Is 3 % voltage drop an NEC requirement?

No. 210.19(A) Informational Note No. 4 and 215.2(A)(1) Informational Note No. 2 say conductors sized so the drop does not exceed 3 % on the branch circuit and 5 % overall “provide reasonable efficiency of operation”. Under 90.5(C) informational notes are not enforceable. Limits become mandatory only where a section states them: 647.4(D) for sensitive electronic equipment (1.5 % branch, 2.5 % total) and 695.7 for fire pumps, plus whatever a local amendment or energy code adds.

Is the length one-way or round trip?

One-way. The formula multiplies by 2 (single-phase, DC) for the return conductor, or by √3 for three-phase. If you enter the total conductor length the answer is doubled. The often-quoted “3 % on 12 AWG at 20 A over 100 ft” is 100 ft of wire, 50 ft each way.

Which method should I trust?

For sizes up to about 1/0 and power factors near 1 the two agree within a few percent and the K-factor is fine. For large conductors, low power factor or steel conduit, reactance matters and Table 9 is the better estimate; that is why the NEC prints it. Neither replaces a measurement on the installed circuit.

Why does aluminum drop more?

Aluminum has about 61 % of copper’s conductivity, so the same size has roughly 1.6 times the resistance; K is 21.2 instead of 12.9. Aluminum feeders are usually two sizes larger than copper for the same drop.

Does the ambient temperature change the drop?

Slightly. Table 8 and Table 9 are at 75 °C conductor temperature. Table 8 Note 2 gives R2 = R1 [1 + α (T2 − 75)] with α = 0.00323 for copper; a lightly loaded conductor at 30 °C has about 15 % less resistance than the table value, so the calculator errs on the safe side.

Voltage drop calculator

Conductor size

Voltage drop · K-factor method3.3%OVER 3 %3.95 V lost over 50 ft one-way at 20 A, 120 V single-phase, 12 AWG copper · 116.0 V at the load · NEC 2017 Chapter 9 Table 8

Length, one-way

Source to load, one way; the formula doubles it for the return conductor

Load current

Actual load, not the breaker size

Voltage

120, 240, or 12/24/48 for DC

K-factor method2 × K 12.9 × 20 A × 50 ft ÷ 6,530 cmil (NEC 2017 Chapter 9 Table 8 at 75 °C, unchanged since 2005); an approximation that ignores reactance: 3.95 V · 3.29 %
Chapter 9 Table 9 impedance · PVC conduit, PF 1.00Ze = R 2 × PF + XL 0.0540 × sin(arccos PF) = 2 Ω/kFT to neutral (NEC 2017 Chapter 9 Table 9, identical to 2005/2014); 60 Hz, 75 °C, three single conductors in conduit: 4.00 V · 3.33 %
Table 8 DC resistance1.98 Ω/kFT × 2 × length × current (NEC 2017 Chapter 9 Table 8); the basis the K-factor is rounded from: 3.96 V · 3.30 %
Smallest size within 3 % (K method)Within 5 %: 12 AWG: 10 AWG
Longest one-way run within 3 % on 12 AWGL = cmil × VD ÷ (2 × K × I): 46 ft

The 3 % (branch) and 5 % (feeder + branch) figures are NEC 210.19(A) Informational Note No. 4 and 215.2(A)(1) Informational Note No. 2: recommendations for reasonable efficiency, not requirements (90.5(C)). Voltage drop is mandatory only where a section says so, for example 647.4(D) sensitive electronic equipment (1.5 % / 2.5 %) and 695.7 fire pumps. Local amendments and energy codes may impose limits.

Conductor

Phase

Power factor

1 for resistive loads; 0.8–0.9 for motors; Table 9 method only

Raceway (Table 9 column)

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Pick the conductor on the gauge, type the one-way length, the load current and the voltage, and read the drop in volts and percent. Two methods are struck side by side: the K-factor shortcut most electricians use (12.9 for copper, 21.2 for aluminum, an approximation that ignores reactance) and the NEC Chapter 9 Table 9 impedance method with your power factor and raceway. The 3 % and 5 % figures are Informational Notes, not requirements; the page says so and names the sections where a limit is mandatory. The rim also shows the smallest size that stays within 3 % and the longest run the chosen size can make.

Worked examples

20 A on 12 AWG copper, 50 ft one-way, 120 V: K method: 2 × 12.9 × 20 × 50 ÷ 6,530 = 3.95 V, 3.29 %. Table 9 in PVC at PF 1: 2 × 20 × 2.0 Ω/kFT × 0.050 = 4.00 V, 3.33 %. Just over the 3 % note; 10 AWG brings it to 2.1 %. This is the classic “100 ft of 12 AWG” figure: 100 ft of conductor, 50 ft each way. Open this run
EV charger: 48 A on 6 AWG copper, 150 ft, 240 V: K method 7.08 V, 2.95 %; Table 9 at PF 1 in PVC 7.06 V, 2.94 %. Within 3 %, but only just: at 175 ft the run passes 3 % and 4 AWG is the next slot. Open this run
Detached garage feeder: 150 A on 2/0 aluminum, 200 ft, 240 V: K method 9.56 V, 3.98 %; Table 9 in PVC 9.60 V, 4.00 %. Over 3 %, under 5 %. 4/0 aluminum is the smallest size within 3 % (2.5 %). Open this run
Three-phase motor: 60 A on 4 AWG copper, 300 ft, 480 V, PF 0.85, steel conduit: K method 9.64 V, 2.01 %. Table 9 in steel: Ze = 0.31 × 0.85 + 0.060 × 0.527 = 0.295 Ω/kFT, VD = √3 × 60 × 0.295 × 0.3 = 9.20 V, 1.92 %. The two methods differ by 5 % here; at large sizes and low power factors they diverge further. Open this run

