Conductor sizing
Estimate voltage drop, percent drop, and end voltage for any run — copper or aluminum, single- or three-phase — and size the conductor to NEC's 3% branch and 5% combined guidance.
Voltage drop
—Enter a run to estimate voltage drop.
| Voltage drop | — |
|---|---|
| Percent drop | — |
| Voltage at load | — |
| Resistance used | — |
The estimate uses the single-conductor formula Vdrop = K × length × current × (R ÷ 1000). K is 2 for single-phase and √3 (about 1.732) for three-phase. R is the conductor's DC resistance in ohms per 1000 ft, taken from NEC Chapter 9, Table 8 for uncoated copper and aluminum. Percent drop is the drop divided by the source voltage.
The NEC suggests (informational note, not a hard rule) a branch circuit stay at or below 3% and the feeder plus branch stay at or below 5%. The calculator flags both. When a run exceeds 3%, it suggests the thinnest conductor that brings it back under 3% so you don't over-buy copper.
The NEC does not set a general, enforceable voltage-drop limit for branch circuits or feeders. The familiar 3% and 5% figures come from Informational Notes — notably NEC 210.19(A) Informational Note No. 4, which recommends limiting branch-circuit drop to 3% and combined feeder-plus-branch drop to 5%. Informational notes are advisory, not enforceable requirements. A few cases do carry mandatory limits, such as fire pump circuits and sensitive electronic equipment under Article 647, and some local jurisdictions and energy codes adopt the 3%/5% guidance as a hard requirement — so check with your AHJ. In practice, treat 3%/5% as the design target: run this calculator for your length, load, and conductor, and if the drop comes in high, step up a wire size.
Based on NEC Ch.9 Table 8 (DC resistance, ~75°C, uncoated conductors).
Voltage drop = K × length (ft) × current (A) × (R ÷ 1000), where K is 2 for single-phase or √3 for three-phase, and R is the conductor's DC resistance in ohms per 1000 ft. Percent drop is the voltage drop divided by the source voltage, times 100.
The NEC's informational notes recommend keeping a branch circuit at or below 3% and the feeder plus branch combined at or below 5%. These are recommendations (FPN), not enforceable rules, but most designers and inspectors treat 3% as the practical target.
For the same size, aluminum has higher resistance than copper, so it drops more voltage. To carry the same load over the same distance with a similar drop, aluminum usually needs to be one or two sizes larger than copper.
In most cases, no. The 3% branch-circuit and 5% combined figures live in NEC Informational Notes (such as 210.19(A) Informational Note No. 4), which are advisory rather than enforceable code. There are exceptions: fire pump circuits have enforceable requirements, and sensitive electronic equipment under Article 647 has a mandatory voltage-drop limit. Some local jurisdictions and energy codes also adopt the 3%/5% guidance as a binding rule, and the authority having jurisdiction (AHJ) has the final say — so confirm locally. Even where it is optional, staying at or under roughly 3% is a sensible target: excessive drop wastes energy and can mean dim lights, sluggish motors, and nuisance tripping.
A common shortcut for single-phase circuits is VD = 2 × K × I × L / CM, where K is roughly 12.9 ohm-cmil/ft for copper at 75°C (about 21.2 for aluminum), I is the load current in amps, L is the one-way run length in feet, and CM is the conductor's circular-mil area. For three-phase, replace the 2 with 1.732. Alternatively, use the approach this calculator takes: pull the conductor's resistance from NEC Chapter 9 Table 8 (ohms per 1,000 ft) and compute VD = 2 × R × I × L / 1,000 (again swapping 2 for 1.732 on three-phase). Both are estimates; actual drop varies with temperature, power factor, and terminations. Divide VD by the source voltage and multiply by 100, then check the percentage against the 3%/5% guidance.
It estimates voltage drop only, using NEC Chapter 9 Table 8 DC resistance. It does not size for ampacity, temperature derating, conduit fill, power factor, or continuous-load factors, and it is not a substitute for a licensed electrician or your local code (AHJ).
Estimate only, based on NEC Chapter 9 Table 8 DC resistance. This is not a substitute for a licensed electrician or your local code (AHJ). It does not fully model ampacity, temperature derating, power factor, or continuous-load factors. Always verify against the current NEC and local code before wiring. Based on NEC Ch.9 Table 8.