult a licensed electrician for actual installations.

Working the rest of the circuit? Use the Ohm’s & Watt’s Law Calculator.

How FieldPad helps in the field

Size the run here, then keep the numbers with the job. FieldPad logs your electrical readings and corrections against the equipment and the client, documents the fix, and turns it into a signed estimate or invoice — offline, on your iPhone.

Voltage Drop Calculator

How the voltage drop calculation works

Worked example

Circular mils reference

A note on the NEC

How FieldPad helps in the field

Stop Re-Running These Numbers by Hand

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