How it is worked out

K-factor method: VD = 2 × K × I × L ÷ CM for single-phase and DC (the current flows out and back), VD = √3 × K × I × L ÷ CM for balanced three-phase. K is 12.9 Ω·cmil/ft for copper and 21.2 for aluminum at 75 °C; CM is the circular-mil area from Chapter 9 Table 8; L is the one-way length in feet. Percent drop is VD ÷ source voltage × 100.

Table 9 method: effective impedance Ze = R × PF + XL × sin(arccos PF), with R and XL in ohms to neutral per 1000 ft from Chapter 9 Table 9 for the conductor material and raceway. VD = 2 × I × Ze × L ÷ 1000 single-phase, √3 × I × Ze × L ÷ 1000 three-phase. The table is for 60 Hz at 75 °C with three single conductors in conduit; other configurations are approximations.

Table 8 DC resistance is shown as a third row because it is what the K-factor is rounded from: VD = 2 × I × R × L ÷ 1000 with R in Ω/kFT. At 12 AWG copper, 1.98 Ω/kFT × 6,530 cmil ÷ 1000 = 12.9.

Longest run within 3 %: L = CM × (0.03 × V) ÷ (2 × K × I). Smallest size within 3 %: the first size on the rim whose K-method drop is at or under 3 %.

The figures behind it

NEC Chapter 9 Table 8, Conductor Properties: Circular-mil areas and DC resistance at 75 °C (ohm/kFT) for 18 AWG to 1000 kcmil, copper uncoated and aluminum. Read from the NFPA-typeset 2017 text and cross-checked cell by cell with a 2014 reproduction; the table is unchanged since at least 2005, so the 2023 values are inferred from that history rather than read from a 2023 copy. Checked 2026-09-05. Source
NEC Chapter 9 Table 9, AC Resistance and Reactance: Ohms to neutral per 1000 ft at 60 Hz, 75 °C, three single conductors in PVC, aluminum or steel conduit, 14 AWG to 1000 kcmil (no 700, 800, 900 kcmil rows; no 14 AWG aluminum). Effective Z is recomputed from R and XL for the entered power factor per Note 2. 2017 text, cross-checked with the 2007 California Electrical Code (NEC 2005) and the 2014 NEC. Source
K-factor 12.9 (copper) and 21.2 (aluminum): Ohm·cmil/ft at 75 °C, the Table 8 resistance per foot multiplied by the area; VD = 2·K·I·L/CM single-phase, √3·K·I·L/CM three-phase. IAEI Magazine, “Voltage Drop Formulas”, March 2017. Source
3 % and 5 % informational notes: NEC 210.19(A) Informational Note No. 4 and 215.2(A)(1) Informational Note No. 2, quoted verbatim from the 2017 text; 90.5(C) says informational notes are not enforceable. Mandatory limits exist only where a section states them (647.4(D), 695.7). Source

Questions people ask

Is 3 % voltage drop an NEC requirement?: No. 210.19(A) Informational Note No. 4 and 215.2(A)(1) Informational Note No. 2 say conductors sized so the drop does not exceed 3 % on the branch circuit and 5 % overall “provide reasonable efficiency of operation”. Under 90.5(C) informational notes are not enforceable. Limits become mandatory only where a section states them: 647.4(D) for sensitive electronic equipment (1.5 % branch, 2.5 % total) and 695.7 for fire pumps, plus whatever a local amendment or energy code adds.
Is the length one-way or round trip?: One-way. The formula multiplies by 2 (single-phase, DC) for the return conductor, or by √3 for three-phase. If you enter the total conductor length the answer is doubled. The often-quoted “3 % on 12 AWG at 20 A over 100 ft” is 100 ft of wire, 50 ft each way.
Which method should I trust?: For sizes up to about 1/0 and power factors near 1 the two agree within a few percent and the K-factor is fine. For large conductors, low power factor or steel conduit, reactance matters and Table 9 is the better estimate; that is why the NEC prints it. Neither replaces a measurement on the installed circuit.
Why does aluminum drop more?: Aluminum has about 61 % of copper’s conductivity, so the same size has roughly 1.6 times the resistance; K is 21.2 instead of 12.9. Aluminum feeders are usually two sizes larger than copper for the same drop.
Does the ambient temperature change the drop?: Slightly. Table 8 and Table 9 are at 75 °C conductor temperature. Table 8 Note 2 gives R2 = R1 [1 + α (T2 − 75)] with α = 0.00323 for copper; a lightly loaded conductor at 30 °C has about 15 % less resistance than the table value, so the calculator errs on the safe side.

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