Voltage Drop — Formulas, NEC Rules, and Worked Examples

Voltage drop is one of the few NEC topics where the code book gives you a recommendation, not a hard rule — and licensing exams test whether you know the formulas, the constants, and what to do when the math forces you to upsize.

What the NEC actually says

Voltage drop is addressed in two informational notes, not in mandatory text:

• 210.19(A)(1) Informational Note No. 4 — branch-circuit conductors should be sized so the voltage drop at the farthest outlet does not exceed 3%, and the combined drop on feeder + branch circuit does not exceed 5%.
• 215.2(A)(1) Informational Note No. 2 — the same 3% / 5% guideline applies to feeders.

"Informational Note" means it's a recommendation, not a code requirement. But most exams treat the 3% / 5% values as the answer, and many state amendments and AHJs adopt them as enforceable. For sensitive equipment (motors, electronic loads), the manufacturer's tolerance is usually tighter than 5%.

One section does impose a hard limit: 647.4(D), sensitive electronic equipment on technical-power systems, requires drop ≤ 1.5% on branch circuits and ≤ 2.5% combined. And NEC 647.4(D) applies only to those technical systems.

The formulas

Two formulas cover almost everything you'll see on the journeyman exam:

Where:

• VD = voltage drop, in volts
• K = resistivity constant of the conductor: 12.9 for copper, 21.2 for aluminum (units: ohm·circular-mil per foot at 75 °C)
• I = current in amperes
• D = one-way distance from source to load, in feet (the formula's "2 ×" handles round-trip on single-phase; on three-phase the 1.732 = √3 handles the line-to-line geometry)
• CM = circular-mil area of the conductor, from Chapter 9 Table 8

Circular mils — the lookup you actually need

From Chapter 9, Table 8 (DC resistance, but the CM column is what we use):

Worked example 1 — 12 AWG Cu, 20 A, 100 ft (single-phase 120 V)

Run a 20 A general-purpose receptacle circuit 100 feet to a detached garage on 12 AWG Cu THWN at 120 V single-phase. What's the voltage drop?

7.9 V on 120 V = 6.6%. That blows the 3% branch-circuit recommendation (3.6 V) and even the 5% combined limit. We need to upsize.

Back-solve for the CM we need at 3% drop (3.6 V):

10 AWG (10,380 CM) is too small. 8 AWG (16,510 CM) clears it. Use 8 AWG Cu if you want to stay under 3% drop on this run.

Worked example 2 — 4/0 Al, 100 A, 250 ft (three-phase 480 V feeder)

A feeder runs 250 ft from a service to a sub-panel at 480 V three-phase, carrying 100 A continuous. Conductor: 4/0 aluminum.

4.34 V on 480 V = 0.90%. Comfortably under the 3% feeder recommendation. 4/0 Al is fine on this feeder run.

If the same 100 A load were on a 100 ft branch circuit downstream — say, 4 AWG Al for the branch (41,740 CM):

Combined 0.90% + 1.83% = 2.73% — under the 5% combined limit. Both runs comply with the NEC recommendation.

Worked example 3 — back-solving for wire size

You need to feed a 50 A motor at 240 V single-phase, 200 ft from the panel, on aluminum. What's the smallest conductor that keeps you under 3% drop (7.2 V)?

Look up CM ≥ 58,888 in the table. 2 AWG Al is too small (66,360 CM is for copper — but in Table 8 for aluminum the same AWG sizes have the same circular-mil area; the K constant captures the resistance difference). 2 AWG (66,360 CM) clears it. Use 2 AWG aluminum for the 50 A motor feeder.

Common pitfalls

• Forgetting to use one-way distance — D in the formula is the one-way distance. The single-phase formula has a 2 in front to make it round-trip; you don't double the distance again.
• Using nominal voltage in the wrong place — VD is the drop in volts; the percentage is VD divided by the system voltage (line-to-neutral for single-phase 120 V circuits, line-to-line for three-phase 480 V feeders).
• Confusing K for copper vs aluminum — 12.9 vs 21.2. Aluminum is roughly 64% less conductive, so K is roughly 64% higher.
• Ignoring the small-conductor rule — your voltage-drop math says 12 AWG would carry the load, but 240.4(D)(5) still caps the OCPD at 20 A regardless of any drop calculation.
• Worrying about voltage drop on motors — motor inrush during starting drops voltage way more than the steady-state formula predicts. NEMA recommends ≤10% drop at starting and ≤5% at full load. For long motor runs, oversizing one or two trade sizes is normal.

Quick-reference cheat sheet