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K is the resistivity of the conductor in ohm-circular-mils per foot (12.9 for copper, 21.2 for aluminum at typical operating temperature), L is the one-way run length in feet, I is the load current in amps, and CM is the conductor cross-section in circular mils. Single-phase uses a factor of 2 (current out and back); three-phase uses √3.
- Circuit: 120 V, 1-phase, 20 A, 100 ft one-way, #10 Cu (10,380 cmil).
- VD = (2 × 12.9 × 100 × 20) ÷ 10,380 = 4.97 V (4.14%) — exceeds 3%.
- Upsize to #8 Cu (16,510 cmil): VD = 3.13 V = 2.6% ✓.
Reference tables
Recommended voltage-drop limits (NEC informational notes)
| Segment | Limit |
|---|---|
| Branch circuit | 3% |
| Feeder + branch (total) | 5% |
K value and common copper conductor areas
| Conductor | Circular mils (CM) | K (Cu) |
|---|---|---|
| #14 | 4,110 | 12.9 |
| #12 | 6,530 | 12.9 |
| #10 | 10,380 | 12.9 |
| #8 | 16,510 | 12.9 |
| #6 | 26,240 | 12.9 |
| 1/0 | 105,600 | 12.9 |
| 4/0 | 211,600 | 12.9 |
A number the code tables do not give: for a 20 A, 120 V, 1-phase circuit on #10 copper, the maximum one-way run at 3% is about 60 ft.
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FAQ
Is 3% voltage drop required by the NEC?
No. The 3% branch / 5% total figures are recommendations in NEC informational notes (210.19(A) and 215.2(A)), not enforceable rules — but most engineers and inspectors treat them as the design target for efficiency and performance.
Why does single-phase use 2 and three-phase use √3?
Single-phase current travels out on one conductor and back on another, so the drop happens across two lengths (factor 2). In a balanced three-phase circuit the return currents cancel, and the line-to-line drop works out to √3 times the one-way drop.
Copper or aluminum — does it matter?
Yes. Aluminum has about 61% of copper conductivity, so its K is 21.2 versus 12.9. For the same current and length, aluminum drops more voltage or needs a larger size.
Does length mean the round trip?
No — use the one-way run length. The factor of 2 (or √3) already accounts for the return path.
What if my drop is still too high after upsizing?
Options are a larger conductor, a higher system voltage, shortening the run, or a step-up/step-down transformer closer to the load. The calculator shows the smallest wire that clears your target.
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