What is voltage drop and why does it matter?

Voltage drop is the reduction in voltage along a conductor caused by the conductor's resistance and the load current. Excessive drop causes motors to run hot, lights to dim, and sensitive equipment to malfunction. The NEC (210.19/215.2 informational notes) recommends a maximum 3% drop on a branch circuit and 5% total from the service entrance to the final outlet. BS 7671 (IEC-derived UK standard) specifies 3% for lighting circuits and 5% for other circuits.

How is voltage drop calculated?

For single-phase AC: VD = (2 × K × I × L) ÷ CM, where K is the resistivity constant (12.9 for copper, 21.2 for aluminium), I is the load current in amps, L is one-way length in feet, and CM is the circular-mil area of the conductor. For three-phase: multiply by √3 instead of 2. The calculator handles both automatically.

What percentage voltage drop is acceptable for a branch circuit?

The NEC recommends — but does not mandate — no more than 3% voltage drop on a branch circuit (NEC 210.19 informational note). A combined feeder + branch drop of 5% total is also recommended. For critical loads such as medical equipment, many designers target 1–2%.

Does voltage drop depend on conductor material?

Yes. Copper has lower resistivity (roughly 1.724 µΩ·cm) than aluminium (roughly 2.82 µΩ·cm), so an aluminium conductor of the same size drops about 64% more voltage than copper. The calculator lets you switch between copper and aluminium conductors.

How do I reduce voltage drop on a long run?

The most effective options are: (1) increase conductor size — upsizing one AWG step roughly halves resistance; (2) reduce the one-way run length if the panel location is flexible; (3) raise the supply voltage (e.g. 240 V instead of 120 V drops proportionally less for the same wattage load); (4) split the load across two smaller circuits.

Is this voltage drop calculator free?

Yes — the voltage drop calculator is completely free to use with no account or signup required. Enter your conductor size, material, circuit length, load current, and voltage, and the results appear instantly.

Does the calculator support both AWG and metric (mm²) conductor sizes?

Yes. You can select AWG sizes (14 AWG through 1000 kcmil) or metric cross-sectional areas in mm². The calculator internally converts mm² to circular mils for the NEC formula.

Can I use this calculator for three-phase circuits?

Yes. Choose "Three-phase" in the circuit type selector and the formula automatically applies the √3 multiplier for balanced three-phase loads.

Voltage Drop Calculator — Free NEC & IEC Tool

Voltage Drop Formula (Single-Phase & Three-Phase)

The single-phase voltage drop formula is VD = (2 × K × I × L) ÷ CM, and the three-phase formula is VD = (1.732 × K × I × L) ÷ CM, where K = 12.9 for copper or 21.2 for aluminum, I is the load current in amperes, L is the one-way circuit length in feet, and CM is the conductor's circular-mil area. The NEC recommends keeping voltage drop to 3% or less on a branch circuit and 5% total including the feeder — a recommendation, not a mandatory limit. The calculator above evaluates the formula instantly; the sections below show the math and a max-run-length chart you can cite directly.

What each term means

VD is the voltage lost between the source and the load, in volts; divide it by the supply voltage for the percentage drop.
2 (single-phase) accounts for the out-and-back path — current flows through both the hot and the return conductor, so the circuit resistance covers twice the one-way length.
1.732 (three-phase) is √3, which replaces the factor 2 for balanced three-phase circuits because the return currents partially cancel between phases.
K is the direct-current resistivity constant in ohm-circular-mils per foot at 75 °C: 12.9 for copper and 21.2 for aluminum.
I is the load current in amperes.
L is the one-way length of the run in feet (the formula's factor of 2 or 1.732 already accounts for the return path).
CM is the conductor cross-sectional area in circular mils: 14 AWG = 4,110 CM, 12 AWG = 6,530 CM, 10 AWG = 10,380 CM, 8 AWG = 16,510 CM, 6 AWG = 26,240 CM.

Worked example: 20 A at 120 V over 100 ft of 12 AWG copper

A 20 A load at 120 V single-phase, 100 ft one-way on 12 AWG copper (6,530 CM):

VD = (2 × 12.9 × 20 × 100) ÷ 6,530 = 51,600 ÷ 6,530 = 7.90 V, which is 7.90 ÷ 120 = 6.6% — more than double the NEC 3% recommendation.

Upsizing to 10 AWG copper (10,380 CM): VD = 51,600 ÷ 10,380 = 4.97 V = 4.1% — better, but still above 3%. To meet the 3% recommendation at this length and load you need 8 AWG copper (16,510 CM): VD = 51,600 ÷ 16,510 = 3.13 V = 2.6%.

Maximum one-way run length at 20 A, 120 V, for 3% voltage drop

Maximum lengths below solve the formula for L with VD = 3.6 V (3% of 120 V), copper conductors, single-phase: L = (3.6 × CM) ÷ (2 × 12.9 × 20).

Copper wire size (AWG)	Circular mils (CM)	Max one-way run at 20 A / 120 V, 3% drop (ft)
14 AWG*	4,110	28
12 AWG	6,530	45
10 AWG	10,380	72
8 AWG	16,510	115
6 AWG	26,240	183

*14 AWG is shown for comparison only — it is limited to 15 A overcurrent protection (NEC 240.4(D)(3)) and may not be used on a 20 A circuit.

12 AWG copper can run about 45 feet one-way at 20 A and 120 V before exceeding 3% voltage drop.
10 AWG copper can run about 72 feet one-way at 20 A and 120 V before exceeding 3% voltage drop.
8 AWG copper can run about 115 feet one-way at 20 A and 120 V before exceeding 3% voltage drop.
6 AWG copper can run about 183 feet one-way at 20 A and 120 V before exceeding 3% voltage drop.
Doubling the voltage doubles the allowable length: at 240 V and the same 20 A, each figure above doubles (12 AWG reaches about 91 ft) because 3% of 240 V is 7.2 V.

What the NEC actually requires

The 3%/5% figures come from Informational Note 4 to NEC 210.19(A) (branch circuits) and Informational Note 2 to NEC 215.2(A) (feeders): a maximum 3% drop on the branch circuit, and 5% combined for feeder plus branch circuit, "provides reasonable efficiency of operation." Because these appear in informational notes, they are recommendations, not enforceable code requirements — an inspector cannot fail an installation solely for exceeding them (exception: certain specific rules such as fire pumps in NEC 695.7 and sensitive equipment per 647.4(D) do impose mandatory limits). They remain the standard design target for professional work.

When voltage drop matters most

Long runs: detached garages, workshops, barns, and gate openers — length is a direct multiplier in the formula, so a 200 ft run drops twice the voltage of a 100 ft run.
Well pumps: often 100–400 ft of cable down the well plus the run to the panel; undervoltage makes pump motors draw more current and overheat.
EV chargers: continuous 32–48 A loads for hours at a time — even a 3% drop wastes energy as heat in the wire on every charge, and long driveway or panel-to-garage runs commonly force an upsize.
Low-voltage landscape lighting: at 12 V, a drop of just 1.2 V is already 10% — the same absolute drop that would be negligible at 120 V. Low-voltage systems need dramatically larger wire or shorter runs.

Last verified against NEC 2023 210.19(A) Informational Note 4 and 215.2(A) Informational Note 2, with K constants and circular-mil areas from NEC Chapter 9, Table 8. Results are for planning; confirm with the current NEC edition and your local authority having jurisdiction.