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Cable Voltage Drop Calculator

Calculate voltage drop across a cable run. Enter supply voltage, load current, wire gauge (AWG), and one-way distance to get voltage drop in volts and percent, power loss, and NEC 3%/5% compliance status. Supports copper and aluminum conductors.

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Formula

Vdrop=I⋅2⋅d⋅R/km1000V_{drop} = I \cdot \frac{2 \cdot d \cdot R_{/km}}{1000}
I— Load current (A)
d— One-way cable length (m)
R_km— Wire resistance per km (from AWG) (Ω/km)
V_drop%— Drop as percentage of supply (%)

How It Works

Voltage drop across a cable is the reduction in voltage between the source (panel/PSU) and the load, caused by conductor resistance. Per Ohm's law, V_drop = I x R_total, where R_total is the round-trip resistance (outgoing + return conductor). For single-phase or DC circuits, the total cable resistance is 2 x length x R_per_km / 1000, doubling the one-way distance because current flows through both conductors. The NEC (National Electrical Code, Article 210.19 Informational Note 4) recommends maximum 3% voltage drop for branch circuits and 5% total (feeder + branch) for acceptable performance. These are recommendations, not requirements, but exceeding them causes reduced equipment performance, increased heating, motor torque reduction, and LED/lighting flicker. Wire resistance depends on material (copper: 1.72e-8 ohm-m, aluminum: 2.82e-8 ohm-m at 20C), cross-sectional area, and temperature. AWG (American Wire Gauge) is logarithmic: each 3-gauge increase doubles the resistance (halves the area). Temperature increases resistance by approximately 0.393%/C for copper above 20C. For long cable runs (solar arrays, EV charging, large motors), voltage drop often requires oversizing conductors beyond the minimum ampacity rating from NEC Table 310.16.

Worked Example

Problem

A 12V DC solar system feeds a 15A load through 30 meters of 12 AWG copper cable. Calculate the voltage drop and determine if it meets the 3% recommendation.

Solution
  1. System parameters: V_supply = 12V, I = 15A, d = 30m (one-way), Wire = 12 AWG copper
  2. 12 AWG copper resistance: 5.211 ohm/km (per NEC Chapter 9, Table 8)
  3. Round-trip distance: 2 x 30m = 60m (DC or single-phase)
  4. Total resistance: R = 2 x 30 x 5.211 / 1000 = 0.3127 ohm
  5. Voltage drop: V_drop = 15A x 0.3127 ohm = 4.69V
  6. Percentage: 4.69 / 12 x 100 = 39.1%
  7. Voltage at load: 12 - 4.69 = 7.31V
  8. Power loss in cable: P = I^2 x R = 225 x 0.3127 = 70.3W
Assessment: 39.1% drop is catastrophic for a 12V system! The system will not function.

Solution: Maximum cable length for 3% drop = (0.03 x 12 x 1000) / (2 x 15 x 5.211) = 2.3m.

Fix: Upgrade to 2 AWG (0.5127 ohm/km): V_drop = 15 x 2 x 30 x 0.5127/1000 = 0.46V = 3.84%. Still marginal. Better fix: Increase system voltage to 48V (then 15A delivers same power at lower current with acceptable drop), or relocate equipment closer to panels.

Practical Tips

  • ✓Quick reference for 120V copper circuits at maximum 3% drop (3.6V): 14 AWG = 15A max 14m, 12 AWG = 20A max 11m, 10 AWG = 30A max 11m. For 240V circuits (7.2V drop budget), distances double. For 12V DC systems, distances are 1/10th of 120V values — this is why low-voltage solar and automotive systems require very thick cables for any meaningful distance.
  • ✓Aluminum conductors have 1.61x the resistance of copper for the same gauge. To get equivalent performance, upsize aluminum by 2 AWG (e.g., use 2 AWG aluminum where you'd use 4 AWG copper). Aluminum is common in service entrance cables (SE, SER) and large feeders because it's lighter and cheaper, despite needing larger conduit. Always use aluminum-rated terminals (AL/CU marked) to prevent galvanic corrosion.
  • ✓For solar installations, voltage drop is especially critical because you lose energy at both low voltage AND high current conditions. A 3% drop at rated current means 3% of generated energy is lost as heat in cables every day. Over 25 years, this adds up to significant lifetime energy loss. Many solar designers target 1-2% drop, accepting the higher upfront wire cost for better lifetime ROI. Use the 'max length for 3%' output to verify your cable sizing.
  • ✓Parallel conductors reduce effective resistance. Two identical cables in parallel halve the resistance (and the voltage drop). NEC 310.10(H) allows paralleling conductors 1/0 AWG and larger. For large loads where a single cable would be impractically thick (e.g., 200A at 100m), using two parallel runs of smaller cable is often more practical and may be cheaper than a single oversized cable. Each parallel conductor must have identical length, material, and termination.

