The formula is Ohm's law, V = I × R, with the resistance worked out for the actual length of wire in the circuit. Everything else is bookkeeping.
| Size | Uncoated copper | Aluminum |
|---|---|---|
| 14 | 3.14 | — |
| 12 | 1.98 | 3.25 |
| 10 | 1.24 | 2.04 |
| 8 | 0.778 | 1.28 |
| 6 | 0.491 | 0.808 |
| 4 | 0.308 | 0.508 |
| 3 | 0.245 | 0.403 |
| 2 | 0.194 | 0.319 |
| 1 | 0.154 | 0.253 |
| 1/0 | 0.122 | 0.201 |
| 2/0 | 0.0967 | 0.159 |
| 3/0 | 0.0766 | 0.126 |
| 4/0 | 0.0608 | 0.1 |
| 250 kcmil | 0.0515 | 0.0847 |
| 350 kcmil | 0.0367 | 0.0605 |
| 500 kcmil | 0.0258 | 0.0424 |
A 20 amp kitchen circuit run 100 feet from the panel, carrying a full 20 A load.
VD = 2 × 20 × 1.98 × 100 ÷ 1000
VD = 2 × 20 × 0.198 = 7.92 volts
Percentage = 7.92 ÷ 120 × 100 = 6.6%
Fails the 3% recommendation badly. Upsize to 10 AWG (4.96 V, 4.1%) or 8 AWG (3.11 V, 2.6%). At 120 V, long runs are punishing — the same circuit at 240 V would be at 3.3%.
An electric range on a 50 A double-pole breaker, 60 feet of 6 AWG copper.
VD = 2 × 50 × 0.491 × 60 ÷ 1000
VD = 2 × 50 × 0.02946 = 2.95 volts
Percentage = 2.95 ÷ 240 × 100 = 1.2%
Comfortably within the 3% recommendation. No upsizing needed.
A feeder to a detached garage, 150 feet one way, carrying a calculated 80 A load.
VD = 2 × 80 × 0.245 × 150 ÷ 1000
VD = 2 × 80 × 0.03675 = 5.88 volts
Percentage = 5.88 ÷ 240 × 100 = 2.5%
Within the 5% feeder-plus-branch allowance with 2.5% left for the branch circuits inside the garage. Note that upsizing here would also require upsizing the equipment grounding conductor under NEC 250.122(B).
A 65 A three-phase motor load 400 feet from the distribution panel.
VD = 1.732 × 65 × 0.194 × 400 ÷ 1000
VD = 1.732 × 65 × 0.0776 = 8.74 volts
Percentage = 8.74 ÷ 480 × 100 = 1.8%
Within 3%. Higher voltages carry the same power at lower current, which is exactly why long runs are done at 480 V rather than 240 V.
Most real problems are not "what is the drop" but "how big a wire do I need" or "how far can I run this". Solve for the term you want:
R = (VD × 1000) ÷ (2 × I × L)
Maximum allowable resistance per 1000 feet. Look up the first conductor in the table above with a resistance at or below this figure.
L = (VD × 1000) ÷ (2 × I × R)
Maximum one-way run for a given conductor. This is what the distance charts tabulate.
VDmax = Vnominal × 0.03
The 3% branch-circuit target: 3.6 V at 120 V, 7.2 V at 240 V, 14.4 V at 480 V.
Worked backwards: a 30 A circuit at 240 V running 200 feet. Allowable drop at 3% is 7.2 V. R = (7.2 × 1000) ÷ (2 × 30 × 200) = 0.60 ohms per 1000 feet. The first copper conductor at or below 0.60 is 6 AWG at 0.491. So a 30 A circuit that only needs 10 AWG for ampacity needs 6 AWG at 200 feet.
Voltage drop appears in the NEC as Informational Notes, not as enforceable rules, in 210.19(A) for branch circuits and 215.2(A) for feeders. The guidance is 3% maximum on the branch circuit and 5% maximum for the feeder and branch circuit combined.
Informational Notes are explicitly not enforceable under NEC 90.5(C), so an inspector cannot fail an installation for a 4% drop on that basis alone. Several other articles do make it mandatory in specific cases — 647.4(D) for sensitive electronic equipment and 695.7 for fire pump feeders.
What excessive drop actually costs you: incandescent lamps dim noticeably and lose far more light output than the voltage lost; motors draw higher current at lower voltage and run hotter, shortening their life; electronics with switching supplies compensate by drawing more current, which makes the drop worse; and every volt dropped is power turned into heat inside your walls.
Our calculator applies this formula along with NEC Table 310.16 ampacity and the 240.4(D) small-conductor limits, and returns the larger of the two requirements.
Open the wire size calculatorFor a single-phase circuit, VD = 2 × I × R × L ÷ 1000, where I is the current in amperes, R is the conductor resistance in ohms per 1000 feet, and L is the one-way run length in feet. The 2 accounts for current travelling out on the hot and back on the neutral. For three-phase, replace the 2 with the square root of 3, about 1.732.
Because current flows through two conductors — out along the ungrounded conductor and back along the neutral or the second hot leg. The physical length of wire in the circuit is twice the one-way run, so the resistance is doubled. In a three-phase circuit the return currents partly cancel, which is why the multiplier drops to the square root of 3.
The NEC Informational Notes to 210.19(A) and 215.2(A) recommend a maximum of 3% on a branch circuit and 5% total across the feeder plus branch circuit combined. These are recommendations rather than enforceable requirements in most installations, but exceeding them causes dim lights, slow motor starts, and wasted energy as heat in the wire.
Divide the voltage drop by the nominal system voltage and multiply by 100. A 4.8 volt drop on a 240 V circuit is 4.8 ÷ 240 × 100 = 2.0%. Always use the nominal system voltage, not the measured voltage at the panel.
It can. Ampacity sets the minimum wire size for safety; voltage drop sets a second minimum for performance on long runs. You use whichever is larger. A 20 A load at 120 V needs 12 AWG for ampacity, but 12 AWG passes 3% drop at about 45 feet one-way; 10 AWG reaches about 73 feet and 8 AWG about 115 feet.