Voltage Drop Calculator
Estimate voltage drop for a copper or aluminum run using the standard V = 2×K×I×L÷CM (single-phase) or √3×K×I×L÷CM (three-phase) formula, see whether it's within the commonly followed 3% branch / 5% total recommendations, or switch to max run length mode to solve for the longest run that stays under 3%.
7.9 V (6.58%)
Voltage at load: 112.1 V
Above the 5% combined recommendation — consider a larger wire gauge or a shorter run.
Circular mils by AWG (NEC Chapter 9, Table 8)
| AWG | Circular mils |
|---|---|
| #14 | 4,110 |
| #12 | 6,530 |
| #10 | 10,380 |
| #8 | 16,510 |
| #6 | 26,240 |
| #4 | 41,740 |
| #3 | 52,620 |
| #2 | 66,360 |
| #1 | 83,690 |
| #1/0 | 105,600 |
| #2/0 | 133,100 |
| #3/0 | 167,800 |
| #4/0 | 211,600 |
Educational estimate, not an engineering design. Final wire sizing and circuit design must be confirmed by a licensed electrician against the National Electrical Code and your local jurisdiction's code.
K constants: Southwire Voltage Drop Calculator · IAEI Magazine. Circular mils: NEC Chapter 9, Table 8. How we calculate →
How do you calculate voltage drop?
For a single-phase (or DC) circuit, the standard formula is V_drop = 2 × K × I × L ÷ CM, where K is a resistivity constant for the conductor material, I is the load current in amps, L is the ONE-WAY circuit length in feet, and CM is the conductor's cross-sectional area in circular mils. The factor of 2 accounts for current traveling to the load and back through the neutral or return conductor.
For a three-phase circuit, the return-path factor is replaced by √3 (≈1.732): V_drop = √3 × K × I × L ÷ CM, reflecting the phase geometry of a balanced three-phase load.
What is the K constant, and why does it differ by material?
K represents the conductor's DC resistance per circular-mil-foot at 75°C: 12.9 for copper and 21.2 for aluminum — aluminum's higher resistivity means an aluminum conductor needs roughly one size larger than copper to carry the same current with the same voltage drop. These constants come directly from the Southwire voltage drop calculator (a wire manufacturer's official tool) and are corroborated by IAEI Magazine's published voltage drop formulas.
What is an acceptable voltage drop under the NEC?
The NEC does not make voltage drop a hard, enforceable limit for most circuits — the guidance lives in Informational Notes attached to NEC 210.19 (branch circuits) and NEC 215.2 (feeders), which NEC 90.5 defines as explanatory, non-enforceable material.
That said, the widely followed recommendation (not a code requirement) is a maximum 3% drop on branch circuits and a combined 5% drop across feeder plus branch circuit. This calculator reports both thresholds as recommendations, not code violations — always confirm with a licensed electrician and your local jurisdiction.
How does circular mils (CM) relate to wire gauge (AWG)?
Circular mils measure a conductor's cross-sectional area — the bigger the number, the thicker the wire and the lower its resistance per foot. NEC Chapter 9, Table 8 gives the official circular-mil area for every standard AWG size: for example, #12 AWG copper is 6,530 cmil, while #4/0 AWG is 211,600 cmil — roughly 32 times the cross-section, which is why heavy-gauge wire carries high current over long runs with far less voltage drop.
Frequently asked questions
What is the voltage drop for #12 AWG copper, 20A, over 100 feet at 120V?
Using V_drop = 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.90V, which is 6.6% of 120V — above the 3% branch-circuit recommendation. To stay under 3% at 20A and 120V, the one-way run on #12 copper should be limited to about 46 feet; a longer run calls for a heavier gauge like #10 or #8 AWG.
What is an acceptable voltage drop percentage?
The commonly followed (though not code-mandated) guideline is a maximum 3% drop on the branch circuit and 5% combined across the feeder and branch circuit — for a 120V circuit, that's about 3.6V and 6V respectively. These are NEC Informational Note recommendations, not enforceable requirements.
Does aluminum wire have more voltage drop than copper?
Yes, for the same gauge: aluminum's K constant (21.2) is about 64% higher than copper's (12.9), so an aluminum conductor of the same AWG size will have roughly 64% more voltage drop than copper carrying the identical load over the identical distance. Aluminum runs are typically sized one or two AWG sizes larger than copper to compensate.
How do you calculate voltage drop for a three-phase circuit?
Replace the factor of 2 (used for single-phase/DC round-trip) with √3 (≈1.732): V_drop = √3 × K × I × L ÷ CM. For example, #6 AWG copper carrying 30A over a 150-foot run at 208V three-phase drops about 3.83V (1.8% — comfortably within the 3% recommendation).
What's the maximum wire run length before voltage drop becomes a problem?
It depends on the gauge, material, current and voltage — use 'Max run length mode' above to solve directly for the longest one-way run that stays within the 3% branch-circuit recommendation for your specific wire and load, rather than guessing and re-checking a fixed length.
Researched & verified by the Calcuris Data & Research Team. How we build and check our tools →