Common Mistakes

  • ✗Forgetting to account for the return conductor (doubling the distance). In a DC or single-phase AC circuit, current flows through BOTH conductors (hot and neutral/return). The total resistance is 2x the one-way cable resistance. This is the most common error and results in actual voltage drop being double the calculated value. Only three-phase balanced loads use a factor of sqrt(3) instead of 2.
  • ✗Using NEC Table 310.16 ampacity ratings to size cables for long runs. Table 310.16 gives the MAXIMUM current a conductor can carry without overheating (thermal limit), but it does NOT account for voltage drop. A 12 AWG wire is rated 20A for ampacity, but over a 50m run at 20A on 120V, the drop is 8.7% (far exceeding 3%). Always check voltage drop separately from ampacity; for long runs, voltage drop usually requires oversizing the conductor.
  • ✗Ignoring temperature effects on resistance. Copper resistance increases ~0.393%/C above 20C. In a hot attic (60C), resistance increases by 15.7%. In a solar installation where cables reach 75C under sun exposure, resistance increases by 21.6%. NEC Table 8 values are at 75C (not 20C) precisely for this reason. For critical calculations (large solar arrays, data centers), use resistance values at expected operating temperature.
  • ✗Applying single-phase voltage drop formulas to three-phase systems. For balanced three-phase loads, V_drop = sqrt(3) x I x R x L / 1000 (line-to-line drop), or equivalently, the multiplier is 1.732 instead of 2. Using the single-phase formula (multiplier = 2) overestimates three-phase drop by 15%. Also note: for three-phase, the 'length' is still one-way distance since current returns through the other two phases.

Frequently Asked Questions

NEC recommends (not requires) maximum 3% voltage drop for branch circuits and 5% total for feeder + branch combined (Article 210.19 Informational Note 4 and 215.2 Informational Note 2). For 120V: 3% = 3.6V drop, 5% = 6V drop. For 240V: 3% = 7.2V, 5% = 12V. For 12V DC (solar/automotive): 3% = 0.36V. Note these are informational notes, not mandatory requirements. However, many local jurisdictions and project specifications enforce them. Sensitive equipment (VFDs, PLCs, medical devices) may require tighter limits of 1-2%.
Five approaches: (1) Increase wire gauge (most common) — each 3-AWG increase halves resistance; (2) Shorten cable run — relocate panel or equipment closer; (3) Increase system voltage — 240V has 1/4 the drop of 120V for same power delivery; (4) Use parallel conductors — two cables halve the drop; (5) Reduce current — use higher voltage or lower power equipment. For existing installations where rewiring is impractical, options 3-5 may be more cost-effective than pulling new cable.
Yes, equally as DC for resistive loads. For AC circuits, impedance (not just resistance) matters: Z = sqrt(R^2 + X_L^2), where X_L is inductive reactance of the cable. For small conductors (14-10 AWG) at 60 Hz, reactance is negligible. For large conductors (4/0+) in steel conduit, inductive reactance can add 20-30% to effective impedance. This calculator uses resistance only, which is accurate for residential/light commercial wiring (14-6 AWG). For large industrial feeders (500 kcmil in conduit), use NEC Chapter 9 Table 9 which includes reactance.
For the same power delivery, a 12V system carries 10x the current of a 120V system (P=V*I). Since voltage drop is V_drop = I*R, the 10x higher current creates 10x the voltage drop in the same cable. Additionally, the 3% budget is only 0.36V at 12V vs 3.6V at 120V. Combined effect: a 12V system needs approximately 100x lower resistance (much thicker, shorter cables) to deliver the same power within the same percentage drop limit. This is why solar arrays, data centers, and EV charging use higher voltages (48V, 380VDC, or 400VAC) for efficiency.

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