Voltage Drop Calculator
Single-phase and three-phase voltage drop by wire size, current, and one-way circuit length, checked against the NEC's informational 3% and 5% guidance.
How this is calculated
This calculator uses the standard K-factor method taught for NEC exam prep and field use:
VD = (2 × K × I × D) ÷ CM (single-phase)
VD = (1.732 × K × I × D) ÷ CM (three-phase)
Where K is the resistivity constant (12.9 for copper, 21.2 for aluminum — approximate ohms·cmil/ft at typical operating temperature), I is load current in amps, D is one-way circuit length in feet, and CM is the conductor's circular-mil area from NEC Chapter 9, Table 8. The 2× (or 1.732× for three-phase) factor accounts for the full circuit, not just one leg.
For a more precise result, an estimator/engineer can instead use actual AC resistance and reactance values from NEC Chapter 9, Table 9, which account for conduit type and power factor — this K-factor method is a widely used simplified approximation, good for field estimating and most residential/light-commercial work.
NEC voltage drop guidance
NEC 210.19(A) and 215.2(A) include informational notes recommending voltage drop be limited to 3% for a branch circuit, and 5% total for branch circuit plus feeder combined. These are informational recommendations for efficient design, not enforceable code requirements — but exceeding them commonly causes dim lighting, motor overheating, and equipment that underperforms its rating.
| AWG / kcmil | Circular mils |
|---|---|
| 14 AWG | 4,110 |
| 12 AWG | 6,530 |
| 10 AWG | 10,380 |
| 8 AWG | 16,510 |
| 6 AWG | 26,240 |
| 4 AWG | 41,740 |
| 3 AWG | 52,620 |
| 2 AWG | 66,360 |
| 1 AWG | 83,690 |
| 1/0 AWG | 105,600 |
| 2/0 AWG | 133,100 |
| 3/0 AWG | 167,800 |
| 4/0 AWG | 211,600 |
| 250 kcmil | 250,000 |
| 300 kcmil | 300,000 |
| 350 kcmil | 350,000 |
| 400 kcmil | 400,000 |
| 500 kcmil | 500,000 |
In-depth guide: Voltage drop formula, with worked examples
How this calculator works
K-factor method: VD = 2 × K × I × D ÷ CM (×1.732 instead of 2 for three-phase), K = 12.9 copper / 21.2 aluminum; circular mils from NEC Chapter 9, Table 8; 3% / 5% guidance from the informational notes to NEC 210.19(A) and 215.2(A). Metric mode: simplified method of the Spanish Guía Técnica de Aplicación del REBT, Anexo 2 (e = 2·P·L/(γ·S·U) single-phase, P·L/(γ·S·U) three-phase, reactance neglected up to 120 mm², γ from its Table 3) and the REBT limits of its Table 6.
Maximum one-way distance for a 3% voltage drop (copper, single-phase)
Feet of one-way circuit length before the drop reaches 3%, using the same K-factor formula as the calculator above (K = 12.9 for copper). Longer runs need the next wire size up.
| Wire · 120 V | 15 A | 20 A | 30 A | 40 A | 50 A |
|---|---|---|---|---|---|
| 14 AWG | 38 ft | 28 ft | 19 ft | 14 ft | 11 ft |
| 12 AWG | 60 ft | 45 ft | 30 ft | 22 ft | 18 ft |
| 10 AWG | 96 ft | 72 ft | 48 ft | 36 ft | 28 ft |
| 8 AWG | 153 ft | 115 ft | 76 ft | 57 ft | 46 ft |
| 6 AWG | 244 ft | 183 ft | 122 ft | 91 ft | 73 ft |
| 4 AWG | 388 ft | 291 ft | 194 ft | 145 ft | 116 ft |
| 2 AWG | 617 ft | 462 ft | 308 ft | 231 ft | 185 ft |
| 1/0 AWG | 982 ft | 736 ft | 491 ft | 368 ft | 294 ft |
| Wire · 240 V | 15 A | 20 A | 30 A | 40 A | 50 A |
|---|---|---|---|---|---|
| 14 AWG | 76 ft | 57 ft | 38 ft | 28 ft | 22 ft |
| 12 AWG | 121 ft | 91 ft | 60 ft | 45 ft | 36 ft |
| 10 AWG | 193 ft | 144 ft | 96 ft | 72 ft | 57 ft |
| 8 AWG | 307 ft | 230 ft | 153 ft | 115 ft | 92 ft |
| 6 AWG | 488 ft | 366 ft | 244 ft | 183 ft | 146 ft |
| 4 AWG | 776 ft | 582 ft | 388 ft | 291 ft | 232 ft |
| 2 AWG | 1,234 ft | 925 ft | 617 ft | 462 ft | 370 ft |
| 1/0 AWG | 1,964 ft | 1,473 ft | 982 ft | 736 ft | 589 ft |
Worked example
A 20 A load on a 120 V single-phase circuit, 100 ft one-way, copper conductors.
12 AWG: VD = (2 × 12.9 × 20 × 100) ÷ 6,530 = 7.90 V, or 6.58% — well past the 5% total guidance. 10 AWG: 4.97 V (4.14%) — under 5% but still over the 3% branch target. 8 AWG: 3.13 V (2.60%) — within 3%, leaving about 116.9 V at the load.
So a run that is "only" a 20 A circuit can still need 8 AWG once the distance gets long, even though 12 AWG is enough for the ampacity.
Frequently asked questions
What is an acceptable voltage drop?
The NEC informational notes in 210.19(A) and 215.2(A) recommend no more than 3% on a branch circuit and 5% total for feeder plus branch circuit. They are design recommendations, not enforceable requirements, but most engineers and inspectors treat them as the target.
Do I use one-way or round-trip distance?
Enter the one-way distance from the source to the load. The formula already multiplies by 2 for single-phase (out and back) or by 1.732 for three-phase.
Why does aluminum show a bigger voltage drop than copper?
Aluminum has higher resistivity — K is about 21.2 versus 12.9 for copper — so the same size conductor drops roughly 64% more voltage. Aluminum runs are usually upsized one or two sizes to compensate.
Is the K-factor method accurate enough?
It is the standard simplified method for field estimating and most residential and light-commercial work. For long feeders or low power factor loads, engineers use the AC resistance and reactance values from NEC Chapter 9, Table 9 instead.
Common mistakes
- Entering the round-trip length. Enter the one-way distance from the panel to the load. The single-phase formula already multiplies by 2 for the return conductor, so doubling it yourself counts the run four times.
- Stopping at the ampacity check. A conductor that is big enough for the current can still drop too much voltage on a long run. A 20 A, 120 V circuit 100 ft long is fine on 12 AWG for ampacity but needs 8 AWG to stay within 3%.
- Checking only the branch circuit. The NEC recommendation is 3% for a branch circuit or feeder and 5% for both together. A feeder that already drops 3% leaves only 2% for the branch circuits it serves.
- Metric mode: using the 20 °C conductivity. The REBT guide's simplified method takes the cable at its maximum service temperature: 70 °C for PVC insulation, 90 °C for XLPE or EPR. The 20 °C value gives a smaller drop than the cable will really have under load.
What it does not cover
AC reactance and power factor on very large conductors.
Sources
- NFPA 70, National Electrical Code (free read-only access)
- Guía Técnica de Aplicación del REBT, Anexo 2: Cálculo de las caídas de tensión (Ministerio de Industria)
- Real Decreto 842/2002, Reglamento electrotécnico para baja tensión (BOE)
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