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Voltage drop calculator
NEC-compliant. Single & three-phase. Copper or aluminum. 14 AWG through 1000 kcmil. Live results with NEC 210.19(A) and 215.2(A) limits flagged inline. No signup, no ads, no tracking. Built by a 20-year electrician and IBEW member.
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Install SparkRef →VD = (2 · K · I · L) / CM (single-phase) · VD = (1.732 · K · I · L) / CM (three-phase) · K = 12.9 (Cu) / 21.2 (Al) · NEC 210.19(A) Inf. Note 4: ≤ 3% branch · 215.2(A) Inf. Note 2: ≤ 5% combined.
How voltage drop is calculated
Voltage drop is the loss of voltage across a conductor due to its resistance. The longer the run, the smaller the conductor, or the higher the current, the more voltage drops between the source and the load. If drop is excessive, motors run hot, lights dim, electronics misbehave, and code-compliance fails.
The formulas
The K-value method is the standard for job-site voltage drop calculation:
- Single phase: VD = (2 × K × I × L) ÷ CM
- Three phase: VD = (1.732 × K × I × L) ÷ CM
Where:
- K is the conductor resistance constant — 12.9 for copper, 21.2 for aluminum
- I is the load current in amps
- L is the one-way distance in feet (the formula's leading factor accounts for the round trip)
- CM is the conductor area in circular mils (look up in NEC Chapter 9 Table 8)
NEC voltage drop limits
Voltage drop is not enforceable code in itself, but two NEC informational notes set the de-facto standard inspectors and engineers use:
- NEC 210.19(A) Informational Note 4: Branch-circuit voltage drop should not exceed 3%.
- NEC 215.2(A) Informational Note 2: Combined feeder + branch-circuit voltage drop should not exceed 5%.
The calculator above flags your result against these thresholds — green for compliant, yellow for between 3% and 5% (acceptable as feeder, not as branch), red for over 5%.
Common scenarios worked out
Three real situations electricians hit often, with the right answer:
1. 20A residential circuit, 100 ft, 12 AWG copper, 240V single phase
VD = (2 × 12.9 × 20 × 100) ÷ 6,530 = 7.9 V (3.3%). Just over the 3% recommendation — upsize to 10 AWG and you're at 2.1%, comfortably under.
2. 60A subpanel feeder, 150 ft, 6 AWG copper, 240V single phase
VD = (2 × 12.9 × 60 × 150) ÷ 26,240 = 8.85 V (3.7%). Acceptable as feeder under 215.2(A) (5%), but if the subpanel feeds branch circuits you've already used most of the combined budget — verify the branches don't exceed the remaining 1.3%.
3. 200A service feeder, 250 ft, 250 kcmil aluminum, 240V single phase
VD = (2 × 21.2 × 200 × 250) ÷ 250,000 = 8.48 V (3.5%). Under the 5% feeder limit — but if this is a long agricultural or shop run, consider upsizing to 350 kcmil (drop falls to 2.5%) for better motor starting and inverter behavior.
Need more than voltage drop?
This calculator is one of several in the SparkRef Progressive Web App. The full app runs offline, installs from your browser (no App Store), and includes:
- Wire ampacity calculator (NEC Table 310.16, with ambient and raceway derating)
- Conduit fill calculator
- Box fill calculator
- Wire size visualizer (16 AWG to 1000 kcmil, drawn to actual scale)
- Trade calculator (feet-inches, fractions, trig, Benfield bend presets)
- NEC article search and reference
Voltage drop FAQ
What is the formula for voltage drop?
Single phase: VD = (2 × K × I × L) ÷ CM. Three phase: VD = (1.732 × K × I × L) ÷ CM. K = 12.9 (Cu), 21.2 (Al). I = amps. L = one-way feet. CM = circular mils (NEC Chapter 9 Table 8).
What is the NEC voltage drop limit?
NEC 210.19(A) recommends ≤ 3% on branch circuits. 215.2(A) recommends ≤ 5% combined feeder + branch. Both are recommendations, not enforceable rules — but most jurisdictions enforce them as standard practice.
Should I use one-way or round-trip distance?
One-way. The formula's leading factor (2 for single phase, 1.732 for three phase) already accounts for the round trip.
Why does aluminum need a larger conductor?
Aluminum has higher resistance per circular mil (K = 21.2) than copper (K = 12.9). For the same current and distance, aluminum drops about 64% more voltage — so you upsize one to two AWG sizes to compensate.
Does temperature affect voltage drop?
Yes. Conductor resistance rises with temperature — about 0.4% per °C above 75°C ambient for copper. For runs near rated ampacity in hot environments, the actual drop can exceed this calculator's estimate by 5–10%. The K-value method assumes 75°C operating temperature.